README.md 75.3 KB
Newer Older
Niels's avatar
Niels committed
1
[![JSON for Modern C++](https://raw.githubusercontent.com/nlohmann/json/master/doc/json.gif)](https://github.com/nlohmann/json/releases)
Niels's avatar
Niels committed
2

Niels's avatar
Niels committed
3
[![Build Status](https://travis-ci.org/nlohmann/json.svg?branch=master)](https://travis-ci.org/nlohmann/json)
4
[![Build Status](https://ci.appveyor.com/api/projects/status/1acb366xfyg3qybk/branch/develop?svg=true)](https://ci.appveyor.com/project/nlohmann/json)
Niels Lohmann's avatar
Niels Lohmann committed
5
[![Build Status](https://circleci.com/gh/nlohmann/json.svg?style=svg)](https://circleci.com/gh/nlohmann/json)
Niels's avatar
Niels committed
6
[![Coverage Status](https://img.shields.io/coveralls/nlohmann/json.svg)](https://coveralls.io/r/nlohmann/json)
7
[![Coverity Scan Build Status](https://scan.coverity.com/projects/5550/badge.svg)](https://scan.coverity.com/projects/nlohmann-json)
8
[![Codacy Badge](https://api.codacy.com/project/badge/Grade/f3732b3327e34358a0e9d1fe9f661f08)](https://www.codacy.com/app/nlohmann/json?utm_source=github.com&utm_medium=referral&utm_content=nlohmann/json&utm_campaign=Badge_Grade)
Niels Lohmann's avatar
Niels Lohmann committed
9
[![Language grade: C/C++](https://img.shields.io/lgtm/grade/cpp/g/nlohmann/json.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/nlohmann/json/context:cpp)
10
[![Fuzzing Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/json.svg)](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:json)
11
[![Try online](https://img.shields.io/badge/try-online-blue.svg)](https://wandbox.org/permlink/TarF5pPn9NtHQjhf)
Niels's avatar
Niels committed
12
[![Documentation](https://img.shields.io/badge/docs-doxygen-blue.svg)](http://nlohmann.github.io/json)
Niels's avatar
Niels committed
13
[![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/nlohmann/json/master/LICENSE.MIT)
14
15
[![GitHub Releases](https://img.shields.io/github/release/nlohmann/json.svg)](https://github.com/nlohmann/json/releases)
[![GitHub Issues](https://img.shields.io/github/issues/nlohmann/json.svg)](http://github.com/nlohmann/json/issues)
16
[![Average time to resolve an issue](http://isitmaintained.com/badge/resolution/nlohmann/json.svg)](http://isitmaintained.com/project/nlohmann/json "Average time to resolve an issue")
17
[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/289/badge)](https://bestpractices.coreinfrastructure.org/projects/289)
Niels's avatar
Niels committed
18

19
20
- [Design goals](#design-goals)
- [Integration](#integration)
21
22
  - [CMake](#cmake)
  - [Package Managers](#package-managers)
23
24
25
26
27
28
- [Examples](#examples)
  - [JSON as first-class data type](#json-as-first-class-data-type)
  - [Serialization / Deserialization](#serialization--deserialization)
  - [STL-like access](#stl-like-access)
  - [Conversion from STL containers](#conversion-from-stl-containers)
  - [JSON Pointer and JSON Patch](#json-pointer-and-json-patch)
29
  - [JSON Merge Patch](#json-merge-patch)
30
  - [Implicit conversions](#implicit-conversions)
Niels Lohmann's avatar
Niels Lohmann committed
31
  - [Conversions to/from arbitrary types](#arbitrary-types-conversions)
32
  - [Specializing enum conversion](#specializing-enum-conversion)
33
  - [Binary formats (BSON, CBOR, MessagePack, and UBJSON)](#binary-formats-bson-cbor-messagepack-and-ubjson)
34
35
- [Supported compilers](#supported-compilers)
- [License](#license)
36
- [Contact](#contact)
37
- [Thanks](#thanks)
38
- [Used third-party tools](#used-third-party-tools)
39
- [Projects using JSON for Modern C++](#projects-using-json-for-modern-c)
40
41
42
- [Notes](#notes)
- [Execute unit tests](#execute-unit-tests)

