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doc/mkdocs/docs/images
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# Configuration options related to the input files
#---------------------------------------------------------------------------
INPUT = ../single_include/nlohmann/json.hpp \
index.md \
faq.md \
binary_formats.md
index.md
INPUT_ENCODING = UTF-8
FILE_PATTERNS =
RECURSIVE = NO

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# Binary formats
![conversion between JSON and binary formats](images/binary.png)
Several formats exist that encode JSON values in a binary format to reduce the size of the encoded value as well as the required effort to parse encoded value. The library implements three formats, namely
- [CBOR](https://tools.ietf.org/html/rfc7049) (Concise Binary Object Representation)
- [MessagePack](https://msgpack.org)
- [UBJSON](http://ubjson.org) (Universal Binary JSON)
## Interface
### JSON to binary format
For each format, the `to_*` functions (i.e., `to_cbor`, `to_msgpack`, and `to_ubjson`) convert a JSON value into the respective binary format. Taking CBOR as example, the concrete prototypes are:
```cpp
static std::vector<uint8_t> to_cbor(const basic_json& j); // 1
static void to_cbor(const basic_json& j, detail::output_adapter<uint8_t> o); // 2
static void to_cbor(const basic_json& j, detail::output_adapter<char> o); // 3
```
The first function creates a byte vector from the given JSON value. The second and third function writes to an output adapter of `uint8_t` and `char`, respectively. Output adapters are implemented for strings, output streams, and vectors.
Given a JSON value `j`, the following calls are possible:
```cpp
std::vector<uint8_t> v;
v = json::to_cbor(j); // 1
json::to_cbor(j, v); // 2
std::string s;
json::to_cbor(j, s); // 3
std::ostringstream oss;
json::to_cbor(j, oss); // 3
```
### Binary format to JSON
Likewise, the `from_*` functions (i.e, `from_cbor`, `from_msgpack`, and `from_ubjson`) convert a binary encoded value into a JSON value. Taking CBOR as example, the concrete prototypes are:
```cpp
static basic_json from_cbor(detail::input_adapter i, const bool strict = true); // 1
static basic_json from_cbor(A1 && a1, A2 && a2, const bool strict = true); // 2
```
Both functions read from an input adapter: the first function takes it directly form argument `i`, whereas the second function creates it from the provided arguments `a1` and `a2`. If the optional parameter `strict` is true, the input must be read completely (or a parse error exception is thrown). If it is false, parsing succeeds even if the input is not completely read.
Input adapters are implemented for input streams, character buffers, string literals, and iterator ranges.
Given several inputs (which we assume to be filled with a CBOR value), the following calls are possible:
```cpp
std::string s;
json j1 = json::from_cbor(s); // 1
std::ifstream is("somefile.cbor", std::ios::binary);
json j2 = json::from_cbor(is); // 1
std::vector<uint8_t> v;
json j3 = json::from_cbor(v); // 1
const char* buff;
std::size_t buff_size;
json j4 = json::from_cbor(buff, buff_size); // 2
```
## Details
### CBOR
The mapping from CBOR to JSON is **incomplete** in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors (parse_error.112):
- byte strings (0x40..0x5F)
- date/time (0xC0..0xC1)
- bignum (0xC2..0xC3)
- decimal fraction (0xC4)
- bigfloat (0xC5)
- tagged items (0xC6..0xD4, 0xD8..0xDB)
- expected conversions (0xD5..0xD7)
- simple values (0xE0..0xF3, 0xF8)
- undefined (0xF7)
CBOR further allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps with keys other than UTF-8 strings are rejected (parse_error.113).
The mapping from JSON to CBOR is **complete** in the sense that any JSON value type can be converted to a CBOR value.
If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the dump() function which serializes NaN or Infinity to null.
The following CBOR types are not used in the conversion:
- byte strings (0x40..0x5F)
- UTF-8 strings terminated by "break" (0x7F)
- arrays terminated by "break" (0x9F)
- maps terminated by "break" (0xBF)
- date/time (0xC0..0xC1)
- bignum (0xC2..0xC3)
- decimal fraction (0xC4)
- bigfloat (0xC5)
- tagged items (0xC6..0xD4, 0xD8..0xDB)
- expected conversions (0xD5..0xD7)
- simple values (0xE0..0xF3, 0xF8)
- undefined (0xF7)
- half and single-precision floats (0xF9-0xFA)
- break (0xFF)
### MessagePack
The mapping from MessagePack to JSON is **incomplete** in the sense that not all MessagePack types can be converted to a JSON value. The following MessagePack types are not supported and will yield parse errors:
- bin 8 - bin 32 (0xC4..0xC6)
- ext 8 - ext 32 (0xC7..0xC9)
- fixext 1 - fixext 16 (0xD4..0xD8)
The mapping from JSON to MessagePack is **complete** in the sense that any JSON value type can be converted to a MessagePack value.
The following values can not be converted to a MessagePack value:
- strings with more than 4294967295 bytes
- arrays with more than 4294967295 elements
- objects with more than 4294967295 elements
The following MessagePack types are not used in the conversion:
- bin 8 - bin 32 (0xC4..0xC6)
- ext 8 - ext 32 (0xC7..0xC9)
- float 32 (0xCA)
- fixext 1 - fixext 16 (0xD4..0xD8)
Any MessagePack output created `to_msgpack` can be successfully parsed by `from_msgpack`.
If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the `dump()` function which serializes NaN or Infinity to `null`.
### UBJSON
The mapping from UBJSON to JSON is **complete** in the sense that any UBJSON value can be converted to a JSON value.
The mapping from JSON to UBJSON is **complete** in the sense that any JSON value type can be converted to a UBJSON value.
The following values can not be converted to a UBJSON value:
- strings with more than 9223372036854775807 bytes (theoretical)
- unsigned integer numbers above 9223372036854775807
The following markers are not used in the conversion:
- `Z`: no-op values are not created.
- `C`: single-byte strings are serialized with S markers.
Any UBJSON output created to_ubjson can be successfully parsed by from_ubjson.
If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the `dump()` function which serializes NaN or Infinity to null.
The optimized formats for containers are supported: Parameter `use_size` adds size information to the beginning of a container and removes the closing marker. Parameter `use_type` further checks whether all elements of a container have the same type and adds the type marker to the beginning of the container. The `use_type` parameter must only be used together with `use_size = true`. Note that `use_size = true` alone may result in larger representations - the benefit of this parameter is that the receiving side is immediately informed on the number of elements of the container.
## Size comparison examples
The following table shows the size compared to the original JSON value for different files from the repository for the different formats.
| format | sample.json | all_unicode.json | floats.json | signed_ints.json | jeopardy.json | canada.json |
| ----------------------- | -----------:| ----------------:| -----------:| ----------------:| -------------:| -----------:|
| JSON | 100.00 % | 100.00 % | 100.00 % | 100.00 % | 100.00 % | 100.00 % |
| CBOR | 87.21 % | 71.18 % | 48.20 % | 44.16 % | 87.96 % | 50.53 % |
| MessagePack | 87.16 % | 71.18 % | 48.20 % | 44.16 % | 87.91 % | 50.56 % |
| UBJSON unoptimized | 88.15 % | 100.00 % | 48.20 % | 44.16 % | 96.58 % | 53.20 % |
| UBJSON size-optimized | 89.26 % | 100.00 % | 48.20 % | 44.16 % | 97.40 % | 58.56 % |
| UBJSON format-optimized | 89.45 % | 100.00 % | 42.85 % | 39.26 % | 94.96 % | 55.93 % |
The results show that there does not exist a "best" encoding. Furthermore, it is not always worthwhile to use UBJSON's optimizations.

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# FAQ
## Parsing
### How can I parse from a string?
```cpp
json j = json::parse("[1,2,3,4]");
```
You can pass string literals (as above), `std::string`, `const char*` or byte containers such as `std::vector<uint8_t>`.
### How can I parse from a file?
```cpp
std::ifstream i("your_file.json");
json j = json::parse(i);
```
## Serialization
### How can I serialize a JSON value
```cpp
std::cout << j << std::endl;
```
This is equivalent to
```cpp
std::string s = j.dump();
std::cout << s << std::endl;
```
### How can I pretty-print a JSON value
```cpp
std::cout << std::setw(4) << j << std::endl;
```
This is equivalent to
```cpp
std::string s = j.dump(4);
std::cout << s << std::endl;
```
The number `4` denotes the number of spaces used for indentation.
## Iterating
### How can I iterate over a JSON value?
```cpp
for (json& val : j)
{
// val is a reference for the current value
}
```
This works with any JSON value, also primitive values like numbers.
### How can I access the keys when iterating over a JSON object?
```cpp
for (auto it = j.begin(); it != j.end(); ++it)
{
// the value
json &val = it.value();
// the key (for objects)
const std::string &key = it.key();
}
```
You can also use an iteration wrapper and use range for:
```cpp
for (auto it : json::iteration_wrapper(j))
{
// the value
json &val = it.value();
// the key (for objects)
const std::string &key = it.key();
}
```

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# serve the site locally
serve: prepare_files
venv/bin/mkdocs serve
# create files that are not versioned inside the mkdocs folder
prepare_files: clean
# build Doxygen
$(MAKE) -C ..
# create subfolders
mkdir docs/images docs/examples
# copy images
cp -vr ../json.gif ../images/range-begin-end.svg ../images/range-rbegin-rend.svg docs/images
# copy examples
cp -vr ../examples/*.cpp ../examples/*.output docs/examples
# clean subfolders
clean:
rm -fr docs/images docs/examples
# publish site to GitHub pages
publish: prepare_files
venv/bin/mkdocs gh-deploy --clean --force
# install a Python virtual environment
install_venv: requirements.txt
python3 -mvenv venv
venv/bin/pip install -r requirements.txt
# uninstall the virtual environment
uninstall_venv: clean
rm -fr venv

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# Arbitrary Types Conversions
Every type can be serialized in JSON, not just STL containers and scalar types. Usually, you would do something along those lines:
```cpp
namespace ns {
// a simple struct to model a person
struct person {
std::string name;
std::string address;
int age;
};
}
ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};
// convert to JSON: copy each value into the JSON object
json j;
j["name"] = p.name;
j["address"] = p.address;
j["age"] = p.age;
// ...
// convert from JSON: copy each value from the JSON object
ns::person p {
j["name"].get<std::string>(),
j["address"].get<std::string>(),
j["age"].get<int>()
};
```
It works, but that's quite a lot of boilerplate... Fortunately, there's a better way:
```cpp
// create a person
ns::person p {"Ned Flanders", "744 Evergreen Terrace", 60};
// conversion: person -> json
json j = p;
std::cout << j << std::endl;
// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}
// conversion: json -> person
auto p2 = j.get<ns::person>();
// that's it
assert(p == p2);
```
## Basic usage
To make this work with one of your types, you only need to provide two functions:
```cpp
using nlohmann::json;
namespace ns {
void to_json(json& j, const person& p) {
j = json{ {"name", p.name}, {"address", p.address}, {"age", p.age} };
}
void from_json(const json& j, person& p) {
j.at("name").get_to(p.name);
j.at("address").get_to(p.address);
j.at("age").get_to(p.age);
}
} // namespace ns
```
That's all! When calling the `json` constructor with your type, your custom `to_json` method will be automatically called.
Likewise, when calling `get<your_type>()` or `get_to(your_type&)`, the `from_json` method will be called.
Some important things:
* Those methods **MUST** be in your type's namespace (which can be the global namespace), or the library will not be able to locate them (in this example, they are in namespace `ns`, where `person` is defined).
* Those methods **MUST** be available (e.g., proper headers must be included) everywhere you use these conversions. Look at [issue 1108](https://github.com/nlohmann/json/issues/1108) for errors that may occur otherwise.
* When using `get<your_type>()`, `your_type` **MUST** be [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). (There is a way to bypass this requirement described later.)
* In function `from_json`, use function [`at()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a93403e803947b86f4da2d1fb3345cf2c.html#a93403e803947b86f4da2d1fb3345cf2c) to access the object values rather than `operator[]`. In case a key does not exist, `at` throws an exception that you can handle, whereas `operator[]` exhibits undefined behavior.
* You do not need to add serializers or deserializers for STL types like `std::vector`: the library already implements these.
## How do I convert third-party types?
This requires a bit more advanced technique. But first, let's see how this conversion mechanism works:
The library uses **JSON Serializers** to convert types to json.
The default serializer for `nlohmann::json` is `nlohmann::adl_serializer` (ADL means [Argument-Dependent Lookup](https://en.cppreference.com/w/cpp/language/adl)).
It is implemented like this (simplified):
```cpp
template <typename T>
struct adl_serializer {
static void to_json(json& j, const T& value) {
// calls the "to_json" method in T's namespace
}
static void from_json(const json& j, T& value) {
// same thing, but with the "from_json" method
}
};
```
This serializer works fine when you have control over the type's namespace. However, what about `boost::optional` or `std::filesystem::path` (C++17)? Hijacking the `boost` namespace is pretty bad, and it's illegal to add something other than template specializations to `std`...
To solve this, you need to add a specialization of `adl_serializer` to the `nlohmann` namespace, here's an example:
```cpp
// partial specialization (full specialization works too)
namespace nlohmann {
template <typename T>
struct adl_serializer<boost::optional<T>> {
static void to_json(json& j, const boost::optional<T>& opt) {
if (opt == boost::none) {
j = nullptr;
} else {
j = *opt; // this will call adl_serializer<T>::to_json which will
// find the free function to_json in T's namespace!
}
}
static void from_json(const json& j, boost::optional<T>& opt) {
if (j.is_null()) {
opt = boost::none;
} else {
opt = j.get<T>(); // same as above, but with
// adl_serializer<T>::from_json
}
}
};
}
```
## How can I use `get()` for non-default constructible/non-copyable types?
There is a way, if your type is [MoveConstructible](https://en.cppreference.com/w/cpp/named_req/MoveConstructible). You will need to specialize the `adl_serializer` as well, but with a special `from_json` overload:
```cpp
struct move_only_type {
move_only_type() = delete;
move_only_type(int ii): i(ii) {}
move_only_type(const move_only_type&) = delete;
move_only_type(move_only_type&&) = default;
int i;
};
namespace nlohmann {
template <>
struct adl_serializer<move_only_type> {
// note: the return type is no longer 'void', and the method only takes
// one argument
static move_only_type from_json(const json& j) {
return {j.get<int>()};
}
// Here's the catch! You must provide a to_json method! Otherwise you
// will not be able to convert move_only_type to json, since you fully
// specialized adl_serializer on that type
static void to_json(json& j, move_only_type t) {
j = t.i;
}
};
}
```
## Can I write my own serializer? (Advanced use)
Yes. You might want to take a look at [`unit-udt.cpp`](https://github.com/nlohmann/json/blob/develop/test/src/unit-udt.cpp) in the test suite, to see a few examples.
If you write your own serializer, you'll need to do a few things:
- use a different `basic_json` alias than `nlohmann::json` (the last template parameter of `basic_json` is the `JSONSerializer`)
- use your `basic_json` alias (or a template parameter) in all your `to_json`/`from_json` methods
- use `nlohmann::to_json` and `nlohmann::from_json` when you need ADL
Here is an example, without simplifications, that only accepts types with a size <= 32, and uses ADL.
```cpp
// You should use void as a second template argument
// if you don't need compile-time checks on T
template<typename T, typename SFINAE = typename std::enable_if<sizeof(T) <= 32>::type>
struct less_than_32_serializer {
template <typename BasicJsonType>
static void to_json(BasicJsonType& j, T value) {
// we want to use ADL, and call the correct to_json overload
using nlohmann::to_json; // this method is called by adl_serializer,
// this is where the magic happens
to_json(j, value);
}
template <typename BasicJsonType>
static void from_json(const BasicJsonType& j, T& value) {
// same thing here
using nlohmann::from_json;
from_json(j, value);
}
};
```
Be **very** careful when reimplementing your serializer, you can stack overflow if you don't pay attention:
```cpp
template <typename T, void>
struct bad_serializer
{
template <typename BasicJsonType>
static void to_json(BasicJsonType& j, const T& value) {
// this calls BasicJsonType::json_serializer<T>::to_json(j, value);
// if BasicJsonType::json_serializer == bad_serializer ... oops!
j = value;
}
template <typename BasicJsonType>
static void to_json(const BasicJsonType& j, T& value) {
// this calls BasicJsonType::json_serializer<T>::from_json(j, value);
// if BasicJsonType::json_serializer == bad_serializer ... oops!
value = j.template get<T>(); // oops!
}
};
```

