A C++11 JSON writer and parser library. It
- parses and serializes directly to and from statically typed C++ objects,
- requires very little boilerplate code,
- is fast and makes use of vectorization,
- supports UTF-8,
- comes with a good suite of tests,
- is deployed and in active use on well over 100 million devices,
- and has API documentation.
spotify-json depends on Google's double-conversion library, which must be linked in to the code that uses spotify-json.
#include <iostream>
#include <map>
#include <string>
#include <spotify/json.hpp>
using namespace spotify::json;
struct Track {
std::string uri;
std::string uid;
std::map<std::string, std::string> metadata;
};
namespace spotify {
namespace json {
// Specialize spotify::json::default_codec_t to specify default behavior when
// encoding and decoding objects of certain types.
template <>
struct default_codec_t<Track> {
static object_t<Track> codec() {
auto codec = object<Track>();
codec.required("uri", &Track::uri);
codec.optional("uid", &Track::uid);
codec.optional("metadata", &Track::metadata);
return codec;
}
};
} // namespace json
} // namespace spotify
int main() {
const auto parsed_track = decode<Track>(R"({ "uri": "spotify:track:xyz", "metadata": { "a": "b" } })");
std::cout << "Parsed track with uri " << parsed_track.uri << std::endl;
Track track;
track.uri = "spotify:track:abc";
track.uid = "a-uid";
const auto json = encode(track);
std::cout << "Encoded the track into " << json << std::endl;
return 0;
}
spotify-json offers a range of codec types that can serialize and parse specific JSON values. There are codecs for each of the basic data types that JSON offers: strings, numbers, arrays, booleans, objects and null.
A codec for integers can be made using
codec::number<int>()
. The codec for strings can be
instantiated with codec::string()
.
Codecs are composable. It is for example possible to construct a codec for
parsing and serialization of JSON arrays of numbers, such as [1,4,2]
:
codec::array<std::vector<int>>(codec::number<int>())
.
Constructing deeply nested codecs manually as above can become tedious. To ease
this pain, default_codec
is a helper function
that makes it easy to construct codecs for built-in types. For example,
default_codec<int>()
is a codec that can parse and serialize numbers, and
default_codec<std::vector<int>>()
is one that works on arrays of numbers.
It is possible to work with JSON objects with arbitrary keys. For example,
default_codec<std::map<std::string, bool>>()
is a codec for JSON objects
with strings as keys and booleans as values.
Parsing is done using the decode
function:
try {
decode(codec::number<int>(), "123") == 123;
decode<int>("123") == 123; // Shortcut for decode(default_codec<int>(), "123")
decode<std::vector<int>>("[1,2,3]") == std::vector{ 1, 2, 3 };
} catch (const decode_exception &e) {
std::cout << "Failed to decode: " << e.what() << std::endl;
}
decode
throws
decode_exception
when parsing fails. There is
also a function try_decode
that doesn't throw on
parse errors:
int result = 0;
if (try_decode(result, "123")) {
result == 123;
} else {
// Decoding failed!
}
Similarly, serialization is done using encode
:
encode(codec::number<int>(), 123) == "123";
encode(123) == "123"; // Shortcut for encode(default_codec<int>(), 123)
encode(std::vector<int>{ 1, 2, 3 }) == "[1,2,3]";
Working with basic types such as numbers, strings, booleans and arrays is all nice and dandy, but most practical applications need to deal with rich JSON schemas that involve objects.
Many JSON libraries work by parsing JSON strings into a tree structure that can be read by the application. In our experience, this approach often leads to large amounts of boilerplate code to extract the information in this tree object into statically typed counterparts that are practical to use in C++. This boilerplate is painful to write, bug-prone and slow due to unnecessary copying. SAX-style event based libraries such as yajl avoid the slowdown but require even more boilerplate.
spotify-json avoids these issues by parsing the JSON directly into statically
typed data structures. To explain how, let's use the example of a basic
two-dimensional coordinate, represented in JSON as {"x":1,"y":2}
. In C++, such
a coordinate may be represented as a struct:
struct Coordinate {
Coordinate() = default;
Coordinate(int x, int y) : x(x), y(y) {}
int x = 0;
int y = 0;
};
With spotify-json, it is possible to construct a codec that can convert
Coordinate
directly to and from JSON:
auto coordinate_codec = object<Coordinate>();
coordinate_codec.required("x", &Coordinate::x);
coordinate_codec.required("y", &Coordinate::y);
The use of required
will cause parsing to fail if the fields are missing.
