json/README.md
2013-12-11 08:49:07 +01:00

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# Yet another JSON class for C++
[![Build Status](https://travis-ci.org/nlohmann/json.png?branch=master)](https://travis-ci.org/nlohmann/json)
## Design goals
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:
- **Trivial integration**. Our whole code consists of just two files: A header file `JSON.h` and a source file `JSON.cc`. That's it. No library, no subproject, no dependencies. The class is written in vanilla C++98 and -- if possible -- uses some features of C++11 such as move constructors. All in all, the class should require no adjustment of your compiler flags or project settings.
- **Intiuitve syntax**. In languages such as Python, JSON feels like a first class data type. We used all the operator magic of C++ to achieve the same feeling in your code. Check out the [examples below](#examples) and you know, what I mean.
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). We use the following C++ data types: `std::string` for strings, `int` or `double` for numbers, `std::map` for objects, `std::vector` for arrays, and `bool` for Booleans. We know that there are more efficient ways to store the values, but we are happy enough right now. And by the way: [Valgrind](http://valgrind.org) says our code is free of leaks.
- **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).
- **Rigourous standard compliance**. We followed the [specification](http://json.org) as close as possible, but did not invest too much in a 100% compliance with respect to Unicode support. As a result, there might be edge cases of false positives and false negatives, but as long as we have a hand-written parser, we won't invest too much to be fully compliant.
## Integration
All you need to do is add
```cpp
#include "JSON.h"
```
to the files you want to use JSON objects. Furthermore, you need to compile the file `JSON.cc` and link it to your binaries.
## Examples
Here are some examples to give you an idea how to use the class:
```cpp
// create an empty structure
JSON j;
// add a number that is stored as double
j["pi"] = 3.141;
// add a Boolean that is stored as bool
j["happy"] = true;
// add a string that is stored as std::string
j["name"] = "Niels";
// add an object inside the object
j["further"]["entry"] = 42;
// add an array that is stored as std::vector (C++11)
j["list"] = { 1, 0, 2 };
```
### Input / Output
```cpp
// create object from stream
JSON j;
j << "{ \"pi\": 3.141, \"happy\": true }";
// write string representation to stream
std::cout << j;
```
These operators work for any subclasses of `std::istream` or `std::ostream`.
### STL-like access
```cpp
// create an array
JSON j;
j.push_back("foo");
j.push_back(1);
j.push_back(true);
// iterate the array
for (JSON::iterator it = j.begin(); it != j.end(); ++it) {
std::cout << *it << '\n';
}
// C++11 style
for (auto element : j) {
std::cout << element << '\n';
}
// getter/setter
std::string tmp = j[0];
j[1] = 42;
// other stuff
j.size(); // 3
j.empty(); // false
j.type(); // JSON::array
// create an object
JSON o;
o["foo"] = 23;
o["bar"] = false;
o["baz"] = 3.141;
// find an entry
if (o.find("foo") != o.end()) {
// there is an entry with key "foo"
}
// iterate the object
for (JSON::iterator it = o.begin(); it != o.end(); ++it) {
std::cout << it.key() << ':' << it.value() << '\n';
}
```
### Implicit conversions
The type of the JSON object is determined automatically by the expression to store. Likewise, the stored value is implicitly converted
```cpp
/// strings
std::string s1 = "Hello, world!";
JSON js = s;
std::string s2 = j;
// Booleans
bool b1 = true;
JSON jb = b1;
bool b2 = jb;
// numbers
int i = 42;
JSON jn = i;
double f = jn;
```