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# BSON
BSON, short for Bin­ary JSON, is a bin­ary-en­coded seri­al­iz­a­tion of JSON-like doc­u­ments. Like JSON, BSON sup­ports the em­bed­ding of doc­u­ments and ar­rays with­in oth­er doc­u­ments and ar­rays. BSON also con­tains ex­ten­sions that al­low rep­res­ent­a­tion of data types that are not part of the JSON spec. For ex­ample, BSON has a Date type and a BinData type.
!!! abstract "References"
- [BSON Website](http://bsonspec.org) - the main source on BSON
- [BSON Specification](http://bsonspec.org/spec.html) - the specification
## Serialization
The library uses the following mapping from JSON values types to BSON types:
JSON value type | value/range | BSON type | marker
--------------- | --------------------------------- | ----------- | ------
null | `null` | null | 0x0A
boolean | `true`, `false` | boolean | 0x08
number_integer | -9223372036854775808..-2147483649 | int64 | 0x12
number_integer | -2147483648..2147483647 | int32 | 0x10
number_integer | 2147483648..9223372036854775807 | int64 | 0x12
number_unsigned | 0..2147483647 | int32 | 0x10
number_unsigned | 2147483648..9223372036854775807 | int64 | 0x12
number_unsigned | 9223372036854775808..18446744073709551615| -- | --
number_float | *any value* | double | 0x01
string | *any value* | string | 0x02
array | *any value* | document | 0x04
object | *any value* | document | 0x03
binary | *any value* | binary | 0x05
!!! warning "Incomplete mapping"
The mapping is **incomplete**, since only JSON-objects (and things
contained therein) can be serialized to BSON.
Also, integers larger than 9223372036854775807 cannot be serialized to BSON,
and the keys may not contain U+0000, since they are serialized a
zero-terminated c-strings.
??? example
```cpp
--8<-- "examples/to_bson.cpp"
```
Output:
```c
--8<-- "examples/to_bson.output"
```
## Deserialization
The library maps BSON record types to JSON value types as follows:
BSON type | BSON marker byte | JSON value type
--------------- | ---------------- | ---------------------------
double | 0x01 | number_float
string | 0x02 | string
document | 0x03 | object
array | 0x04 | array
binary | 0x05 | binary
undefined | 0x06 | *unsupported*
ObjectId | 0x07 | *unsupported*
boolean | 0x08 | boolean
UTC Date-Time | 0x09 | *unsupported*
null | 0x0A | null
Regular Expr. | 0x0B | *unsupported*
DB Pointer | 0x0C | *unsupported*
JavaScript Code | 0x0D | *unsupported*
Symbol | 0x0E | *unsupported*
JavaScript Code | 0x0F | *unsupported*
int32 | 0x10 | number_integer
Timestamp | 0x11 | *unsupported*
128-bit decimal float | 0x13 | *unsupported*
Max Key | 0x7F | *unsupported*
Min Key | 0xFF | *unsupported*
!!! warning "Incomplete mapping"
The mapping is **incomplete**. The unsupported mappings are indicated in the table above.
??? example
```cpp
--8<-- "examples/from_bson.cpp"
```
Output:
```json
--8<-- "examples/from_bson.output"
```

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# CBOR
The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation.
!!! abstract "References"
- [CBOR Website](http://cbor.io) - the main source on CBOR
- [CBOR Playground](http://cbor.me) - an interactive webpage to translate between JSON and CBOR
- [RFC 7049](https://tools.ietf.org/html/rfc7049) - the CBOR specification
## Serialization
The library uses the following mapping from JSON values types to CBOR types according to the CBOR specification (RFC 7049):
JSON value type | value/range | CBOR type | first byte
--------------- | ------------------------------------------ | ---------------------------------- | ---------------
null | `null` | Null | 0xF6
boolean | `true` | True | 0xF5
boolean | `false` | False | 0xF4
number_integer | -9223372036854775808..-2147483649 | Negative integer (8 bytes follow) | 0x3B
number_integer | -2147483648..-32769 | Negative integer (4 bytes follow) | 0x3A
number_integer | -32768..-129 | Negative integer (2 bytes follow) | 0x39
number_integer | -128..-25 | Negative integer (1 byte follow) | 0x38
number_integer | -24..-1 | Negative integer | 0x20..0x37
number_integer | 0..23 | Integer | 0x00..0x17
number_integer | 24..255 | Unsigned integer (1 byte follow) | 0x18
number_integer | 256..65535 | Unsigned integer (2 bytes follow) | 0x19
number_integer | 65536..4294967295 | Unsigned integer (4 bytes follow) | 0x1A
number_integer | 4294967296..18446744073709551615 | Unsigned integer (8 bytes follow) | 0x1B
number_unsigned | 0..23 | Integer | 0x00..0x17
number_unsigned | 24..255 | Unsigned integer (1 byte follow) | 0x18
number_unsigned | 256..65535 | Unsigned integer (2 bytes follow) | 0x19
number_unsigned | 65536..4294967295 | Unsigned integer (4 bytes follow) | 0x1A
number_unsigned | 4294967296..18446744073709551615 | Unsigned integer (8 bytes follow) | 0x1B
number_float | *any value representable by a float* | Single-Precision Float | 0xFA
number_float | *any value NOT representable by a float* | Double-Precision Float | 0xFB
string | *length*: 0..23 | UTF-8 string | 0x60..0x77
string | *length*: 23..255 | UTF-8 string (1 byte follow) | 0x78
string | *length*: 256..65535 | UTF-8 string (2 bytes follow) | 0x79
string | *length*: 65536..4294967295 | UTF-8 string (4 bytes follow) | 0x7A
string | *length*: 4294967296..18446744073709551615 | UTF-8 string (8 bytes follow) | 0x7B
array | *size*: 0..23 | array | 0x80..0x97
array | *size*: 23..255 | array (1 byte follow) | 0x98
array | *size*: 256..65535 | array (2 bytes follow) | 0x99
array | *size*: 65536..4294967295 | array (4 bytes follow) | 0x9A
array | *size*: 4294967296..18446744073709551615 | array (8 bytes follow) | 0x9B
object | *size*: 0..23 | map | 0xA0..0xB7
object | *size*: 23..255 | map (1 byte follow) | 0xB8
object | *size*: 256..65535 | map (2 bytes follow) | 0xB9
object | *size*: 65536..4294967295 | map (4 bytes follow) | 0xBA
object | *size*: 4294967296..18446744073709551615 | map (8 bytes follow) | 0xBB
binary | *size*: 0..23 | byte string | 0x40..0x57
binary | *size*: 23..255 | byte string (1 byte follow) | 0x58
binary | *size*: 256..65535 | byte string (2 bytes follow) | 0x59
binary | *size*: 65536..4294967295 | byte string (4 bytes follow) | 0x5A
binary | *size*: 4294967296..18446744073709551615 | byte string (8 bytes follow) | 0x5B
!!! success "Complete mapping"
The mapping is **complete** in the sense that any JSON value type can be converted to a CBOR value.
!!! info "NaN/infinity handling"
If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the normal JSON serialization which serializes NaN or Infinity to `null`.
!!! info "Unused CBOR types"
The following CBOR types are not used in the conversion:
- UTF-8 strings terminated by "break" (0x7F)
- arrays terminated by "break" (0x9F)
- maps terminated by "break" (0xBF)
- byte strings terminated by "break" (0x5F)
- date/time (0xC0..0xC1)
- bignum (0xC2..0xC3)
- decimal fraction (0xC4)
- bigfloat (0xC5)
- tagged items (0xC6..0xD4, 0xD8..0xDB)
- expected conversions (0xD5..0xD7)
- simple values (0xE0..0xF3, 0xF8)
- undefined (0xF7)
- half-precision floats (0xF9)
- break (0xFF)
??? example
```cpp
--8<-- "examples/to_cbor.cpp"
```
Output:
```c
--8<-- "examples/to_cbor.output"
```
## Deserialization
The library maps CBOR types to JSON value types as follows:
CBOR type | JSON value type | first byte
---------------------- | --------------- | ----------
Integer | number_unsigned | 0x00..0x17
Unsigned integer | number_unsigned | 0x18
Unsigned integer | number_unsigned | 0x19
Unsigned integer | number_unsigned | 0x1A
Unsigned integer | number_unsigned | 0x1B
Negative integer | number_integer | 0x20..0x37
Negative integer | number_integer | 0x38
Negative integer | number_integer | 0x39
Negative integer | number_integer | 0x3A
Negative integer | number_integer | 0x3B
Byte string | binary | 0x40..0x57
Byte string | binary | 0x58
Byte string | binary | 0x59
Byte string | binary | 0x5A
Byte string | binary | 0x5B
UTF-8 string | string | 0x60..0x77
UTF-8 string | string | 0x78
UTF-8 string | string | 0x79
UTF-8 string | string | 0x7A
UTF-8 string | string | 0x7B
UTF-8 string | string | 0x7F
array | array | 0x80..0x97
array | array | 0x98
array | array | 0x99
array | array | 0x9A
array | array | 0x9B
array | array | 0x9F
map | object | 0xA0..0xB7
map | object | 0xB8
map | object | 0xB9
map | object | 0xBA
map | object | 0xBB
map | object | 0xBF
False | `false` | 0xF4
True | `true` | 0xF5
Null | `null` | 0xF6
Half-Precision Float | number_float | 0xF9
Single-Precision Float | number_float | 0xFA
Double-Precision Float | number_float | 0xFB
!!! warning "Incomplete mapping"
The mapping is **incomplete** in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:
- date/time (0xC0..0xC1)
- bignum (0xC2..0xC3)
- decimal fraction (0xC4)
- bigfloat (0xC5)
- tagged items (0xC6..0xD4, 0xD8..0xDB)
- expected conversions (0xD5..0xD7)
- simple values (0xE0..0xF3, 0xF8)
- undefined (0xF7)
!!! warning "Object keys"
CBOR allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps with keys other than UTF-8 strings are rejected.
??? example
```cpp
--8<-- "examples/from_cbor.cpp"
```
Output:
```json
--8<-- "examples/from_cbor.output"
```

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# Overview
Though JSON is a ubiquitous data format, it is not a very compact format suitable for data exchange, for instance over a network. Hence, the library supports
- [BSON](bson.md) (Binary JSON),
- [CBOR](cbor.md) (Concise Binary Object Representation),
- [MessagePack](messagepack.md), and
- [UBJSON](ubjson.md) (Universal Binary JSON)
to efficiently encode JSON values to byte vectors and to decode such vectors.
## Comparison
### Completeness
| Format | Serialization | Deserialization |
| ----------- |---------------------------------------------- | -------------------------------------------- |
| BSON | incomplete: top-level value must be an object | incomplete, but all JSON types are supported |
| CBOR | complete | incomplete, but all JSON types are supported |
| MessagePack | complete | complete |
| UBJSON | complete | complete |
### Binary values
| Format | Binary values | Binary subtypes |
| ----------- | ------------- | --------------- |
| BSON | supported | supported |
| CBOR | supported | not supported |
| MessagePack | supported | supported |
| UBJSON | not supported | not supported |
See [binary values](../binary_values.md) for more information.
### Sizes
| Format | canada.json | twitter.json | citm_catalog.json | jeopardy.json |
| ------------------ | ----------- | ------------ | ----------------- | ------------- |
| BSON | 85,8 % | 95,2 % | 95,8 % | 106,7 % |
| CBOR | 50,5 % | 86,3 % | 68,4 % | 88,0 % |
| MessagePack | 50,6 % | 86,0 % | 68,5 % | 87,9 % |
| UBJSON | 53,2 % | 91,3 % | 78,2 % | 96,6 % |
| UBJSON (size) | 58,6 % | 92,3 % | 86,8 % | 97,4 % |
| UBJSON (size+type) | 55,9 % | 92,3 % | 85,0 % | 95,0 % |
Sizes compared to minified JSON value.