Niels's avatar
Niels committed
43
44
45
46
## 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:

47
- **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.
Niels's avatar
Niels committed
48

49
- **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.
Niels's avatar
Niels committed
50

Manvendra Singh's avatar
Manvendra Singh committed
51
- **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).
Niels's avatar
Niels committed
52
53
54

Other aspects were not so important to us:

Niels's avatar
Niels committed
55
- **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.
Niels's avatar
Niels committed
56

Niels Lohmann's avatar
Niels Lohmann committed
57
- **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.
Niels's avatar
Niels committed
58

Niels's avatar
cleanup    
Niels committed
59
See the [contribution guidelines](https://github.com/nlohmann/json/blob/master/.github/CONTRIBUTING.md#please-dont) for more information.
Niels's avatar
Niels committed
60

Niels's avatar
Niels committed
61

Niels's avatar
Niels committed
62
63
## Integration

Niels Lohmann's avatar
Niels Lohmann committed
64
[`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
Niels's avatar
Niels committed
65
66

```cpp
Patrik Huber's avatar
Patrik Huber committed
67
#include <nlohmann/json.hpp>
Niels's avatar
Niels committed
68
69
70
71
72

// for convenience
using json = nlohmann::json;
```

Wilson's avatar
Wilson committed
73
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).
Niels's avatar
Niels committed
74

Wilson's avatar
Wilson committed
75
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`.
76

77
78
79
80
81
82
83
### 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:
Niels Lohmann's avatar
Niels Lohmann committed
84

85
86
```cmake
# CMakeLists.txt
87
find_package(nlohmann_json 3.2.0 REQUIRED)
88
89
90
91
92
...
add_library(foo ...)
...
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
```
Niels Lohmann's avatar
Niels Lohmann committed
93

94
95
The package configuration file, `nlohmann_jsonConfig.cmake`, can be used either from an install tree or directly out of the build tree.

96
97
98
#### 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:
Niels Lohmann's avatar
Niels Lohmann committed
99

100
101
102
103
104
```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 "")

105
106
107
108
# 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 "")

109
# Don't use include(nlohmann_json/CMakeLists.txt) since that carries with it
Mark Beckwith's avatar
Mark Beckwith committed
110
# unintended consequences that will break the build.  It's generally
111
112
113
114
115
116
117
118
119
120
# 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)
```

#### Supporting Both
Niels Lohmann's avatar
Niels Lohmann committed
121

122
To allow your project to support either an externally supplied or an embedded JSON library, you can use a pattern akin to the following:
Niels Lohmann's avatar
Niels Lohmann committed
123

124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
``` 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()
...
```
Niels Lohmann's avatar
Niels Lohmann committed
149

150
`thirdparty/nlohmann_json` is then a complete copy of this source tree.
151

152
153
### Package Managers

154
:beer: If you are using OS X and [Homebrew](http://brew.sh), just type `brew tap nlohmann/json` and `brew install nlohmann-json` and you're set. If you want the bleeding edge rather than the latest release, use `brew install nlohmann-json --HEAD`.
Niels's avatar
Niels committed
155

156
If you are using the [Meson Build System](http://mesonbuild.com), then 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`.
Jay Sistar's avatar
Jay Sistar committed
157

158
159
If you are using [Conan](https://www.conan.io/) to manage your dependencies, merely add `jsonformoderncpp/x.y.z@vthiery/stable` to your `conanfile.py`'s requires, where `x.y.z` is the release version you want to use. Please file issues [here](https://github.com/vthiery/conan-jsonformoderncpp/issues) if you experience problems with the packages.

Niels Lohmann's avatar
Niels Lohmann committed
160
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.
Axel Huebl's avatar
Axel Huebl committed
161

Jorrit Wronski's avatar
Jorrit Wronski committed
162
If you are using [hunter](https://github.com/ruslo/hunter/) on your project for external dependencies, then you can use the [nlohmann_json package](https://docs.hunter.sh/en/latest/packages/pkg/nlohmann_json.html). Please see the hunter project for any issues regarding the packaging.
gregmarr's avatar
gregmarr committed
163

njlr's avatar
njlr committed
164
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).
165

gregmarr's avatar
gregmarr committed
166
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.
167

Wilson's avatar
Wilson committed
168
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`).
169

170
171
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).