There is also an optional
method. For more information, see
object_t
's API documentation.
This codec can be used with encode
and decode
:
encode(coordinate_codec, Coordinate(10, 0)) == R"({"x":10,"y":0})";
const Coordinate coord = decode(coordinate_codec, R"({ "x": 12, "y": 13 })");
coord.x == 12;
coord.y == 13;
Objects can be nested. To demonstrate this, let's introduce another data type:
struct Player {
std::string name;
std::string instrument;
Coordinate position;
};
A codec for Player
might be created with
auto player_codec = object<Player>();
player_codec.required("name", &Player::name);
player_codec.required("instrument", &Player::instrument);
// Because there is no default_codec for Coordinate, we need to pass in the
// codec explicitly:
player_codec.required("position", &Player::position, coordinate_codec);
// Let's use it:
Player player;
player.name = "Daniel";
player.instrument = "guitar";
encode(player_codec, player) == R"({"name":"Daniel","instrument":"guitar","position":{"x":0,"y":0}})";
Since codecs are just normal objects, it is possible to create and use
several different codecs for any given data type. This makes it possibile to
parameterize parsing and do other fancy things, but for most data types there
will only really exist one codec. For these cases, it is possible to extend
the default_codec
helper to support your own data types.
namespace spotify {
namespace json {
template <>
struct default_codec_t<Coordinate> {
static object_t<Coordinate> codec() {
auto codec = object<Coordinate>();
codec.required("x", &Coordinate::x);
codec.required("y", &Coordinate::y);
return codec;
}
};
template <>
struct default_codec_t<Player> {
static object_t<Player> codec() {
auto codec = object<Player>();
codec.required("name", &Player::name);
codec.required("instrument", &Player::instrument);
codec.required("position", &Player::position);
return codec;
}
};
} // namespace json
} // namespace spotify
Coordinate
and Player
can now be used like any other type that spotify-json
supports out of the box:
encode(Coordinate(10, 0)) == R"({"x":10,"y":0})";
decode<std::vector<Coordinate>>(R"([{ "x": 1, "y": -1 }])") == std::vector<Coordinate>{ Coordinate(1, -1) };
Player player;
player.name = "Martin";
player.instrument = "drums";
encode(player) == R"({"name":"Martin","instrument":"drums","position":{"x":0,"y":0}})";
The examples above cover the most commonly used parts of spotify-json. The library supports more things that sometimes come in handy:
- Most STL containers, including
array
,vector
,deque
,list
,set
,unordered_set
,pair
,tuple
,map
andunordered_map
- C++
enum
s and similar types - Arbitrary conversion logic, for example when a raw binary hash in C++ is represented as a hex coded string in JSON
- Dealing with versioning
- Ignoring values that are of the wrong type instead of failing the parse
- Values wrapped in
unique_ptr
s andshared_ptr
s boost::optional
boost::chrono
andstd::chrono
types- Dealing with virtual classes / type erasure
- Floating point numbers with lossless serialize/parse roundtrip
If your project is built with CMake, it is easy to use spotify-json. Here is an example of how it can be done:
- Add spotify-json as a git submodule under
vendor/
- Add the following lines to the
CMakeLists.txt
of your project:
add_subdirectory(vendor/spotify-json)
target_link_libraries([YOUR TARGET] spotify-json)
- CMake (http://www.cmake.org)
- Boost (http://www.boost.org)
export BOOST_ROOT=/path/to/boost
export BOOST_LIBRARYDIR=/path/to/boost/lib/
mkdir build
cd build
cmake -G <generator-name> ..
Run "cmake --help" for a list of generators available on your system.
cd build
ctest -j 8
This project adheres to the Open Code of Conduct. By participating, you are expected to honor this code.