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# MessagePack
MessagePack is an efficient binary serialization format. It lets you exchange data among multiple languages like JSON. But it's faster and smaller. Small integers are encoded into a single byte, and typical short strings require only one extra byte in addition to the strings themselves.
!!! abstract "References"
- [MessagePack website](https://msgpack.org)
- [MessagePack specification](https://github.com/msgpack/msgpack/blob/master/spec.md)
## Serialization
The library uses the following mapping from JSON values types to MessagePack types according to the MessagePack specification:
JSON value type | value/range | MessagePack type | first byte
--------------- | --------------------------------- | ---------------- | ----------
null | `null` | nil | 0xC0
boolean | `true` | true | 0xC3
boolean | `false` | false | 0xC2
number_integer | -9223372036854775808..-2147483649 | int64 | 0xD3
number_integer | -2147483648..-32769 | int32 | 0xD2
number_integer | -32768..-129 | int16 | 0xD1
number_integer | -128..-33 | int8 | 0xD0
number_integer | -32..-1 | negative fixint | 0xE0..0xFF
number_integer | 0..127 | positive fixint | 0x00..0x7F
number_integer | 128..255 | uint 8 | 0xCC
number_integer | 256..65535 | uint 16 | 0xCD
number_integer | 65536..4294967295 | uint 32 | 0xCE
number_integer | 4294967296..18446744073709551615 | uint 64 | 0xCF
number_unsigned | 0..127 | positive fixint | 0x00..0x7F
number_unsigned | 128..255 | uint 8 | 0xCC
number_unsigned | 256..65535 | uint 16 | 0xCD
number_unsigned | 65536..4294967295 | uint 32 | 0xCE
number_unsigned | 4294967296..18446744073709551615 | uint 64 | 0xCF
number_float | *any value* | float 64 | 0xCB
string | *length*: 0..31 | fixstr | 0xA0..0xBF
string | *length*: 32..255 | str 8 | 0xD9
string | *length*: 256..65535 | str 16 | 0xDA
string | *length*: 65536..4294967295 | str 32 | 0xDB
array | *size*: 0..15 | fixarray | 0x90..0x9F
array | *size*: 16..65535 | array 16 | 0xDC
array | *size*: 65536..4294967295 | array 32 | 0xDD
object | *size*: 0..15 | fix map | 0x80..0x8F
object | *size*: 16..65535 | map 16 | 0xDE
object | *size*: 65536..4294967295 | map 32 | 0xDF
binary | *size*: 0..255 | bin 8 | 0xC4
binary | *size*: 256..65535 | bin 16 | 0xC5
binary | *size*: 65536..4294967295 | bin 32 | 0xC6
!!! success "Complete mapping"
The mapping is **complete** in the sense that any JSON value type can be converted to a MessagePack value.
Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.
!!! warning "Size constraints"
The following values can **not** be converted to a MessagePack value:
- strings with more than 4294967295 bytes
- byte strings with more than 4294967295 bytes
- arrays with more than 4294967295 elements
- objects with more than 4294967295 elements
!!! info "Unused MessagePack types"
The following MessagePack types are not used in the conversion: float 32 (0xCA)
!!! info "NaN/infinity handling"
If NaN or Infinity are stored inside a JSON number, they are serialized properly. function which serializes NaN or Infinity to `null`.
??? example
```cpp
--8<-- "examples/to_msgpack.cpp"
```
Output:
```c
--8<-- "examples/to_msgpack.output"
```
## Deserialization
The library maps MessagePack types to JSON value types as follows:
MessagePack type | JSON value type | first byte
---------------- | --------------- | ----------
positive fixint | number_unsigned | 0x00..0x7F
fixmap | object | 0x80..0x8F
fixarray | array | 0x90..0x9F
fixstr | string | 0xA0..0xBF
nil | `null` | 0xC0
false | `false` | 0xC2
true | `true` | 0xC3
float 32 | number_float | 0xCA
float 64 | number_float | 0xCB
uint 8 | number_unsigned | 0xCC
uint 16 | number_unsigned | 0xCD
uint 32 | number_unsigned | 0xCE
uint 64 | number_unsigned | 0xCF
int 8 | number_integer | 0xD0
int 16 | number_integer | 0xD1
int 32 | number_integer | 0xD2
int 64 | number_integer | 0xD3
str 8 | string | 0xD9
str 16 | string | 0xDA
str 32 | string | 0xDB
array 16 | array | 0xDC
array 32 | array | 0xDD
map 16 | object | 0xDE
map 32 | object | 0xDF
bin 8 | binary | 0xC4
bin 16 | binary | 0xC5
bin 32 | binary | 0xC6
ext 8 | binary | 0xC7
ext 16 | binary | 0xC8
ext 32 | binary | 0xC9
fixext 1 | binary | 0xD4
fixext 2 | binary | 0xD5
fixext 4 | binary | 0xD6
fixext 8 | binary | 0xD7
fixext 16 | binary | 0xD8
negative fixint | number_integer | 0xE0-0xFF
!!! info
Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.
??? example
```cpp
--8<-- "examples/from_msgpack.cpp"
```
Output:
```json
--8<-- "examples/from_msgpack.output"
```

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# UBJSON
Universal Binary JSON (UBJSON) is a binary form directly imitating JSON, but requiring fewer bytes of data. It aims to achieve the generality of JSON, combined with being much easier to process than JSON.
!!! abstract "References"
- [UBJSON Website](http://ubjson.org)
## Serialization
The library uses the following mapping from JSON values types to UBJSON types according to the UBJSON specification:
JSON value type | value/range | UBJSON type | marker
--------------- | --------------------------------- | ----------- | ------
null | `null` | null | `Z`
boolean | `true` | true | `T`
boolean | `false` | false | `F`
number_integer | -9223372036854775808..-2147483649 | int64 | `L`
number_integer | -2147483648..-32769 | int32 | `l`
number_integer | -32768..-129 | int16 | `I`
number_integer | -128..127 | int8 | `i`
number_integer | 128..255 | uint8 | `U`
number_integer | 256..32767 | int16 | `I`
number_integer | 32768..2147483647 | int32 | `l`
number_integer | 2147483648..9223372036854775807 | int64 | `L`
number_unsigned | 0..127 | int8 | `i`
number_unsigned | 128..255 | uint8 | `U`
number_unsigned | 256..32767 | int16 | `I`
number_unsigned | 32768..2147483647 | int32 | `l`
number_unsigned | 2147483648..9223372036854775807 | int64 | `L`
number_float | *any value* | float64 | `D`
string | *with shortest length indicator* | string | `S`
array | *see notes on optimized format* | array | `[`
object | *see notes on optimized format* | map | `{`
!!! success "Complete mapping"
The mapping is **complete** in the sense that any JSON value type can be converted to a UBJSON value.
Any UBJSON output created by `to_ubjson` can be successfully parsed by `from_ubjson`.
!!! warning "Size constraints"
The following values can **not** be converted to a UBJSON value:
- strings with more than 9223372036854775807 bytes (theoretical)
- unsigned integer numbers above 9223372036854775807
!!! info "Unused UBJSON markers"
The following markers are not used in the conversion:
- `Z`: no-op values are not created.
- `C`: single-byte strings are serialized with `S` markers.
!!! info "NaN/infinity handling"
If NaN or Infinity are stored inside a JSON number, they are
serialized properly. This behavior differs from the `dump()`
function which serializes NaN or Infinity to `null`.
!!! info "Optimized formats"
The optimized formats for containers are supported: Parameter
`use_size` adds size information to the beginning of a container and
removes the closing marker. Parameter `use_type` further checks
whether all elements of a container have the same type and adds the
type marker to the beginning of the container. The `use_type`
parameter must only be used together with `use_size = true`.
Note that `use_size = true` alone may result in larger representations -
the benefit of this parameter is that the receiving side is
immediately informed on the number of elements of the container.
!!! info "Binary values"
If the JSON data contains the binary type, the value stored is a list
of integers, as suggested by the UBJSON documentation. In particular,
this means that serialization and the deserialization of a JSON
containing binary values into UBJSON and back will result in a
different JSON object.
??? example
```cpp
--8<-- "examples/to_ubjson.cpp"
```
Output:
```c
--8<-- "examples/to_ubjson.output"
```
## Deserialization
The library maps UBJSON types to JSON value types as follows:
UBJSON type | JSON value type | marker
----------- | --------------------------------------- | ------
no-op | *no value, next value is read* | `N`
null | `null` | `Z`
false | `false` | `F`
true | `true` | `T`
float32 | number_float | `d`
float64 | number_float | `D`
uint8 | number_unsigned | `U`
int8 | number_integer | `i`
int16 | number_integer | `I`
int32 | number_integer | `l`
int64 | number_integer | `L`
string | string | `S`
char | string | `C`
array | array (optimized values are supported) | `[`
object | object (optimized values are supported) | `{`
!!! success "Complete mapping"
The mapping is **complete** in the sense that any UBJSON value can be converted to a JSON value.
??? example
```cpp
--8<-- "examples/from_ubjson.cpp"
```
Output:
```json
--8<-- "examples/from_ubjson.output"
```

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# Binary Values
The library implements several [binary formats](binary_formats/index.md) that encode JSON in an efficient way. Most of these formats support binary values; that is, values that have semantics define outside the library and only define a sequence of bytes to be stored.
JSON itself does not have a binary value. As such, binary values are an extension that this library implements to store values received by a binary format. Binary values are never created by the JSON parser, and are only part of a serialized JSON text if they have been created manually or via a binary format.
## API for binary values
```plantuml
class json::binary_t {
-- setters --
+void set_subtype(std::uint8_t subtype)
+void clear_subtype()
-- getters --
+std::uint8_t subtype() const
+bool has_subtype() const
}
"std::vector<uint8_t>" <|-- json::binary_t
```
By default, binary values are stored as `std::vector<std::uint8_t>`. This type can be changed by providing a template parameter to the `basic_json` type. To store binary subtypes, the storage type is extended and exposed as `json::binary_t`:
```cpp
auto binary = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE});
auto binary_with_subtype = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE}, 42);
```
There are several convenience functions to check and set the subtype:
```cpp
binary.has_subtype(); // returns false
binary_with_subtype.has_subtype(); // returns true
binary_with_subtype.clear_subtype();
binary_with_subtype.has_subtype(); // returns true
binary_with_subtype.set_subtype(42);
binary.set_subtype(23);
binary.subtype(); // returns 23
```
As `json::binary_t` is subclassing `std::vector<std::uint8_t>`, all member functions are available:
```cpp
binary.size(); // returns 4
binary[1]; // returns 0xFE
```
JSON values can be constructed from `json::binary_t`:
```cpp
json j = binary;
```
Binary values are primitive values just like numbers or strings:
```cpp
j.is_binary(); // returns true
j.is_primitive(); // returns true
```
Given a binary JSON value, the `binary_t` can be accessed by reference as via `get_binary()`:
```cpp
j.get_binary().has_subtype(); // returns true
j.get_binary().size(); // returns 4
```
For convencience, binary JSON values can be constructed via `json::binary`:
```cpp
auto j2 = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 23);
auto j3 = json::binary({0xCA, 0xFE, 0xBA, 0xBE});
j2 == j; // returns true
j3.get_binary().has_subtype(); // returns false
```
## Serialization
Binary values are serialized differently according to the formats.
### JSON
JSON does not have a binary type, and this library does not introduce a new type as this would break conformance. Instead, binary values are serialized as an object with two keys: `bytes` holds an array of integers, and `subtype` is an integer or `null`.
??? example
Code:
```cpp
// create a binary value of subtype 42
json j;
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
// serialize to standard output
std::cout << j.dump(2) << std::endl;
```
Output:
```json
{
"binary": {
"bytes": [202, 254, 186, 190],
"subtype": 42
}
}
```
!!! warning "No roundtrip for binary values"
The JSON parser will not parse the objects generated by binary values back to binary values. This is by design to remain standards compliant. Serializing binary values to JSON is only implemented for debugging purposes.
### BSON
[BSON](binary_formats/bson.md) supports binary values and subtypes. If a subtype is given, it is used and added as unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00 is used.
??? example
Code:
```cpp
// create a binary value of subtype 42
json j;
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
// convert to BSON
auto v = json::to_bson(j);
```
`v` is a `std::vector<std::uint8t>` with the following 22 elements:
```c
0x16 0x00 0x00 0x00 // number of bytes in the document
0x05 // binary value
0x62 0x69 0x6E 0x61 0x72 0x79 0x00 // key "binary" + null byte
0x04 0x00 0x00 0x00 // number of bytes
0x2a // subtype
0xCA 0xFE 0xBA 0xBE // content
0x00 // end of the document
```
Note that the serialization preserves the subtype, and deserializing `v` would yield the following value:
```json
{
"binary": {
"bytes": [202, 254, 186, 190],
"subtype": 42
}
}
```
### CBOR
[CBOR](binary_formats/cbor.md) supports binary values, but no subtypes. Any binary value will be serialized as byte strings. The library will choose the smallest representation using the length of the byte array.
??? example
Code:
```cpp
// create a binary value of subtype 42 (will be ignored by CBOR)
json j;
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
// convert to CBOR
auto v = json::to_cbor(j);
```
`v` is a `std::vector<std::uint8t>` with the following 13 elements:
```c
0xA1 // map(1)
0x66 // text(6)
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
0x44 // bytes(4)
0xCA 0xFE 0xBA 0xBE // content
```
Note the subtype (42) is **not** serialized, and deserializing `v` would yield the following value:
```json
{
"binary": {
"bytes": [202, 254, 186, 190],
"subtype": null
}
}
```
### MessagePack
[MessagePack](binary_formats/messagepack.md) supports binary values and subtypes. If a subtype is given, the ext family is used. The library will choose the smallest representation among fixext1, fixext2, fixext4, fixext8, ext8, ext16, and ext32. The subtype is then added as singed 8-bit integer.
If no subtype is given, the bin family (bin8, bin16, bin32) is used.
??? example
Code:
```cpp
// create a binary value of subtype 42
json j;
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
// convert to MessagePack
auto v = json::to_msgpack(j);
```
`v` is a `std::vector<std::uint8t>` with the following 14 elements:
```c
0x81 // fixmap1
0xA6 // fixstr6
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
0xD6 // fixext4
0x2A // subtype
0xCA 0xFE 0xBA 0xBE // content
```
Note that the serialization preserves the subtype, and deserializing `v` would yield the following value:
```json
{
"binary": {
"bytes": [202, 254, 186, 190],
"subtype": 42
}
}
```
### UBJSON
[UBJSON](binary_formats/ubjson.md) neither supports binary values nor subtypes, and proposes to serialize binary values as array of uint8 values. This translation is implemented by the library.
??? example
Code:
```cpp
// create a binary value of subtype 42 (will be ignored in UBJSON)
json j;
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
// convert to UBJSON
auto v = json::to_msgpack(j);
```
`v` is a `std::vector<std::uint8t>` with the following 20 elements:
```c
0x7B // '{'
0x69 0x06 // i 6 (length of the key)
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
0x5B // '['
0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE // content (each byte prefixed with 'U')
0x5D // ']'
0x7D // '}'
```
The following code uses the type and size optimization for UBJSON:
```cpp
// convert to UBJSON using the size and type optimization
auto v = json::to_ubjson(j, true, true);
```
The resulting vector has 23 elements; the optimization is not effective for examples with few values:
```c
0x7B // '{'
0x24 // '$' type of the object elements
0x5B // '[' array
0x23 0x69 0x01 // '#' i 1 number of object elements
0x69 0x06 // i 6 (length of the key)
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
0x24 0x55 // '$' 'U' type of the array elements: unsinged integers
0x23 0x69 0x04 // '#' i 4 number of array elements
0xCA 0xFE 0xBA 0xBE // content
```
Note that subtype (42) is **not** serialized and that UBJSON has **no binary type**, and deserializing `v` would yield the following value:
```json
{
"binary": [202, 254, 186, 190]
}
```