172
173
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).

174
175
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).

176
177
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.

Niels's avatar
Niels committed
178
179
## Examples

180
Beside the examples below, you may want to check the [documentation](https://nlohmann.github.io/json/) where each function contains a separate code example (e.g., check out [`emplace()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a5338e282d1d02bed389d852dd670d98d.html#a5338e282d1d02bed389d852dd670d98d)). All [example files](https://github.com/nlohmann/json/tree/develop/doc/examples) can be compiled and executed on their own (e.g., file [emplace.cpp](https://github.com/nlohmann/json/blob/develop/doc/examples/emplace.cpp)).
181

182
183
### JSON as first-class data type

Niels's avatar
Niels committed
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
Here are some examples to give you an idea how to use the class.

Assume you want to create the JSON object

```json
{
  "pi": 3.141,
  "happy": true,
  "name": "Niels",
  "nothing": null,
  "answer": {
    "everything": 42
  },
  "list": [1, 0, 2],
  "object": {
    "currency": "USD",
    "value": 42.99
  }
}
```

Niels Lohmann's avatar
Niels Lohmann committed
205
With this library, you could write:
Niels's avatar
Niels committed
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248

```cpp
// create an empty structure (null)
json j;

// add a number that is stored as double (note the implicit conversion of j to an object)
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 another null object by passing nullptr
j["nothing"] = nullptr;

// add an object inside the object
j["answer"]["everything"] = 42;

// add an array that is stored as std::vector (using an initializer list)
j["list"] = { 1, 0, 2 };

// add another object (using an initializer list of pairs)
j["object"] = { {"currency", "USD"}, {"value", 42.99} };

// instead, you could also write (which looks very similar to the JSON above)
json j2 = {
  {"pi", 3.141},
  {"happy", true},
  {"name", "Niels"},
  {"nothing", nullptr},
  {"answer", {
    {"everything", 42}
  }},
  {"list", {1, 0, 2}},
  {"object", {
    {"currency", "USD"},
    {"value", 42.99}
  }}
};
```

249
Note that in all these cases, you never need to "tell" the compiler which JSON value type you want to use. If you want to be explicit or express some edge cases, the functions [`json::array()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a9ad7ec0bc1082ed09d10900fbb20a21f.html#a9ad7ec0bc1082ed09d10900fbb20a21f) and [`json::object()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_aaf509a7c029100d292187068f61c99b8.html#aaf509a7c029100d292187068f61c99b8) will help:
Niels's avatar
Niels committed
250
251

```cpp
Niels's avatar
Niels committed
252
// a way to express the empty array []
Niels's avatar
Niels committed
253
254
json empty_array_explicit = json::array();

Niels's avatar
Niels committed
255
256
// ways to express the empty object {}
json empty_object_implicit = json({});
Niels's avatar
Niels committed
257
258
259
json empty_object_explicit = json::object();

// a way to express an _array_ of key/value pairs [["currency", "USD"], ["value", 42.99]]
260
json array_not_object = json::array({ {"currency", "USD"}, {"value", 42.99} });
Niels's avatar
Niels committed
261
262
263
264
```

### Serialization / Deserialization

Niels Lohmann's avatar
Niels Lohmann committed
265
266
#### To/from strings

Niels Lohmann's avatar
Niels Lohmann committed
267
You can create a JSON value (deserialization) by appending `_json` to a string literal:
Niels's avatar
Niels committed
268
269
270
271
272

```cpp
// create object from string literal
json j = "{ \"happy\": true, \"pi\": 3.141 }"_json;

Niels's avatar
Niels committed
273
// or even nicer with a raw string literal
Niels's avatar
Niels committed
274
275
276
277
278
279
auto j2 = R"(
  {
    "happy": true,
    "pi": 3.141
  }
)"_json;
280
281
```

