413 lines
17 KiB
Markdown
413 lines
17 KiB
Markdown
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[](https://travis-ci.org/nlohmann/json)
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[](https://ci.appveyor.com/project/nlohmann/json)
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[](https://coveralls.io/r/nlohmann/json)
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[](http://melpon.org/wandbox/permlink/WSW3gHHE4UcZ9K3G)
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[](http://nlohmann.github.io/json)
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[](https://raw.githubusercontent.com/nlohmann/json/master/LICENSE.MIT)
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[](https://github.com/nlohmann/json/releases)
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[](http://github.com/nlohmann/json/issues)
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## Design goals
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There are myriads of [JSON](http://json.org) libraries out there, and each may even have its reason to exist. Our class had these design goals:
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- **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 know, what I mean.
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- **Trivial integration**. Our whole code consists of a single header file `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.
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- **Serious testing**. Our class is heavily [unit-tested](https://github.com/nlohmann/json/blob/master/test/json_unit.cc) and covers [100%](https://coveralls.io/r/nlohmann/json) of the code, including all exceptional behavior. Furthermore, we checked with [Valgrind](http://valgrind.org) that there are no memory leaks.
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Other aspects were not so important to us:
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- **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` 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.
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- **Speed**. We currently implement the parser as naive [recursive descent parser](http://en.wikipedia.org/wiki/Recursive_descent_parser) with hand coded string handling. It is fast enough, but a [LALR-parser](http://en.wikipedia.org/wiki/LALR_parser) with a decent regular expression processor should be even faster (but would consist of more files which makes the integration harder).
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See the [https://github.com/nlohmann/json/blob/master/CONTRIBUTING.md#please-dont](contribution guidelines) for more information.
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## Integration
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The single required source, file `json.hpp` is in the `src` directory or [released here](https://github.com/nlohmann/json/releases). All you need to do is add
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```cpp
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#include "json.hpp"
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// for convenience
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using json = nlohmann::json;
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```
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to the files you want to use JSON objects. That's it. Do not forget to set the necessary switches to enable C++11 (e.g., `-std=c++11` for GCC and Clang).
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## Supported compilers
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Though it's 2016 already, the support for C++11 is still a bit sparse. Currently, the following compilers are known to work:
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- GCC 4.9 - 6.0 (and possibly later)
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- Clang 3.4 - 3.8 (and possibly later)
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- Microsoft Visual C++ 14.0 RC (and possibly later)
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I would be happy to learn about other compilers/versions.
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For GCC running on MinGW or Android SDK, the error `'to_string' is not a member of 'std'` (or similarly, for `strtod`) may occur. Note this is not an issue with the code, but rather with the compiler itself. 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.
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## Examples
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Here are some examples to give you an idea how to use the class.
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Assume you want to create the JSON object
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```json
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{
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"pi": 3.141,
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"happy": true,
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"name": "Niels",
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"nothing": null,
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"answer": {
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"everything": 42
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},
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"list": [1, 0, 2],
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"object": {
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"currency": "USD",
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"value": 42.99
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}
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}
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```
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With the JSON class, you could write:
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```cpp
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// create an empty structure (null)
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json j;
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// add a number that is stored as double (note the implicit conversion of j to an object)
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j["pi"] = 3.141;
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// add a Boolean that is stored as bool
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j["happy"] = true;
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// add a string that is stored as std::string
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j["name"] = "Niels";
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// add another null object by passing nullptr
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j["nothing"] = nullptr;
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// add an object inside the object
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j["answer"]["everything"] = 42;
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// add an array that is stored as std::vector (using an initializer list)
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j["list"] = { 1, 0, 2 };
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// add another object (using an initializer list of pairs)
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j["object"] = { {"currency", "USD"}, {"value", 42.99} };
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// instead, you could also write (which looks very similar to the JSON above)
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json j2 = {
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{"pi", 3.141},
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{"happy", true},
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{"name", "Niels"},
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{"nothing", nullptr},
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{"answer", {
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{"everything", 42}
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}},
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{"list", {1, 0, 2}},
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{"object", {
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{"currency", "USD"},
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{"value", 42.99}
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}}
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};
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```
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Note that in all these cases, you never need to "tell" the compiler which JSON value you want to use. If you want to be explicit or express some edge cases, the functions `json::array` and `json::object` will help:
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```cpp
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// a way to express the empty array []
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json empty_array_explicit = json::array();
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// ways to express the empty object {}
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json empty_object_implicit = json({});
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json empty_object_explicit = json::object();
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// a way to express an _array_ of key/value pairs [["currency", "USD"], ["value", 42.99]]
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json array_not_object = { json::array({"currency", "USD"}), json::array({"value", 42.99}) };
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```
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### Serialization / Deserialization
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You can create an object (deserialization) by appending `_json` to a string literal:
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```cpp
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// create object from string literal
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json j = "{ \"happy\": true, \"pi\": 3.141 }"_json;
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// or even nicer (thanks http://isocpp.org/blog/2015/01/json-for-modern-cpp)
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auto j2 = R"(
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{
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"happy": true,
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"pi": 3.141
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}
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)"_json;
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// or explicitly
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auto j3 = json::parse("{ \"happy\": true, \"pi\": 3.141 }");
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```
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You can also get a string representation (serialize):
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```cpp
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// explicit conversion to string
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std::string s = j.dump(); // {\"happy\":true,\"pi\":3.141}
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// serialization with pretty printing
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// pass in the amount of spaces to indent
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std::cout << j.dump(4) << std::endl;
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// {
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// "happy": true,
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// "pi": 3.141
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// }
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```
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You can also use streams to serialize and deserialize:
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```cpp
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// deserialize from standard input
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json j;
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std::cin >> j;
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// serialize to standard output
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std::cout << j;
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// the setw manipulator was overloaded to set the indentation for pretty printing
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std::cout << std::setw(4) << j << std::endl;
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```
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These operators work for any subclasses of `std::istream` or `std::ostream`.