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# Specializing enum conversion
By default, enum values are serialized to JSON as integers. In some cases this could result in undesired behavior. If an enum is modified or re-ordered after data has been serialized to JSON, the later de-serialized JSON data may be undefined or a different enum value than was originally intended.
It is possible to more precisely specify how a given enum is mapped to and from JSON as shown below:
```cpp
// example enum type declaration
enum TaskState {
TS_STOPPED,
TS_RUNNING,
TS_COMPLETED,
TS_INVALID=-1,
};
// map TaskState values to JSON as strings
NLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {
{TS_INVALID, nullptr},
{TS_STOPPED, "stopped"},
{TS_RUNNING, "running"},
{TS_COMPLETED, "completed"},
})
```
The `NLOHMANN_JSON_SERIALIZE_ENUM()` macro declares a set of `to_json()` / `from_json()` functions for type `TaskState` while avoiding repetition and boilerplate serialization code.
## Usage
```cpp
// enum to JSON as string
json j = TS_STOPPED;
assert(j == "stopped");
// json string to enum
json j3 = "running";
assert(j3.get<TaskState>() == TS_RUNNING);
// undefined json value to enum (where the first map entry above is the default)
json jPi = 3.14;
assert(jPi.get<TaskState>() == TS_INVALID );
```
## Notes
Just as in [Arbitrary Type Conversions](#arbitrary-types-conversions) above,
- `NLOHMANN_JSON_SERIALIZE_ENUM()` MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it and it will default to integer serialization.
- It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.
Other Important points:
- When using `get<ENUM_TYPE>()`, undefined JSON values will default to the first pair specified in your map. Select this default pair carefully.
- If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the map will be returned when converting to or from JSON.

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# Iterators
## Overview
A `basic_json` value is a container and allows access via iterators. Depending on the value type, `basic_json` stores zero or more values.
As for other containers, `begin()` returns an iterator to the first value and `end()` returns an iterator to the value following the last value. The latter iterator is a placeholder and cannot be dereferenced. In case of null values, empty arrays, or empty objects, `begin()` will return `end()`.
![Illustration from cppreference.com](../images/range-begin-end.svg)
### Iteration order for objects
When iterating over objects, values are ordered with respect to the `object_comparator_t` type which defaults to `std::less`. See the [types documentation](types.md#key-order) for more information.
??? example
```cpp
// create JSON object {"one": 1, "two": 2, "three": 3}
json j;
j["one"] = 1;
j["two"] = 2;
j["three"] = 3;
for (auto it = j.begin(); it != j.end(); ++it)
{
std::cout << *it << std::endl;
}
```
Output:
```json
1
3
2
```
The reason for the order is the lexicographic ordering of the object keys "one", "three", "two".
### Access object key during iteration
The JSON iterators have two member functions, `key()` and `value()` to access the object key and stored value, respectively. When calling `key()` on a non-object iterator, an [invalid_iterator.207](../home/exceptions.md#jsonexceptioninvalid_iterator207) exception is thrown.
??? example
```cpp
// create JSON object {"one": 1, "two": 2, "three": 3}
json j;
j["one"] = 1;
j["two"] = 2;
j["three"] = 3;
for (auto it = j.begin(); it != j.end(); ++it)
{
std::cout << it.key() << " : " << it.value() << std::endl;
}
```
Output:
```json
one : 1
three : 3
two : 2
```
### Range-based for loops
C++11 allows to use range-based for loops to iterate over a container.
```cpp
for (auto it : j_object)
{
// "it" is of type json::reference and has no key() member
std::cout << "value: " << it << '\n';
}
```
For this reason, the `items()` function allows to access `iterator::key()` and `iterator::value()` during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.
```cpp
for (auto& el : j_object.items())
{
std::cout << "key: " << el.key() << ", value:" << el.value() << '\n';
}
```
The items() function also allows to use structured bindings (C++17):
```cpp
for (auto& [key, val] : j_object.items())
{
std::cout << "key: " << key << ", value:" << val << '\n';
}
```
!!! note
When iterating over an array, `key()` will return the index of the element as string. For primitive types (e.g., numbers), `key()` returns an empty string.
!!! warning
Using `items()` on temporary objects is dangerous. Make sure the object's lifetime exeeds the iteration. See <https://github.com/nlohmann/json/issues/2040> for more information.
### Reverse iteration order
`rbegin()` and `rend()` return iterators in the reverse sequence.
![Illustration from cppreference.com](../images/range-rbegin-rend.svg)
??? example
```cpp
json j = {1, 2, 3, 4};
for (auto it = j.begin(); it != j.end(); ++it)
{
std::cout << *it << std::endl;
}
```
Output:
```json
4
3
2
1
```
### Iterating strings and binary values
Note that "value" means a JSON value in this setting, not values stored in the underlying containers. That is, `*begin()` returns the complete string or binary array and is also safe the underlying string or binary array is empty.
??? example
```cpp
json j = "Hello, world";
for (auto it = j.begin(); it != j.end(); ++it)
{
std::cout << *it << std::endl;
}
```
Output:
```json
"Hello, world"
```
## Iterator invalidation
| Operations | invalidated iterators |
| ---------- | --------------------- |
| `clear` | all |

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# JSON Patch and Diff
## Patches
JSON Patch ([RFC 6902](https://tools.ietf.org/html/rfc6902)) defines a JSON document structure for expressing a sequence of operations to apply to a JSON) document. With the `patch` function, a JSON Patch is applied to the current JSON value by executing all operations from the patch.
??? example
The following code shows how a JSON patch is applied to a value.
```cpp
--8<-- "examples/patch.cpp"
```
Output:
```json
--8<-- "examples/patch.output"
```
## Diff
The library can also calculate a JSON patch (i.e., a **diff**) given two JSON values.
!!! success "Invariant"
For two JSON values *source* and *target*, the following code yields always true:
```cüü
source.patch(diff(source, target)) == target;
```
??? example
The following code shows how a JSON patch is created as a diff for two JSON values.
```cpp
--8<-- "examples/diff.cpp"
```
Output:
```json
--8<-- "examples/diff.output"
```

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# JSON Pointer
The library supports **JSON Pointer** ([RFC 6901](https://tools.ietf.org/html/rfc6901)) as alternative means to address structured values.
```cpp
// a JSON value
json j_original = R"({
"baz": ["one", "two", "three"],
"foo": "bar"
})"_json;
// access members with a JSON pointer (RFC 6901)
j_original["/baz/1"_json_pointer];
// "two"
```

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# JSON Merge Patch
The library supports JSON Merge Patch ([RFC 7386](https://tools.ietf.org/html/rfc7386)) as a patch format.
The merge patch format is primarily intended for use with the HTTP PATCH method as a means of describing a set of modifications to a target resource's content. This function applies a merge patch to the current JSON value.
Instead of using [JSON Pointer](json_pointer.md) to specify values to be manipulated, it describes the changes using a syntax that closely mimics the document being modified.
??? example
The following code shows how a JSON Merge Patch is applied to a JSON document.
```cpp
--8<-- "examples/merge_patch.cpp"
```
Output:
```json
--8<-- "examples/merge_patch.output"
```

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# Overview
!!! note
This page is under construction.
## Input
## SAX vs. DOM parsing
## Exceptions
See [parsing and exceptions](parse_exceptions.md).

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# Parsing and exceptions
When the input is not valid JSON, an exception of type [`parse_error`](../../home/exceptions.md#parse-errors) is thrown. This exception contains the position in the input where the error occurred, together with a diagnostic message and the last read input token. The exceptions page contains a [list of examples for parse error exceptions](../../home/exceptions.md#parse-errors). In case you process untrusted input, always enclose your code with a `#!cpp try`/`#!cpp catch` block, like
```cpp
json j;
try
{
j = json::parse(my_input);
}
catch (json::exception::parse_error& ex)
{
std::cerr << "parse error at byte " << ex.byte << std::endl;
}
```
In case exceptions are undesired or not supported by the environment, there are different ways to proceed:
## Switch off exceptions
The `parse()` function accepts as last parameter a `#!cpp bool` variable `allow_exceptions` which controls whether an exception is thrown when a parse error occurs (`#!cpp true`, default) or whether a discarded value should be returned (`#!cpp false`).
```cpp
json j = json::parse(my_input, nullptr, false);
if (j.is_discarded())
{
std::cerr << "parse error" << std::endl;
}
```
Note there is no diagnostic information available in this scenario.
## Use accept() function
Alternatively, function `accept()` can be used which does not return a `json` value, but a `#!cpp bool` indicating whether the input is valid JSON.
```cpp
if (!json::accept(my_input))
{
std::cerr << "parse error" << std::endl;
}
```
Again, there is no diagnostic information available.
## User-defined SAX interface
Finally, you can implement the [SAX interface](sax_interface.md) and decide what should happen in case of a parse error.
This function has the following interface:
```cpp
bool parse_error(std::size_t position,
const std::string& last_token,
const json::exception& ex);
```
The return value indicates whether the parsing should continue, so the function should usually return `#!cpp false`.
??? example
```cpp
#include <iostream>
#include "json.hpp"
using json = nlohmann::json;
class sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json>
{
public:
sax_no_exception(json& j)
: nlohmann::detail::json_sax_dom_parser<json>(j, false)
{}
bool parse_error(std::size_t position,
const std::string& last_token,
const json::exception& ex)
{
std::cerr << "parse error at input byte " << position << "\n"
<< ex.what() << "\n"
<< "last read: \"" << last_token << "\""
<< std::endl;
return false;
}
};
int main()
{
std::string myinput = "[1,2,3,]";
json result;
sax_no_exception sax(result);
bool parse_result = json::sax_parse(myinput, &sax);
if (!parse_result)
{
std::cerr << "parsing unsuccessful!" << std::endl;
}
std::cout << "parsed value: " << result << std::endl;
}
```
Output:
```
parse error at input byte 8
[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
last read: "3,]"
parsing unsuccessful!
parsed value: [1,2,3]
```

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# Parser Callbacks
## Overview
With a parser callback function, the result of parsing a JSON text can be influenced. When passed to `parse`, it is called on certain events
(passed as `parse_event_t` via parameter `event`) with a set recursion depth `depth` and context JSON value `parsed`. The return value of the
callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.
The type of the callback function is:
```cpp
template<typename BasicJsonType>
using parser_callback_t =
std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;
```
## Callback event types
We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values
of the parameters `depth`, `event`, and `parsed`.
parameter `event` | description | parameter `depth` | parameter `parsed`
------------------ | ----------- | ------------------ | -------------------
`parse_event_t::object_start` | the parser read `{` and started to process a JSON object | depth of the parent of the JSON object | a JSON value with type discarded
`parse_event_t::key` | the parser read a key of a value in an object | depth of the currently parsed JSON object | a JSON string containing the key
`parse_event_t::object_end` | the parser read `}` and finished processing a JSON object | depth of the parent of the JSON object | the parsed JSON object
`parse_event_t::array_start` | the parser read `[` and started to process a JSON array | depth of the parent of the JSON array | a JSON value with type discarded
`parse_event_t::array_end` | the parser read `]` and finished processing a JSON array | depth of the parent of the JSON array | the parsed JSON array
`parse_event_t::value` | the parser finished reading a JSON value | depth of the value | the parsed JSON value
??? example
When parsing the following JSON text,
```json
{
"name": "Berlin",
"location": [
52.519444,
13.406667
]
}
```
these calls are made to the callback function:
| event | depth | parsed |
| -------------- | ----- | ------ |
| `object_start` | 0 | *discarded* |
| `key` | 1 | `#!json "name"` |
| `value` | 1 | `#!json "Berlin"` |
| `key` | 1 | `#!json "location"` |
| `array_start` | 1 | *discarded* |
| `value` | 2 | `#!json 52.519444` |
| `value` | 2 | `#!json 13.406667` |
| `array_end` | 1 | `#!json [52.519444,13.406667]` |
| `object_end` | 0 | `#!json {"location":[52.519444,13.406667],"name":"Berlin"}` |
## Return value
Discarding a value (i.e., returning `#!c false`) has different effects depending on the context in which function was called:
- Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.
- In case a value outside a structured type is skipped, it is replaced with `#!json null`. This case happens if the top-level element is skipped.
??? example
The example below demonstrates the `parse()` function with and without callback function.
```cpp
--8<-- "examples/parse__string__parser_callback_t.cpp"
```
Output:
```json
--8<-- "examples/parse__string__parser_callback_t.output"
```

