# PicoJSON - a C++ JSON parser / serializer
Copyright © 2009-2010 Cybozu Labs, Inc.
Copyright © 2011-2015 Kazuho Oku
Licensed under [2-clause BSD license](http://opensource.org/licenses/BSD-2-Clause)
## Version
1.3.1-dev [](https://travis-ci.org/kazuho/picojson)
## Introduction
PicoJSON is a tiny JSON parser / serializer for C++ with following properties:
- header-file only
- no external dependencies (only uses standard C++ libraries)
- STL-frendly (arrays are represented by using std::vector, objects are std::map)
- provides both pull interface and streaming (event-based) interface
## Reading JSON using the pull interface
There are several ways to use the pull (DOM-like) interface of picojson.
The easiest way is to use the two-argument `parse` function.
```
std::string json = "[ \"hello JSON\" ]";
picojson::value v;
std::string err = picojson::parse(v, json);
if (! err.empty()) {
std:cerr << err << std::endl;
}
```
Four-argument `parse` function accepts a pair of iterators, and returns the end position of the input.
```
const char* json = "{\"a\":1}";
picojson::value v;
std::string err;
const char* json_end = picojson::parse(v, json, json + strlen(json), &err);
if (! err.empty()) {
std::cerr << err << std::endl;
}
```
```
std::istream_iterator input(std::cin);
picojson::value v;
std::string err;
input = picojson::parse(v, input, std::istream_iterator(), &err);
if (! err.empty()) {
std::cerr << err << std::endl;
}
```
It is also possible to use the `>>` operator to parse the input, however this interface is not thread-safe.
```
picosjon::value v;
std::cin >> v;
std::string err = picojson::get_last_error();
```
## Accessing the values
Values of a JSON object is represented as instances of picojson::value class.
namespace picojson {
class value {
...
public:
typedef std::vector<value> array;
typedef std::map<std::string, value> object;
value(); // create a null object
explicit value(bool b); // create a boolean object
explicit value(double n); // create a number object
explicit value(const std::string& s); // create a string object
explicit value(const array& a); // create an array object
explicit value(const object& o); // create an "object"
bool is<picojson::null>() const; // check if the object is "null"
bool is<bool>() const; // check if the object is a boolean
const bool& get<bool>() const; // const accessor (usable only if the object is a boolean)
bool& get<bool>(); // non-const accessor (usable only if the object is a boolean)
bool is<double>() const; // check if the object is a number
const double& get<double>() const; // const accessor (usable only if the object is a number)
double& get<double>(); // non-const accessor (usable only if the object is a number)
bool is<std::string>() const; // check if the object is a string
const std::string& get<std::string>() const;
// const accessor (usable only if the object is a string)
std::string& get<std::string>(); // non-const accessor (usable only if the object is a string)
bool is<array>() const; // check if the object is an array
const array& get<array>() const; // const accessor (usable only if the object is an array)
array& get<array>(); // non-const accessor (usable only if the object is an array)
bool is<object>() const; // check if the object is an "object"
const object& get<object>() const; // const accessor (usable only if the object is an object)
object& get<object>(); // non-const accessor (usable only if the object is an array)
bool evaluate_as_boolean() const; // evaluates the object as a boolean
std::string serialize() const; // returns the object in JSON representation
template void serialize(Iter os) const;
// serializes the object in JSON representation through an output iterator
std::string to_str() const; // returns the object in string (for casual use)
};
}
The code below parses a JSON string and prints the contents of the object.
picojson::value v;
// parse the input
std::cin >> v;
std::string err = picojson::get_last_error();
if (! err.empty()) {
std::cerr << err << std::endl;
exit(1);
}
// check if the type of the value is "object"
if (! v.is<picojson::object>()) {
std::cerr << "JSON is not an object" << std::endl;
exit(2);
}
// obtain a const reference to the map, and print the contents
const picojson::value::object& obj = v.get<picojson::object>();
for (picojson::value::object::const_iterator i = obj.begin();
i != obj.end();
++i) {
std::cout << i->first << ': ' << i->second.to_str() << std::endl;
}
Please note that the type check is mandatory; do not forget to check the type of the object by calling is<type>() before accessing the value by calling get<type>().
## Reading JSON using the streaming (event-driven) interface
Please refer to the implementation of picojson::default_parse_context and picojson::null_parse_context. There is also an example (examples/streaming.cc) .
## Serializing to JSON
Instances of the picojson::value class can be serialized in three ways, to ostream, to std::string, or to an output iterator.
picojson::value v;
...
std::cout << v;
picojson::value v;
...
std::string json = v.serialize();
picojson::value v;
...
v.serialize(std::ostream_iterator(std::cout));
## Experimental support for int64_t
Experimental suport for int64_t becomes available if the code is compiled with preprocessor macro `PICOJSON_USE_INT64`.
Turning on the feature will cause following changes to picojson:
- new constructor `picojson::value(int64_t)` is defined
- `is()` and `get()` become available
- numerics in JSON within the bounds of int64_t and not using `.` nor `e`/`E` are considered as int64 type
- the values are also avaliable as `double`s as well (i.e. all values which are `.is() == true` are also `.is() == true`)
- int64 values are converted to double once `get()` is called
Enabling the feature should not cause compatibility problem with code that do not use the feature.
## Further reading
Examples can be found in the examples directory, and on the [Wiki](https://github.com/kazuho/picojson/wiki). Please add your favorite examples to the Wiki.