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### STL-like access
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We designed the JSON class to behave just like an STL container. In fact, it satisfies the [**ReversibleContainer**](http://en.cppreference.com/w/cpp/concept/ReversibleContainer) requirement.
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```cpp
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// create an array using push_back
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json j;
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j.push_back("foo");
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j.push_back(1);
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j.push_back(true);
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// iterate the array
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for (json::iterator it = j.begin(); it != j.end(); ++it) {
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std::cout << *it << '\n';
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}
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// range-based for
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for (auto element : j) {
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std::cout << element << '\n';
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}
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// getter/setter
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const std::string tmp = j[0];
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j[1] = 42;
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bool foo = j.at(2);
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// other stuff
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j.size(); // 3 entries
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j.empty(); // false
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j.type(); // json::value_t::array
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j.clear(); // the array is empty again
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// convenience type checkers
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j.is_null();
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j.is_boolean();
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j.is_number();
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j.is_object();
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j.is_array();
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j.is_string();
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// comparison
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j == "[\"foo\", 1, true]"_json; // true
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// create an object
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json o;
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o["foo"] = 23;
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o["bar"] = false;
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o["baz"] = 3.141;
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// special iterator member functions for objects
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for (json::iterator it = o.begin(); it != o.end(); ++it) {
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std::cout << it.key() << " : " << it.value() << "\n";
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}
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// find an entry
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if (o.find("foo") != o.end()) {
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// there is an entry with key "foo"
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}
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// or simpler using count()
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int foo_present = o.count("foo"); // 1
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int fob_present = o.count("fob"); // 0
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// delete an entry
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o.erase("foo");
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```
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### Conversion from STL containers
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Any sequence container (`std::array`, `std::vector`, `std::deque`, `std::forward_list`, `std::list`) whose values can be used to construct JSON types (e.g., integers, floating point numbers, Booleans, string types, or again STL containers described in this section) can be used to create a JSON array. The same holds for similar associative containers (`std::set`, `std::multiset`, `std::unordered_set`, `std::unordered_multiset`), but in these cases the order of the elements of the array depends how the elements are ordered in the respective STL container.
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```cpp
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std::vector<int> c_vector {1, 2, 3, 4};
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json j_vec(c_vector);
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// [1, 2, 3, 4]
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std::deque<double> c_deque {1.2, 2.3, 3.4, 5.6};
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json j_deque(c_deque);
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// [1.2, 2.3, 3.4, 5.6]
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std::list<bool> c_list {true, true, false, true};
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json j_list(c_list);
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// [true, true, false, true]
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std::forward_list<int64_t> c_flist {12345678909876, 23456789098765, 34567890987654, 45678909876543};
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json j_flist(c_flist);
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// [12345678909876, 23456789098765, 34567890987654, 45678909876543]
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std::array<unsigned long, 4> c_array {{1, 2, 3, 4}};
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json j_array(c_array);
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// [1, 2, 3, 4]
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std::set<std::string> c_set {"one", "two", "three", "four", "one"};
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json j_set(c_set); // only one entry for "one" is used
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// ["four", "one", "three", "two"]
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std::unordered_set<std::string> c_uset {"one", "two", "three", "four", "one"};
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json j_uset(c_uset); // only one entry for "one" is used
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// maybe ["two", "three", "four", "one"]
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std::multiset<std::string> c_mset {"one", "two", "one", "four"};
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json j_mset(c_mset); // only one entry for "one" is used
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// maybe ["one", "two", "four"]
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std::unordered_multiset<std::string> c_umset {"one", "two", "one", "four"};
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json j_umset(c_umset); // both entries for "one" are used
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// maybe ["one", "two", "one", "four"]
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```
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Likewise, any associative key-value containers (`std::map`, `std::multimap`, `std::unordered_map`, `std::unordered_multimap`) whose keys are can construct an `std::string` and whose values can be used to construct JSON types (see examples above) can be used to to create a JSON object. Note that in case of multimaps only one key is used in the JSON object and the value depends on the internal order of the STL container.