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# SAX Interface
The library uses a SAX-like interface with the following functions:
```plantuml
interface json::sax_t {
+ {abstract} bool null()
+ {abstract} bool boolean(bool val)
+ {abstract} bool number_integer(number_integer_t val)
+ {abstract} bool number_unsigned(number_unsigned_t val)
+ {abstract} bool number_float(number_float_t val, const string_t& s)
+ {abstract} bool string(string_t& val)
+ {abstract} bool start_object(std::size_t elements)
+ {abstract} bool end_object()
+ {abstract} bool start_array(std::size_t elements)
+ {abstract} bool end_array()
+ {abstract} bool key(string_t& val)
+ {abstract} bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex)
}
```
```cpp
// called when null is parsed
bool null();
// called when a boolean is parsed; value is passed
bool boolean(bool val);
// called when a signed or unsigned integer number is parsed; value is passed
bool number_integer(number_integer_t val);
bool number_unsigned(number_unsigned_t val);
// called when a floating-point number is parsed; value and original string is passed
bool number_float(number_float_t val, const string_t& s);
// called when a string is parsed; value is passed and can be safely moved away
bool string(string_t& val);
// called when an object or array begins or ends, resp. The number of elements is passed (or -1 if not known)
bool start_object(std::size_t elements);
bool end_object();
bool start_array(std::size_t elements);
bool end_array();
// called when an object key is parsed; value is passed and can be safely moved away
bool key(string_t& val);
// called when a parse error occurs; byte position, the last token, and an exception is passed
bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex);
```
The return value of each function determines whether parsing should proceed.
To implement your own SAX handler, proceed as follows:
1. Implement the SAX interface in a class. You can use class `nlohmann::json_sax<json>` as base class, but you can also use any class where the functions described above are implemented and public.
2. Create an object of your SAX interface class, e.g. `my_sax`.
3. Call `#!cpp bool json::sax_parse(input, &my_sax);` where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your SAX interface.
Note the `sax_parse` function only returns a `#!cpp bool` indicating the result of the last executed SAX event. It does not return `json` value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your `parse_error` implementation. Internally, the SAX interface is used for the DOM parser (class `json_sax_dom_parser`) as well as the acceptor (`json_sax_acceptor`), see file `json_sax.hpp`.

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# Types
This page gives an overview how JSON values are stored and how this can be configured.
## Overview
By default, JSON values are stored as follows:
| JSON type | C++ type |
| --------- | -------- |
| object | `std::map<std::string, basic_json>` |
| array | `std::vector<basic_json>` |
| null | `std::nullptr_t` |
| string | `std::string` |
| boolean | `bool` |
| number | `std::int64_t`, `std::uint64_t`, and `double` |
Note there are three different types for numbers - when parsing JSON text, the best fitting type is chosen.
## Storage
```plantuml
enum value_t {
null
object
array
string
boolean
number_integer
number_unsigned
number_float
binary
discarded
}
class json_value << (U,orchid) >> {
object_t* object
array_t* array
string_t* string
binary_t* binary
boolean_t boolean
number_integer_t number_integer
number_unsigned_t number_unsigned
number_float_t number_float
}
class basic_json {
-- type and value --
value_t m_type
json_value m_value
-- derived types --
+ <u>typedef</u> object_t
+ <u>typedef</u> array_t
+ <u>typedef</u> binary_t
+ <u>typedef</u> boolean_t
+ <u>typedef</u> number_integer_t
+ <u>typedef</u> number_unsigned_t
+ <u>typedef</u> number_float_t
}
basic_json .. json_value
basic_json .. value_t
```
## Template arguments
The data types to store a JSON value are derived from the template arguments passed to class `basic_json`:
```cpp
template<
template<typename U, typename V, typename... Args> class ObjectType = std::map,
template<typename U, typename... Args> class ArrayType = std::vector,
class StringType = std::string,
class BooleanType = bool,
class NumberIntegerType = std::int64_t,
class NumberUnsignedType = std::uint64_t,
class NumberFloatType = double,
template<typename U> class AllocatorType = std::allocator,
template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,
class BinaryType = std::vector<std::uint8_t>
>
class basic_json;
```
Type `json` is an alias for `basic_json<>` and uses the default types.
From the template arguments, the following types are derived:
```cpp
using object_comparator_t = std::less<>;
using object_t = ObjectType<StringType, basic_json, object_comparator_t,
AllocatorType<std::pair<const StringType, basic_json>>>;
using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
using string_t = StringType;
using boolean_t = BooleanType;
using number_integer_t = NumberIntegerType;
using number_unsigned_t = NumberUnsignedType;
using number_float_t = NumberFloatType;
using binary_t = nlohmann::byte_container_with_subtype<BinaryType>;
```
## Objects
[RFC 7159](http://rfc7159.net/rfc7159) describes JSON objects as follows:
> An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.
### Default type
With the default values for *ObjectType* (`std::map`), *StringType* (`std::string`), and *AllocatorType* (`std::allocator`), the default value for `object_t` is:
```cpp
std::map<
std::string, // key_type
basic_json, // value_type
std::less<>, // key_compare
std::allocator<std::pair<const std::string, basic_json>> // allocator_type
>
```
### Behavior
The choice of `object_t` influences the behavior of the JSON class. With the default type, objects have the following behavior:
- When all names are unique, objects will be interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings.
- When the names within an object are not unique, it is unspecified which one of the values for a given key will be chosen. For instance, `#!json {"key": 2, "key": 1}` could be equal to either `#!json {"key": 1}` or `#!json {"key": 2}`.
- Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see `dump`) in this order. For instance, both `#!json {"b": 1, "a": 2}` and `#!json {"a": 2, "b": 1}` will be stored and serialized as `#!json {"a": 2, "b": 1}`.
- When comparing objects, the order of the name/value pairs is irrelevant. This makes objects interoperable in the sense that they will not be affected by these differences. For instance, `#!json {"b": 1, "a": 2}` and `#!json {"a": 2, "b": 1}` will be treated as equal.
### Key order
The order name/value pairs are added to the object is *not* preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as `std::map` with `std::less` is used by default. Please note this behavior conforms to [RFC 7159](http://rfc7159.net/rfc7159), because any order implements the specified "unordered" nature of JSON objects.
### Limits
[RFC 7159](http://rfc7159.net/rfc7159) specifies:
> An implementation may set limits on the maximum depth of nesting.
In this class, the object's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the `max_size` function of a JSON object.
### Storage
Objects are stored as pointers in a `basic_json` type. That is, for any access to object values, a pointer of type `object_t*` must be dereferenced.
## Arrays
[RFC 7159](http://rfc7159.net/rfc7159) describes JSON arrays as follows:
> An array is an ordered sequence of zero or more values.
### Default type
With the default values for *ArrayType* (`std::vector`) and *AllocatorType* (`std::allocator`), the default value for `array_t` is:
```cpp
std::vector<
basic_json, // value_type
std::allocator<basic_json> // allocator_type
>
```
### Limits
[RFC 7159](http://rfc7159.net/rfc7159) specifies:
> An implementation may set limits on the maximum depth of nesting.
In this class, the array's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the `max_size` function of a JSON array.
### Storage
Arrays are stored as pointers in a `basic_json` type. That is, for any access to array values, a pointer of type `array_t*` must be dereferenced.
## Strings
[RFC 7159](http://rfc7159.net/rfc7159) describes JSON strings as follows:
> A string is a sequence of zero or more Unicode characters.
Unicode values are split by the JSON class into byte-sized characters during deserialization.
### Default type
With the default values for *StringType* (`std::string`), the default value for `string_t` is `#!cpp std::string`.
### Encoding
Strings are stored in UTF-8 encoding. Therefore, functions like `std::string::size()` or `std::string::length()` return the number of **bytes** in the string rather than the number of characters or glyphs.
### String comparison
[RFC 7159](http://rfc7159.net/rfc7159) states:
> Software implementations are typically required to test names of object members for equality. Implementations that transform the textual representation into sequences of Unicode code units and then perform the comparison numerically, code unit by code unit, are interoperable in the sense that implementations will agree in all cases on equality or inequality of two strings. For example, implementations that compare strings with escaped characters unconverted may incorrectly find that `"a\\b"` and `"a\u005Cb"` are not equal.
This implementation is interoperable as it does compare strings code unit by code unit.
### Storage
String values are stored as pointers in a `basic_json` type. That is, for any access to string values, a pointer of type `string_t*` must be dereferenced.
## Booleans
[RFC 7159](http://rfc7159.net/rfc7159) implicitly describes a boolean as a type which differentiates the two literals `true` and `false`.
### Default type
With the default values for *BooleanType* (`#!cpp bool`), the default value for `boolean_t` is `#!cpp bool`.
### Storage
Boolean values are stored directly inside a `basic_json` type.
## Numbers
[RFC 7159](http://rfc7159.net/rfc7159) describes numbers as follows:
> The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.
This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer or a floating-point number. Therefore, three different types, `number_integer_t`, `number_unsigned_t`, and `number_float_t` are used.
### Default types
With the default values for *NumberIntegerType* (`std::int64_t`), the default value for `number_integer_t` is `std::int64_t`.
With the default values for *NumberUnsignedType* (`std::uint64_t`), the default value for `number_unsigned_t` is `std::uint64_t`.
With the default values for *NumberFloatType* (`#!cpp double`), the default value for `number_float_t` is `#!cpp double`.
### Default behavior
- The restrictions about leading zeros is not enforced in C++. Instead, leading zeros in integer literals lead to an interpretation as octal number. Internally, the value will be stored as decimal number. For instance, the C++ integer literal `#!c 010` will be serialized to `#!c 8`. During deserialization, leading zeros yield an error.
- Not-a-number (NaN) values will be serialized to `#!json null`.
### Limits
[RFC 7159](http://rfc7159.net/rfc7159) specifies:
> An implementation may set limits on the range and precision of numbers.
When the default type is used, the maximal integer number that can be stored is `#!c 9223372036854775807` (`INT64_MAX`) and the minimal integer number that can be stored is `#!c -9223372036854775808` (`INT64_MIN`). Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization, too large or small integer numbers will be automatically be stored as `number_unsigned_t` or `number_float_t`.
When the default type is used, the maximal unsigned integer number that can be stored is `#!c 18446744073709551615` (`UINT64_MAX`) and the minimal integer number that can be stored is `#!c 0`. Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization, too large or small integer numbers will be automatically be stored as `number_integer_t` or `number_float_t`.
[RFC 7159](http://rfc7159.net/rfc7159) further states:
> Note that when such software is used, numbers that are integers and are in the range $[-2^{53}+1, 2^{53}-1]$ are interoperable in the sense that implementations will agree exactly on their numeric values.
As this range is a subrange of the exactly supported range [`INT64_MIN`, `INT64_MAX`], this class's integer type is interoperable.
[RFC 7159](http://rfc7159.net/rfc7159) states:
> This specification allows implementations to set limits on the range and precision of numbers accepted. Since software that implements IEEE 754-2008 binary64 (double precision) numbers is generally available and widely used, good interoperability can be achieved by implementations that expect no more precision or range than these provide, in the sense that implementations will approximate JSON numbers within the expected precision.
This implementation does exactly follow this approach, as it uses double precision floating-point numbers. Note values smaller than `#!c -1.79769313486232e+308` and values greater than `#!c 1.79769313486232e+308` will be stored as NaN internally and be serialized to `#!json null`.
### Storage
Integer number values, unsigned integer number values, and floating-point number values are stored directly inside a `basic_json` type.

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# Contributor Covenant Code of Conduct
## Our Pledge
In the interest of fostering an open and welcoming environment, we as contributors and maintainers pledge to making participation in our project and our community a harassment-free experience for everyone, regardless of age, body size, disability, ethnicity, gender identity and expression, level of experience, nationality, personal appearance, race, religion, or sexual identity and orientation.
## Our Standards
Examples of behavior that contributes to creating a positive environment include:
* Using welcoming and inclusive language
* Being respectful of differing viewpoints and experiences
* Gracefully accepting constructive criticism
* Focusing on what is best for the community
* Showing empathy towards other community members
Examples of unacceptable behavior by participants include:
* The use of sexualized language or imagery and unwelcome sexual attention or advances
* Trolling, insulting/derogatory comments, and personal or political attacks
* Public or private harassment
* Publishing others' private information, such as a physical or electronic address, without explicit permission
* Other conduct which could reasonably be considered inappropriate in a professional setting
## Our Responsibilities
Project maintainers are responsible for clarifying the standards of acceptable behavior and are expected to take appropriate and fair corrective action in response to any instances of unacceptable behavior.
Project maintainers have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct, or to ban temporarily or permanently any contributor for other behaviors that they deem inappropriate, threatening, offensive, or harmful.
## Scope
This Code of Conduct applies both within project spaces and in public spaces when an individual is representing the project or its community. Examples of representing a project or community include using an official project e-mail address, posting via an official social media account, or acting as an appointed representative at an online or offline event. Representation of a project may be further defined and clarified by project maintainers.
## Enforcement
Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by contacting the project team at mail@nlohmann.me. The project team will review and investigate all complaints, and will respond in a way that it deems appropriate to the circumstances. The project team is obligated to maintain confidentiality with regard to the reporter of an incident. Further details of specific enforcement policies may be posted separately.
Project maintainers who do not follow or enforce the Code of Conduct in good faith may face temporary or permanent repercussions as determined by other members of the project's leadership.
## Attribution
This Code of Conduct is adapted from the [Contributor Covenant][homepage], version 1.4, available at [http://contributor-covenant.org/version/1/4][version]
[homepage]: http://contributor-covenant.org
[version]: http://contributor-covenant.org/version/1/4/

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# Design goals
There are myriads of [JSON](https://json.org) libraries out there, and each may even have its reason to exist. Our class had these design goals:
- **Intuitive syntax**. In languages such as Python, JSON feels like a first class data type. We used all the operator magic of modern C++ to achieve the same feeling in your code. Check out the [examples below](#examples) and you'll know what I mean.
- **Trivial integration**. Our whole code consists of a single header file [`json.hpp`](https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json.hpp). That's it. No library, no subproject, no dependencies, no complex build system. The class is written in vanilla C++11. All in all, everything should require no adjustment of your compiler flags or project settings.
- **Serious testing**. Our class is heavily [unit-tested](https://github.com/nlohmann/json/tree/develop/test/src) and covers [100%](https://coveralls.io/r/nlohmann/json) of the code, including all exceptional behavior. Furthermore, we checked with [Valgrind](http://valgrind.org) and the [Clang Sanitizers](https://clang.llvm.org/docs/index.html) that there are no memory leaks. [Google OSS-Fuzz](https://github.com/google/oss-fuzz/tree/master/projects/json) additionally runs fuzz tests against all parsers 24/7, effectively executing billions of tests so far. To maintain high quality, the project is following the [Core Infrastructure Initiative (CII) best practices](https://bestpractices.coreinfrastructure.org/projects/289).
Other aspects were not so important to us:
- **Memory efficiency**. Each JSON object has an overhead of one pointer (the maximal size of a union) and one enumeration element (1 byte). The default generalization uses the following C++ data types: `std::string` for strings, `int64_t`, `uint64_t` or `double` for numbers, `std::map` for objects, `std::vector` for arrays, and `bool` for Booleans. However, you can template the generalized class `basic_json` to your needs.
- **Speed**. There are certainly [faster JSON libraries](https://github.com/miloyip/nativejson-benchmark#parsing-time) out there. However, if your goal is to speed up your development by adding JSON support with a single header, then this library is the way to go. If you know how to use a `std::vector` or `std::map`, you are already set.
See the [contribution guidelines](https://github.com/nlohmann/json/blob/master/.github/CONTRIBUTING.md#please-dont) for more information.

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# Exceptions
## Overview
### Base type
All exceptions inherit from class `json::exception` (which in turn inherits from `std::exception`). It is used as the base class for all exceptions thrown by the `basic_json` class. This class can hence be used as "wildcard" to catch exceptions.
```plantuml
std::exception <|-- json::exception
json::exception <|-- json::parse_error
json::exception <|-- json::invalid_iterator
json::exception <|-- json::type_error
json::exception <|-- json::out_of_range
json::exception <|-- json::other_error
interface std::exception {}
class json::exception {
+ const int id
+ const char* what() const
}
class json::parse_error {
+ const std::size_t byte
}
```
### Switch off exceptions
Exceptions are used widely within the library. They can, however, be switched off with either using the compiler flag `-fno-exceptions` or by defining the symbol `JSON_NOEXCEPTION`. In this case, exceptions are replaced by `abort()` calls. You can further control this behavior by defining `JSON_THROW_USER` (overriding `#!cpp throw`), `JSON_TRY_USER` (overriding `#!cpp try`), and `JSON_CATCH_USER` (overriding `#!cpp catch`).
Note that `JSON_THROW_USER` should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield undefined behavior.
## Parse errors
This exception is thrown by the library when a parse error occurs. Parse errors
can occur during the deserialization of JSON text, CBOR, MessagePack, as well
as when using JSON Patch.
Exceptions have ids 1xx.
!!! info "Byte index"
Member `byte` holds the byte index of the last read character in the input
file.
For an input with n bytes, 1 is the index of the first character and n+1
is the index of the terminating null byte or the end of file. This also
holds true when reading a byte vector (CBOR or MessagePack).
??? example
The following code shows how a `parse_error` exception can be caught.
```cpp
--8<-- "examples/parse_error.cpp"
```
Output:
```
--8<-- "examples/parse_error.output"
```
### json.exception.parse_error.101
This error indicates a syntax error while deserializing a JSON text. The error message describes that an unexpected token (character) was encountered, and the member `byte` indicates the error position.
!!! failure "Example message"
Input ended prematurely:
```
[json.exception.parse_error.101] parse error at 2: unexpected end of input; expected string literal
```
No input:
```
[json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - unexpected end of input; expected '[', '{', or a literal
```
Control character was not escaped:
```
[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0009 (HT) must be escaped to \u0009 or \\; last read: '"<U+0009>'"
```
String was not closed:
```
[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: missing closing quote; last read: '"'
```
Invalid number format:
```
[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1E'
```
`\u` was not be followed by four hex digits:
```
[json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing value - invalid string: '\u' must be followed by 4 hex digits; last read: '"\u01"'
```
Invalid UTF-8 surrogate pair:
```
[json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: surrogate U+DC00..U+DFFF must follow U+D800..U+DBFF; last read: '"\uD7FF\uDC00'"
```
Invalid UTF-8 byte:
```
[json.exception.parse_error.101] parse error at line 3, column 24: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '"vous \352t'
```
!!! tip
- Make sure the input is correctly read. Try to write the input to standard output to check if, for instance, the input file was successfully openened.
- Paste the input to a JSON validator like <http://jsonlint.com> or a tool like [jq](https://stedolan.github.io/jq/).
### json.exception.parse_error.102
JSON uses the `\uxxxx` format to describe Unicode characters. Code points above above 0xFFFF are split into two `\uxxxx` entries ("surrogate pairs"). This error indicates that the surrogate pair is incomplete or contains an invalid code point.
!!! failure "Example message"
```
parse error at 14: missing or wrong low surrogate
```
### json.exception.parse_error.103
Unicode supports code points up to 0x10FFFF. Code points above 0x10FFFF are invalid.
!!! failure "Example message"
```
parse error: code points above 0x10FFFF are invalid
```
### json.exception.parse_error.104
[RFC 6902](https://tools.ietf.org/html/rfc6902) requires a JSON Patch document to be a JSON document that represents an array of objects.
!!! failure "Example message"
```
[json.exception.parse_error.104] parse error: JSON patch must be an array of objects
```
### json.exception.parse_error.105
An operation of a JSON Patch document must contain exactly one "op" member, whose value indicates the operation to perform. Its value must be one of "add", "remove", "replace", "move", "copy", or "test"; other values are errors.
!!! failure "Example message"
```
[json.exception.parse_error.105] parse error: operation 'add' must have member 'value'
```
```
[json.exception.parse_error.105] parse error: operation 'copy' must have string member 'from'
```
```
[json.exception.parse_error.105] parse error: operation value 'foo' is invalid
```
### json.exception.parse_error.106
An array index in a JSON Pointer ([RFC 6901](https://tools.ietf.org/html/rfc6901)) may be `0` or any number without a leading `0`.
!!! failure "Example message"
```
[json.exception.parse_error.106] parse error: array index '01' must not begin with '0'
```
### json.exception.parse_error.107
A JSON Pointer must be a Unicode string containing a sequence of zero or more reference tokens, each prefixed by a `/` character.
!!! failure "Example message"
```
[json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'foo'
```
### json.exception.parse_error.108
In a JSON Pointer, only `~0` and `~1` are valid escape sequences.
!!! failure "Example message"
```
[json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'
```
### json.exception.parse_error.109
A JSON Pointer array index must be a number.
!!! failure "Example message"
```
[json.exception.parse_error.109] parse error: array index 'one' is not a number
```
```
[json.exception.parse_error.109] parse error: array index '+1' is not a number
```
### json.exception.parse_error.110
When parsing CBOR or MessagePack, the byte vector ends before the complete value has been read.
!!! failure "Example message"
```
[json.exception.parse_error.110] parse error at byte 5: syntax error while parsing CBOR string: unexpected end of input
```
```
[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing UBJSON value: expected end of input; last byte: 0x5A
```
### json.exception.parse_error.112
Not all types of CBOR or MessagePack are supported. This exception occurs if an unsupported byte was read.
!!! failure "Example message"
```
[json.exception.parse_error.112] parse error at byte 1: syntax error while parsing CBOR value: invalid byte: 0x1C
```
### json.exception.parse_error.113
While parsing a map key, a value that is not a string has been read.
!!! failure "Example message"
```
[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0xFF
```
```
[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack string: expected length specification (0xA0-0xBF, 0xD9-0xDB); last byte: 0xFF
```
```
[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing UBJSON char: byte after 'C' must be in range 0x00..0x7F; last byte: 0x82
```
### json.exception.parse_error.114
The parsing of the corresponding BSON record type is not implemented (yet).
!!! failure "Example message"
```
[json.exception.parse_error.114] parse error at byte 5: Unsupported BSON record type 0xFF
```
## Iterator errors
This exception is thrown if iterators passed to a library function do not match
the expected semantics.
Exceptions have ids 2xx.
??? example
The following code shows how an `invalid_iterator` exception can be caught.
```cpp
--8<-- "examples/invalid_iterator.cpp"
```
Output:
```
--8<-- "examples/invalid_iterator.output"
```
### json.exception.invalid_iterator.201
The iterators passed to constructor `basic_json(InputIT first, InputIT last)` are not compatible, meaning they do not belong to the same container. Therefore, the range (`first`, `last`) is invalid.
!!! failure "Example message"
```
[json.exception.invalid_iterator.201] iterators are not compatible
```
### json.exception.invalid_iterator.202
In an erase or insert function, the passed iterator @a pos does not belong to the JSON value for which the function was called. It hence does not define a valid position for the deletion/insertion.
!!! failure "Example message"
```
[json.exception.invalid_iterator.202] iterator does not fit current value
```
```
[json.exception.invalid_iterator.202] iterators first and last must point to objects
```
### json.exception.invalid_iterator.203
Either iterator passed to function `erase(IteratorType` first, IteratorType last) does not belong to the JSON value from which values shall be erased. It hence does not define a valid range to delete values from.
!!! failure "Example message"
```
[json.exception.invalid_iterator.203] iterators do not fit current value
```
### json.exception.invalid_iterator.204
When an iterator range for a primitive type (number, boolean, or string) is passed to a constructor or an erase function, this range has to be exactly (`begin(),` `end()),` because this is the only way the single stored value is expressed. All other ranges are invalid.
!!! failure "Example message"
```
[json.exception.invalid_iterator.204] iterators out of range
```
### json.exception.invalid_iterator.205
When an iterator for a primitive type (number, boolean, or string) is passed to an erase function, the iterator has to be the `begin()` iterator, because it is the only way to address the stored value. All other iterators are invalid.
!!! failure "Example message"
```
[json.exception.invalid_iterator.205] iterator out of range
```
### json.exception.invalid_iterator.206
The iterators passed to constructor `basic_json(InputIT first, InputIT last)` belong to a JSON null value and hence to not define a valid range.
!!! failure "Example message"
```
[json.exception.invalid_iterator.206] cannot construct with iterators from null
```
### json.exception.invalid_iterator.207
The `key()` member function can only be used on iterators belonging to a JSON object, because other types do not have a concept of a key.
!!! failure "Example message"
```
[json.exception.invalid_iterator.207] cannot use key() for non-object iterators
```
### json.exception.invalid_iterator.208
The `operator[]` to specify a concrete offset cannot be used on iterators belonging to a JSON object, because JSON objects are unordered.
!!! failure "Example message"
```
[json.exception.invalid_iterator.208] cannot use operator[] for object iterators
```
### json.exception.invalid_iterator.209
The offset operators (`+`, `-`, `+=`, `-=`) cannot be used on iterators belonging to a JSON object, because JSON objects are unordered.
!!! failure "Example message"
```
[json.exception.invalid_iterator.209] cannot use offsets with object iterators
```
### json.exception.invalid_iterator.210
The iterator range passed to the insert function are not compatible, meaning they do not belong to the same container. Therefore, the range (`first`, `last`) is invalid.
!!! failure "Example message"
```
[json.exception.invalid_iterator.210] iterators do not fit
```
### json.exception.invalid_iterator.211
The iterator range passed to the insert function must not be a subrange of the container to insert to.
!!! failure "Example message"
```
[json.exception.invalid_iterator.211] passed iterators may not belong to container
```
### json.exception.invalid_iterator.212
When two iterators are compared, they must belong to the same container.
!!! failure "Example message"
```
[json.exception.invalid_iterator.212] cannot compare iterators of different containers
```
### json.exception.invalid_iterator.213
The order of object iterators cannot be compared, because JSON objects are unordered.
!!! failure "Example message"
```
[json.exception.invalid_iterator.213] cannot compare order of object iterators
```
### json.exception.invalid_iterator.214
Cannot get value for iterator: Either the iterator belongs to a null value or it is an iterator to a primitive type (number, boolean, or string), but the iterator is different to `begin()`.
!!! failure "Example message"
```
[json.exception.invalid_iterator.214] cannot get value
```
## Type errors
This exception is thrown in case of a type error; that is, a library function is executed on a JSON value whose type does not match the expected semantics.
Exceptions have ids 3xx.
??? example
The following code shows how a `type_error` exception can be caught.
```cpp
--8<-- "examples/type_error.cpp"
```
Output:
```
--8<-- "examples/type_error.output"
```
### json.exception.type_error.301
To create an object from an initializer list, the initializer list must consist only of a list of pairs whose first element is a string. When this constraint is violated, an array is created instead.
!!! failure "Example message"
```
[json.exception.type_error.301] cannot create object from initializer list
```
### json.exception.type_error.302
During implicit or explicit value conversion, the JSON type must be compatible to the target type. For instance, a JSON string can only be converted into string types, but not into numbers or boolean types.
!!! failure "Example message"
```
[json.exception.type_error.302] type must be object, but is null
```
```
[json.exception.type_error.302] type must be string, but is object
```
### json.exception.type_error.303
To retrieve a reference to a value stored in a `basic_json` object with `get_ref`, the type of the reference must match the value type. For instance, for a JSON array, the `ReferenceType` must be `array_t &`.
!!! failure "Example message"
```
[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is object
```
```
[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number"
```
### json.exception.type_error.304
The `at()` member functions can only be executed for certain JSON types.
!!! failure "Example message"
```
[json.exception.type_error.304] cannot use at() with string
```
```
[json.exception.type_error.304] cannot use at() with number
```
### json.exception.type_error.305
The `operator[]` member functions can only be executed for certain JSON types.
!!! failure "Example message"
```
[json.exception.type_error.305] cannot use operator[] with a string argument with array
```
```
[json.exception.type_error.305] cannot use operator[] with a numeric argument with object
```
### json.exception.type_error.306
The `value()` member functions can only be executed for certain JSON types.
!!! failure "Example message"
```
[json.exception.type_error.306] cannot use value() with number
```
### json.exception.type_error.307
The `erase()` member functions can only be executed for certain JSON types.
!!! failure "Example message"
```
[json.exception.type_error.307] cannot use erase() with string
```
### json.exception.type_error.308
The `push_back()` and `operator+=` member functions can only be executed for certain JSON types.
!!! failure "Example message"
```
[json.exception.type_error.308] cannot use push_back() with string
```
### json.exception.type_error.309
The `insert()` member functions can only be executed for certain JSON types.
!!! failure "Example message"
```
[json.exception.type_error.309] cannot use insert() with array
```
```
[json.exception.type_error.309] cannot use insert() with number
```
### json.exception.type_error.310
The `swap()` member functions can only be executed for certain JSON types.
!!! failure "Example message"
```
[json.exception.type_error.310] cannot use swap() with number
```
### json.exception.type_error.311
The `emplace()` and `emplace_back()` member functions can only be executed for certain JSON types.
!!! failure "Example message"
```
[json.exception.type_error.311] cannot use emplace() with number
```
```
[json.exception.type_error.311] cannot use emplace_back() with number
```
### json.exception.type_error.312
The `update()` member functions can only be executed for certain JSON types.
!!! failure "Example message"
```
[json.exception.type_error.312] cannot use update() with array
```
### json.exception.type_error.313
The `unflatten` function converts an object whose keys are JSON Pointers back into an arbitrary nested JSON value. The JSON Pointers must not overlap, because then the resulting value would not be well defined.
!!! failure "Example message"
```
[json.exception.type_error.313] invalid value to unflatten
```
### json.exception.type_error.314
The `unflatten` function only works for an object whose keys are JSON Pointers.
!!! failure "Example message"
Calling `unflatten()` on an array `#!json [1,2,3]`:
```
[json.exception.type_error.314] only objects can be unflattened
```
### json.exception.type_error.315
The `unflatten()` function only works for an object whose keys are JSON Pointers and whose values are primitive.
!!! failure "Example message"
Calling `unflatten()` on an object `#!json {"/1", [1,2,3]}`:
```
[json.exception.type_error.315] values in object must be primitive
```
### json.exception.type_error.316
The `dump()` function only works with UTF-8 encoded strings; that is, if you assign a `std::string` to a JSON value, make sure it is UTF-8 encoded.
!!! failure "Example message"
Calling `dump()` on a JSON value containing an ISO 8859-1 encoded string:
```
[json.exception.type_error.316] invalid UTF-8 byte at index 15: 0x6F
```
!!! tip
- Store the source file with UTF-8 encoding.
- Pass an error handler as last parameter to the `dump()` function to avoid this exception:
- `json::error_handler_t::replace` will replace invalid bytes sequences with `U+FFFD`
- `json::error_handler_t::ignore` will silently ignore invalid byte sequences
### json.exception.type_error.317
The dynamic type of the object cannot be represented in the requested serialization format (e.g. a raw `true` or `null` JSON object cannot be serialized to BSON)
!!! failure "Example message"
Serializing `#!json null` to BSON:
```
[json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is null
```
Serializing `#!json [1,2,3]` to BSON:
```
[json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is array
```
!!! tip
Encapsulate the JSON value in an object. That is, instead of serializing `#!json true`, serialize `#!json {"value": true}`
## Out of range
This exception is thrown in case a library function is called on an input parameter that exceeds the expected range, for instance in case of array indices or nonexisting object keys.
Exceptions have ids 4xx.
??? example
The following code shows how an `out_of_range` exception can be caught.
```cpp
--8<-- "examples/out_of_range.cpp"
```
Output:
```
--8<-- "examples/out_of_range.output"
```
### json.exception.out_of_range.401
The provided array index `i` is larger than `size-1`.
!!! failure "Example message"
```
array index 3 is out of range
```
### json.exception.out_of_range.402
The special array index `-` in a JSON Pointer never describes a valid element of the array, but the index past the end. That is, it can only be used to add elements at this position, but not to read it.
!!! failure "Example message"
```
array index '-' (3) is out of range
```
### json.exception.out_of_range.403
The provided key was not found in the JSON object.
!!! failure "Example message"
```
key 'foo' not found
```
### json.exception.out_of_range.404
A reference token in a JSON Pointer could not be resolved.
!!! failure "Example message"
```
unresolved reference token 'foo'
```
### json.exception.out_of_range.405
The JSON Patch operations 'remove' and 'add' can not be applied to the root element of the JSON value.
!!! failure "Example message"
```
JSON pointer has no parent
```
### json.exception.out_of_range.406
A parsed number could not be stored as without changing it to NaN or INF.
!!! failure "Example message"
```
number overflow parsing '10E1000'
```
### json.exception.out_of_range.407
UBJSON and BSON only support integer numbers up to 9223372036854775807.
!!! failure "Example message"
```
number overflow serializing '9223372036854775808'
```
### json.exception.out_of_range.408
The size (following `#`) of an UBJSON array or object exceeds the maximal capacity.
!!! failure "Example message"
```
excessive array size: 8658170730974374167
```
### json.exception.out_of_range.409
Key identifiers to be serialized to BSON cannot contain code point U+0000, since the key is stored as zero-terminated c-string.
!!! failure "Example message"
```
BSON key cannot contain code point U+0000 (at byte 2)
```
## Further exceptions
This exception is thrown in case of errors that cannot be classified with the
other exception types.
Exceptions have ids 5xx.
??? example
The following code shows how an `other_error` exception can be caught.
```cpp
--8<-- "examples/other_error.cpp"
```
Output:
```
--8<-- "examples/other_error.output"
```
### json.exception.other_error.501
A JSON Patch operation 'test' failed. The unsuccessful operation is also printed.
!!! failure "Example message"
Executing `#!json {"op":"test", "path":"/baz", "value":"bar"}` on `#!json {"baz": "qux"}`:
```
[json.exception.other_error.501] unsuccessful: {"op":"test","path":"/baz","value":"bar"}
```