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```cpp
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std::map<std::string, int> c_map { {"one", 1}, {"two", 2}, {"three", 3} };
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json j_map(c_map);
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// {"one": 1, "two": 2, "three": 3}
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std::unordered_map<const char*, double> c_umap { {"one", 1.2}, {"two", 2.3}, {"three", 3.4} };
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json j_umap(c_umap);
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// {"one": 1.2, "two": 2.3, "three": 3.4}
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std::multimap<std::string, bool> c_mmap { {"one", true}, {"two", true}, {"three", false}, {"three", true} };
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json j_mmap(c_mmap); // only one entry for key "three" is used
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// maybe {"one": true, "two": true, "three": true}
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std::unordered_multimap<std::string, bool> c_ummap { {"one", true}, {"two", true}, {"three", false}, {"three", true} };
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json j_ummap(c_ummap); // only one entry for key "three" is used
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// maybe {"one": true, "two": true, "three": true}
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```
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### Implicit conversions
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The type of the JSON object is determined automatically by the expression to store. Likewise, the stored value is implicitly converted.
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```cpp
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/// strings
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std::string s1 = "Hello, world!";
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json js = s1;
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std::string s2 = js;
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// Booleans
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bool b1 = true;
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json jb = b1;
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bool b2 = jb;
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// numbers
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int i = 42;
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json jn = i;
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double f = jn;
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// etc.
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```
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You can also explicitly ask for the value:
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```cpp
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std::string vs = js.get<std::string>();
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bool vb = jb.get<bool>();
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int vi = jn.get<int>();
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// etc.
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```
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## License
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<img align="right" src="http://opensource.org/trademarks/opensource/OSI-Approved-License-100x137.png">
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The class is licensed under the [MIT License](http://opensource.org/licenses/MIT):
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Copyright © 2013-2016 [Niels Lohmann](http://nlohmann.me)
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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:
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The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
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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.
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## Thanks
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I deeply appreciate the help of the following people.
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- [Teemperor](https://github.com/Teemperor) implemented CMake support and lcov integration, realized escape and Unicode handling in the string parser, and fixed the JSON serialization.
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- [elliotgoodrich](https://github.com/elliotgoodrich) fixed an issue with double deletion in the iterator classes.
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- [kirkshoop](https://github.com/kirkshoop) made the iterators of the class composable to other libraries.
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- [wancw](https://github.com/wanwc) fixed a bug that hindered the class to compile with Clang.
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- Tomas Åblad found a bug in the iterator implementation.
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- [Joshua C. Randall](https://github.com/jrandall) fixed a bug in the floating-point serialization.
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- [Aaron Burghardt](https://github.com/aburgh) implemented code to parse streams incrementally. Furthermore, he greatly improved the parser class by allowing the definition of a filter function to discard undesired elements while parsing.
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- [Daniel Kopeček](https://github.com/dkopecek) fixed a bug in the compilation with GCC 5.0.
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- [Florian Weber](https://github.com/Florianjw) fixed a bug in and improved the performance of the comparison operators.
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- [Eric Cornelius](https://github.com/EricMCornelius) pointed out a bug in the handling with NaN and infinity values. He also improved the performance of the string escaping.
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- [易思龙](https://github.com/likebeta) implemented a conversion from anonymous enums.
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- [kepkin](https://github.com/kepkin) patiently pushed forward the support for Microsoft Visual studio.
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- [gregmarr](https://github.com/gregmarr) simplified the implementation of reverse iterators and helped with numerous hints and improvements.
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- [Caio Luppi](https://github.com/caiovlp) fixed a bug in the Unicode handling.
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- [dariomt](https://github.com/dariomt) fixed some typos in the examples.
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- [Daniel Frey](https://github.com/d-frey) cleaned up some pointers and implemented exception-safe memory allocation.
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- [Colin Hirsch](https://github.com/ColinH) took care of a small namespace issue.
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- [Huu Nguyen](https://github.com/whoshuu) correct a variable name in the documentation.
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- [Silverweed](https://github.com/silverweed) overloaded `parse()` to accept an rvalue reference.
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- [dariomt](https://github.com/dariomt) fixed a subtlety in MSVC type support and implemented the `get_ref()` function to get a reference to stored values.
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- [ZahlGraf](https://github.com/ZahlGraf) added a workaround that allows compilation using Android NDK.
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- [whackashoe](https://github.com/whackashoe) replaced a function that was marked as unsafe by Visual Studio.
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- [406345](https://github.com/406345) fixed two small warnings.
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- [Glen Fernandes](https://github.com/glenfe) noted a potential portability problem in the `has_mapped_type` function.
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- [Corbin Hughes](https://github.com/nibroc) fixed some typos in the contribution guidelines.
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- [twelsby](https://github.com/twelsby) fixed the array subscript operator, an issue that failed the MSVC build, and floating-point parsing/dumping.
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Thanks a lot for helping out!
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## Notes
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- The code contains numerous debug assertions which can be switched off by defining the preprocessor macro `NDEBUG`, see the [documentation of `assert`](http://en.cppreference.com/w/cpp/error/assert).
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## Execute unit tests
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To compile and run the tests, you need to execute
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```sh
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$ make
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$ ./json_unit "*"
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===============================================================================
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All tests passed (3343329 assertions in 29 test cases)
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```
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For more information, have a look at the file [.travis.yml](https://github.com/nlohmann/json/blob/master/.travis.yml).
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