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# Frequently Asked Questions (FAQ)
## Limitations
### Comments
!!! question "Questions"
- Why does the library not support comments?
- Can you add support for JSON5/JSONC/HOCON so that comments are supported?
This library does not support comments. It does so for three reasons:
1. Comments are not part of the [JSON specification](https://tools.ietf.org/html/rfc8259). You may argue that `//` or `/* */` are allowed in JavaScript, but JSON is not JavaScript.
2. This was not an oversight: Douglas Crockford [wrote on this](https://plus.google.com/118095276221607585885/posts/RK8qyGVaGSr) in May 2012:
> I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.
> Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.
3. It is dangerous for interoperability if some libraries would add comment support while others don't. Please check [The Harmful Consequences of the Robustness Principle](https://tools.ietf.org/html/draft-iab-protocol-maintenance-01) on this.
This library will not support comments in the future. If you wish to use comments, I see three options:
1. Strip comments before using this library.
2. Use a different JSON library with comment support.
3. Use a format that natively supports comments (e.g., YAML or JSON5).
### Relaxed parsing
!!! question
- Can you add an option to ignore trailing commas?
For the same reason this library does not support [comments](#comments), this library also does not support any feature which would jeopardize interoperability.
### Parse errors reading non-ASCII characters
!!! question "Questions"
- Why is the parser complaining about a Chinese character?
- Does the library support Unicode?
- I get an exception `[json.exception.parse_error.101] parse error at line 1, column 53: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '"Testé$')"`
The library supports **Unicode input** as follows:
- Only **UTF-8** encoded input is supported which is the default encoding for JSON according to [RFC 8259](https://tools.ietf.org/html/rfc8259.html#section-8.1).
- `std::u16string` and `std::u32string` can be parsed, assuming UTF-16 and UTF-32 encoding, respectively. These encodings are not supported when reading from files or other input containers.
- Other encodings such as Latin-1 or ISO 8859-1 are **not** supported and will yield parse or serialization errors.
- [Unicode noncharacters](http://www.unicode.org/faq/private_use.html#nonchar1) will not be replaced by the library.
- Invalid surrogates (e.g., incomplete pairs such as `\uDEAD`) will yield parse errors.
- The strings stored in the library are UTF-8 encoded. When using the default string type (`std::string`), note that its length/size functions return the number of stored bytes rather than the number of characters or glyphs.
- When you store strings with different encodings in the library, calling [`dump()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a50ec80b02d0f3f51130d4abb5d1cfdc5.html#a50ec80b02d0f3f51130d4abb5d1cfdc5) may throw an exception unless `json::error_handler_t::replace` or `json::error_handler_t::ignore` are used as error handlers.
In most cases, the parser is right to complain, because the input is not UTF-8 encoded. This is especially true for Microsoft Windows where Latin-1 or ISO 8859-1 is often the standard encoding.
### Key name in exceptions
!!! question
Can I get the key of the object item that caused an exception?
No, this is not possible. See <https://github.com/nlohmann/json/issues/932> for a longer discussion.
## Serialization issues
### Order of object keys
!!! question "Questions"
- Why are object keys sorted?
- Why is the insertion order of object keys not preserved?
By default, the library does not preserve the **insertion order of object elements**. This is standards-compliant, as the [JSON standard](https://tools.ietf.org/html/rfc8259.html) defines objects as "an unordered collection of zero or more name/value pairs".
If you do want to preserve the insertion order, you can specialize the object type with containers like [`tsl::ordered_map`](https://github.com/Tessil/ordered-map) ([integration](https://github.com/nlohmann/json/issues/546#issuecomment-304447518)) or [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)).
### Number precision
!!! question
- It seems that precision is lost when serializing a double.
- Can I change the precision for floating-point serialization?
The library uses `std::numeric_limits<number_float_t>::digits10` (15 for IEEE `double`s) digits for serialization. This value is sufficient to guarantee roundtripping. If one uses more than this number of digits of precision, then string -> value -> string is not guaranteed to round-trip.
!!! quote "[cppreference.com](https://en.cppreference.com/w/cpp/types/numeric_limits/digits10)"
The value of `std::numeric_limits<T>::digits10` is the number of base-10 digits that can be represented by the type T without change, that is, any number with this many significant decimal digits can be converted to a value of type T and back to decimal form, without change due to rounding or overflow.
!!! tip
The website https://float.exposed gives a good insight into the internal storage of floating-point numbers.
## Compilation issues
### Android SDK
!!! question
Why does the code not compile with Android SDK?
Android defaults to using very old compilers and C++ libraries. To fix this, add the following to your `Application.mk`. This will switch to the LLVM C++ library, the Clang compiler, and enable C++11 and other features disabled by default.
```ini
APP_STL := c++_shared
NDK_TOOLCHAIN_VERSION := clang3.6
APP_CPPFLAGS += -frtti -fexceptions
```
The code compiles successfully with [Android NDK](https://developer.android.com/ndk/index.html?hl=ml), Revision 9 - 11 (and possibly later) and [CrystaX's Android NDK](https://www.crystax.net/en/android/ndk) version 10.
### Missing STL function
!!! question "Questions"
- Why do I get a compilation error `'to_string' is not a member of 'std'` (or similarly, for `strtod` or `strtof`)?
- Why does the code not compile with MinGW or Android SDK?
This is not an issue with the code, but rather with the compiler itself. On Android, see above to build with a newer environment. For MinGW, please refer to [this site](http://tehsausage.com/mingw-to-string) and [this discussion](https://github.com/nlohmann/json/issues/136) for information on how to fix this bug. For Android NDK using `APP_STL := gnustl_static`, please refer to [this discussion](https://github.com/nlohmann/json/issues/219).

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# License
<img align="right" src="http://opensource.org/trademarks/opensource/OSI-Approved-License-100x137.png">
The class is licensed under the [MIT License](http://opensource.org/licenses/MIT):
Copyright &copy; 2013-2019 [Niels Lohmann](http://nlohmann.me)
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
* * *
The class contains the UTF-8 Decoder from Bjoern Hoehrmann which is licensed under the [MIT License](http://opensource.org/licenses/MIT) (see above). Copyright &copy; 2008-2009 [Björn Hoehrmann](http://bjoern.hoehrmann.de/) <bjoern@hoehrmann.de>
The class contains a slightly modified version of the Grisu2 algorithm from Florian Loitsch which is licensed under the [MIT License](http://opensource.org/licenses/MIT) (see above). Copyright &copy; 2009 [Florian Loitsch](http://florian.loitsch.com/)
The class contains a copy of [Hedley](https://nemequ.github.io/hedley/) from Evan Nemerson which is licensed as [CC0-1.0](http://creativecommons.org/publicdomain/zero/1.0/).

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# Sponsors
You can sponsor this library at [GitHub Sponsors](https://github.com/sponsors/nlohmann).
## Named Sponsors
- [Michael Hartmann](https://github.com/reFX-Mike)
- [Stefan Hagen](https://github.com/sthagen)
- [Steve Sperandeo](https://github.com/homer6)
Thanks everyone!

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import shutil
import os.path
def copy_doxygen(*args, **kwargs):
shutil.copytree('../html', os.path.join(kwargs['config']['site_dir'], 'doxygen'))
print('Copy Doxygen complete')

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# JSON for Modern C++
!!! note
This page is under construction. You probably want to see the [Doxygen documentation](doxygen).
![](images/json.gif)

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# CMake
You can also use the `nlohmann_json::nlohmann_json` interface target in CMake. This target populates the appropriate usage requirements for `INTERFACE_INCLUDE_DIRECTORIES` to point to the appropriate include directories and `INTERFACE_COMPILE_FEATURES` for the necessary C++11 flags.
## External
To use this library from a CMake project, you can locate it directly with `find_package()` and use the namespaced imported target from the generated package configuration:
```cmake
# CMakeLists.txt
find_package(nlohmann_json 3.2.0 REQUIRED)
...
add_library(foo ...)
...
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
```
The package configuration file, `nlohmann_jsonConfig.cmake`, can be used either from an install tree or directly out of the build tree.
## Embedded
To embed the library directly into an existing CMake project, place the entire source tree in a subdirectory and call `add_subdirectory()` in your `CMakeLists.txt` file:
```cmake
# Typically you don't care so much for a third party library's tests to be
# run from your own project's code.
set(JSON_BuildTests OFF CACHE INTERNAL "")
# If you only include this third party in PRIVATE source files, you do not
# need to install it when your main project gets installed.
# set(JSON_Install OFF CACHE INTERNAL "")
# Don't use include(nlohmann_json/CMakeLists.txt) since that carries with it
# unintended consequences that will break the build. It's generally
# discouraged (although not necessarily well documented as such) to use
# include(...) for pulling in other CMake projects anyways.
add_subdirectory(nlohmann_json)
...
add_library(foo ...)
...
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
```
## Embedded (FetchContent)
Since CMake v3.11,
[FetchContent](https://cmake.org/cmake/help/v3.11/module/FetchContent.html) can
be used to automatically download the repository as a dependency at configure type.
Example:
```cmake
include(FetchContent)
FetchContent_Declare(json
GIT_REPOSITORY https://github.com/nlohmann/json
GIT_TAG v3.7.3)
FetchContent_GetProperties(json)
if(NOT json_POPULATED)
FetchContent_Populate(json)
add_subdirectory(${json_SOURCE_DIR} ${json_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
```
!!! Note
The repository <https://github.com/nlohmann/json> download size is huge.
It contains all the dataset used for the benchmarks. You might want to depend on
a smaller repository. For instance, you might want to replace the URL above by
<https://github.com/ArthurSonzogni/nlohmann_json_cmake_fetchcontent>.
## Supporting Both
To allow your project to support either an externally supplied or an embedded JSON library, you can use a pattern akin to the following:
``` cmake
# Top level CMakeLists.txt
project(FOO)
...
option(FOO_USE_EXTERNAL_JSON "Use an external JSON library" OFF)
...
add_subdirectory(thirdparty)
...
add_library(foo ...)
...
# Note that the namespaced target will always be available regardless of the
# import method
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
```
```cmake
# thirdparty/CMakeLists.txt
...
if(FOO_USE_EXTERNAL_JSON)
find_package(nlohmann_json 3.2.0 REQUIRED)
else()
set(JSON_BuildTests OFF CACHE INTERNAL "")
add_subdirectory(nlohmann_json)
endif()
...
```
`thirdparty/nlohmann_json` is then a complete copy of this source tree.

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project(json_example)
cmake_minimum_required(VERSION 2.8.12)
add_definitions("-std=c++11")
include(${CMAKE_BINARY_DIR}/conanbuildinfo.cmake)
conan_basic_setup()
add_executable(json_example example.cpp)
target_link_libraries(json_example ${CONAN_LIBS})

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[requires]
nlohmann_json/3.7.3
[generators]
cmake

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#include <nlohmann/json.hpp>
#include <iostream>
using json = nlohmann::json;
int main()
{
std::cout << json::meta() << std::endl;
}

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#include <nlohmann/json.hpp>
#include <iostream>
using json = nlohmann::json;
int main()
{
std::cout << json::meta() << std::endl;
}

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# Integration
[`json.hpp`](https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json.hpp) is the single required file in `single_include/nlohmann` or [released here](https://github.com/nlohmann/json/releases). You need to add
```cpp
#include <nlohmann/json.hpp>
// for convenience
using json = nlohmann::json;
```
to the files you want to process JSON and set the necessary switches to enable C++11 (e.g., `-std=c++11` for GCC and Clang).
You can further use file [`include/nlohmann/json_fwd.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/json_fwd.hpp) for forward-declarations. The installation of json_fwd.hpp (as part of cmake's install step), can be achieved by setting `-DJSON_MultipleHeaders=ON`.

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# Package Managers
Throughout this page, we will describe how to compile the example file `example.cpp` below.
```cpp
--8<-- "integration/example.cpp"
```
## Homebrew
If you are using OS X and [Homebrew](http://brew.sh), just type
```sh
brew tap nlohmann/json
brew install nlohmann_json
```
and you're set. If you want the bleeding edge rather than the latest release, use
```sh
brew tap nlohmann/json
brew install nlohmann_json --HEAD
```
instead.
!!! example
1. Create the following file:
=== "example.cpp"
```cpp
--8<-- "integration/example.cpp"
```
2. Install the package
```sh
brew tap nlohmann/json
brew install nlohmann_json
```
3. Determine the include path, which defaults to `/usr/local/Cellar/nlohmann_json/$version/include`, where `$version` is the version of the library, e.g. `3.7.3`. The path of the library can be determined with
```sh
brew list nlohmann_json
```
4. Compile the code. For instance, the code can be compiled using Clang with
```sh
clang++ example.cpp -I/usr/local/Cellar/nlohmann_json/3.7.3/include -std=c++11 -o example
```
## Meson
If you are using the [Meson Build System](http://mesonbuild.com), add this source tree as a [meson subproject](https://mesonbuild.com/Subprojects.html#using-a-subproject). You may also use the `include.zip` published in this project's [Releases](https://github.com/nlohmann/json/releases) to reduce the size of the vendored source tree. Alternatively, you can get a wrap file by downloading it from [Meson WrapDB](https://wrapdb.mesonbuild.com/nlohmann_json), or simply use `meson wrap install nlohmann_json`. Please see the meson project for any issues regarding the packaging.
The provided meson.build can also be used as an alternative to cmake for installing `nlohmann_json` system-wide in which case a pkg-config file is installed. To use it, simply have your build system require the `nlohmann_json` pkg-config dependency. In Meson, it is preferred to use the [`dependency()`](https://mesonbuild.com/Reference-manual.html#dependency) object with a subproject fallback, rather than using the subproject directly.
## Conan
If you are using [Conan](https://www.conan.io/) to manage your dependencies, merely add `nlohmann_json/x.y.z` to your `conanfile`'s requires, where `x.y.z` is the release version you want to use. Please file issues [here](https://github.com/conan-io/conan-center-index/issues) if you experience problems with the packages.
!!! example
1. Create the following files:
=== "Conanfile.txt"
```ini
--8<-- "integration/conan/Conanfile.txt"
```
=== "CMakeLists.txt"
```cmake
--8<-- "integration/conan/CMakeLists.txt"
```
=== "example.cpp"
```cpp
--8<-- "integration/conan/example.cpp"
```
2. Build:
```sh
mkdir build
cd build
conan install ..
cmake ..
cmake --build .
```
## Spack
If you are using [Spack](https://www.spack.io/) to manage your dependencies, you can use the [`nlohmann-json` package](https://spack.readthedocs.io/en/latest/package_list.html#nlohmann-json). Please see the [spack project](https://github.com/spack/spack) for any issues regarding the packaging.
## Hunter
If you are using [hunter](https://github.com/cpp-pm/hunter) on your project for external dependencies, then you can use the [nlohmann_json package](https://hunter.readthedocs.io/en/latest/packages/pkg/nlohmann_json.html). Please see the hunter project for any issues regarding the packaging.
## Buckaroo
If you are using [Buckaroo](https://buckaroo.pm), you can install this library's module with `buckaroo add github.com/buckaroo-pm/nlohmann-json`. Please file issues [here](https://github.com/buckaroo-pm/nlohmann-json). There is a demo repo [here](https://github.com/njlr/buckaroo-nholmann-json-example).
## vcpkg
If you are using [vcpkg](https://github.com/Microsoft/vcpkg/) on your project for external dependencies, then you can use the [nlohmann-json package](https://github.com/Microsoft/vcpkg/tree/master/ports/nlohmann-json). Please see the vcpkg project for any issues regarding the packaging.
## cget
If you are using [cget](http://cget.readthedocs.io/en/latest/), you can install the latest development version with `cget install nlohmann/json`. A specific version can be installed with `cget install nlohmann/json@v3.1.0`. Also, the multiple header version can be installed by adding the `-DJSON_MultipleHeaders=ON` flag (i.e., `cget install nlohmann/json -DJSON_MultipleHeaders=ON`).
## CocoaPods
If you are using [CocoaPods](https://cocoapods.org), you can use the library by adding pod `"nlohmann_json", '~>3.1.2'` to your podfile (see [an example](https://bitbucket.org/benman/nlohmann_json-cocoapod/src/master/)). Please file issues [here](https://bitbucket.org/benman/nlohmann_json-cocoapod/issues?status=new&status=open).
## NuGet
If you are using [NuGet](https://www.nuget.org), you can use the package [nlohmann.json](https://www.nuget.org/packages/nlohmann.json/). Please check [this extensive description](https://github.com/nlohmann/json/issues/1132#issuecomment-452250255) on how to use the package. Please files issues [here](https://github.com/hnkb/nlohmann-json-nuget/issues).
## Conda
If you are using [conda](https://conda.io/), you can use the package [nlohmann_json](https://github.com/conda-forge/nlohmann_json-feedstock) from [conda-forge](https://conda-forge.org) executing `conda install -c conda-forge nlohmann_json`. Please file issues [here](https://github.com/conda-forge/nlohmann_json-feedstock/issues).
## MSYS2
If you are using [MSYS2](http://www.msys2.org/), your can use the [mingw-w64-nlohmann-json](https://packages.msys2.org/base/mingw-w64-nlohmann-json) package, just type `pacman -S mingw-w64-i686-nlohmann-json` or `pacman -S mingw-w64-x86_64-nlohmann-json` for installation. Please file issues [here](https://github.com/msys2/MINGW-packages/issues/new?title=%5Bnlohmann-json%5D) if you experience problems with the packages.
## build2
If you are using [`build2`](https://build2.org), you can use the [`nlohmann-json`](https://cppget.org/nlohmann-json) package from the public repository http://cppget.org or directly from the [package's sources repository](https://github.com/build2-packaging/nlohmann-json). In your project's `manifest` file, just add `depends: nlohmann-json` (probably with some [version constraints](https://build2.org/build2-toolchain/doc/build2-toolchain-intro.xhtml#guide-add-remove-deps)). If you are not familiar with using dependencies in `build2`, [please read this introduction](https://build2.org/build2-toolchain/doc/build2-toolchain-intro.xhtml).
Please file issues [here](https://github.com/build2-packaging/nlohmann-json) if you experience problems with the packages.
## wsjcpp
If you are using [`wsjcpp`](http://wsjcpp.org), you can use the command `wsjcpp install "https://github.com/nlohmann/json:develop"` to get the latest version. Note you can change the branch ":develop" to an existing tag or another branch.

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# Project information
site_name: JSON for Modern C++
site_author: Niels Lohmann
site_url: https://squidfunk.github.io/mkdocs-material/
# Repository
repo_name: nlohmann/json
repo_url: https://github.com/nlohmann/json
edit_uri: edit/develop/doc/mkdocs/docs
# Copyright
copyright: Copyright &copy; 2013 - 2020 Niels Lohmann
# Configuration
theme:
name: material
language: en
palette:
primary: indigo
accent: indigo
font:
text: Roboto
code: Roboto Mono
features:
- tabs
- instant
nav:
- Home:
- index.md
- home/license.md
- "Code of Conduct": home/code_of_conduct.md
- "FAQ": home/faq.md
- home/exceptions.md
- home/releases.md
- home/design_goals.md
- home/sponsors.md
- Features:
- features/arbitrary_types.md
- Binary Formats:
- features/binary_formats/index.md
- features/binary_formats/bson.md
- features/binary_formats/cbor.md
- features/binary_formats/messagepack.md
- features/binary_formats/ubjson.md
- features/binary_values.md
- features/iterators.md
- features/json_pointer.md
- features/json_patch.md
- features/merge_patch.md
- features/enum_conversion.md
- Parsing:
- features/parsing/index.md
- features/parsing/parse_exceptions.md
- features/parsing/parser_callbacks.md
- features/parsing/sax_interface.md
- features/types.md
- Integration:
- integration/index.md
- integration/cmake.md
- integration/package_managers.md
- Doxygen:
- doxygen/index.html
# Extras
extra:
social:
- icon: fontawesome/brands/github
link: https://github.com/nlohmann
- icon: fontawesome/brands/twitter
link: https://twitter.com/nlohmann
- icon: fontawesome/brands/linkedin
link: https://www.linkedin.com/in/nielslohmann/
- icon: fontawesome/brands/xing
link: https://www.xing.com/profile/Niels_Lohmann
- icon: fontawesome/brands/paypal
link: https://www.paypal.me/nlohmann
# Extensions
markdown_extensions:
- admonition
- codehilite:
guess_lang: false
- toc:
permalink: true
- pymdownx.arithmatex
- pymdownx.betterem:
smart_enable: all
- pymdownx.caret
- pymdownx.critic
- pymdownx.details
- pymdownx.emoji:
emoji_index: !!python/name:materialx.emoji.twemoji
emoji_generator: !!python/name:materialx.emoji.to_svg
- pymdownx.inlinehilite
- pymdownx.magiclink
- pymdownx.mark
#- pymdownx.smartsymbols
- pymdownx.superfences
- pymdownx.tasklist:
custom_checkbox: true
- pymdownx.tabbed
- pymdownx.tilde
- pymdownx.snippets:
base_path: docs
check_paths: true
- plantuml_markdown:
format: svg
plugins:
- search:
separator: '[\s\-\.]+'
- mkdocs-simple-hooks:
hooks:
on_post_build: "docs.hooks:copy_doxygen"
- minify:
minify_html: true
extra_javascript:
- https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js?config=TeX-MML-AM_CHTML

29
doc/mkdocs/requirements.txt Normal file
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click>=7.1.2
future>=0.18.2
htmlmin>=0.1.12
httplib2>=0.18.1
importlib-metadata>=1.6.0
Jinja2>=2.11.2
joblib>=0.15.1
jsmin>=2.2.2
livereload>=2.6.1
lunr>=0.5.8
Markdown>=3.2.2
markdown-include>=0.5.1
MarkupSafe>=1.1.1
mkdocs>=1.1.2
mkdocs-material>=5.2.1
mkdocs-material-extensions>=1.0
mkdocs-minify-plugin>=0.3.0
mkdocs-simple-hooks>=0.1.1
nltk>=3.5
plantuml>=0.3.0
plantuml-markdown>=3.2.2
Pygments>=2.6.1
pymdown-extensions>=7.1
PyYAML>=5.3.1
regex>=2020.5.14
six>=1.15.0
tornado>=6.0.4
tqdm>=4.46.0
zipp>=3.1.0