Niels Lohmann's avatar
Niels Lohmann committed
282
Note that without appending the `_json` suffix, the passed string literal is not parsed, but just used as JSON string value. That is, `json j = "{ \"happy\": true, \"pi\": 3.141 }"` would just store the string `"{ "happy": true, "pi": 3.141 }"` rather than parsing the actual object.
Niels's avatar
Niels committed
283

284
The above example can also be expressed explicitly using [`json::parse()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_afd4ef1ac8ad50a5894a9afebca69140a.html#afd4ef1ac8ad50a5894a9afebca69140a):
285
286
287

```cpp
// parse explicitly
Niels's avatar
Niels committed
288
289
290
auto j3 = json::parse("{ \"happy\": true, \"pi\": 3.141 }");
```

Niels Lohmann's avatar
Niels Lohmann committed
291
You can also get a string representation of a JSON value (serialize):
Niels's avatar
Niels committed
292
293
294
295
296
297
298
299
300
301
302
303
304
305

```cpp
// explicit conversion to string
std::string s = j.dump();    // {\"happy\":true,\"pi\":3.141}

// serialization with pretty printing
// pass in the amount of spaces to indent
std::cout << j.dump(4) << std::endl;
// {
//     "happy": true,
//     "pi": 3.141
// }
```

306
307
308
309
310
311
Note the difference between serialization and assignment:

```cpp
// store a string in a JSON value
json j_string = "this is a string";

312
313
314
315
316
// retrieve the string value
auto cpp_string = j_string.get<std::string>();
// retrieve the string value (alternative when an variable already exists)
std::string cpp_string2;
j_string.get_to(cpp_string2);
317
318
319
320
321

// retrieve the serialized value (explicit JSON serialization)
std::string serialized_string = j_string.dump();

// output of original string
322
std::cout << cpp_string << " == " << cpp_string2 << " == " << j_string.get<std::string>() << '\n';
323
324
325
326
// output of serialized value
std::cout << j_string << " == " << serialized_string << std::endl;
```

327
[`.dump()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a50ec80b02d0f3f51130d4abb5d1cfdc5.html#a50ec80b02d0f3f51130d4abb5d1cfdc5) always returns the serialized value, and [`.get<std::string>()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_aa6602bb24022183ab989439e19345d08.html#aa6602bb24022183ab989439e19345d08) returns the originally stored string value.
328

329
Note the library only supports UTF-8. 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.
330

Niels Lohmann's avatar
Niels Lohmann committed
331
332
#### To/from streams (e.g. files, string streams)

Niels's avatar
Niels committed
333
334
335
336
337
You can also use streams to serialize and deserialize:

```cpp
// deserialize from standard input
json j;
Niels's avatar
Niels committed
338
std::cin >> j;
Niels's avatar
Niels committed
339
340
341
342
343
344
345
346

// serialize to standard output
std::cout << j;

// the setw manipulator was overloaded to set the indentation for pretty printing
std::cout << std::setw(4) << j << std::endl;
```

Niels Lohmann's avatar
Niels Lohmann committed
347
348
349
350
351
352
353
354
355
356
357
358
These operators work for any subclasses of `std::istream` or `std::ostream`. Here is the same example with files:

```cpp
// read a JSON file
std::ifstream i("file.json");
json j;
i >> j;

// write prettified JSON to another file
std::ofstream o("pretty.json");
o << std::setw(4) << j << std::endl;
```
Niels's avatar
Niels committed
359

Kevin Dixon's avatar
Kevin Dixon committed
360
361
Please note that setting the exception bit for `failbit` is inappropriate for this use case. It will result in program termination due to the `noexcept` specifier in use.

Niels Lohmann's avatar
Niels Lohmann committed
362
363
#### Read from iterator range

Niels Lohmann's avatar
Niels Lohmann committed
364
You can also parse JSON from an iterator range; that is, from any container accessible by iterators whose content is stored as contiguous byte sequence, for instance a `std::vector<std::uint8_t>`:
Niels Lohmann's avatar
Niels Lohmann committed
365
366

```cpp
367
std::vector<std::uint8_t> v = {'t', 'r', 'u', 'e'};
Niels Lohmann's avatar
Niels Lohmann committed
368
369
370
371
372
373
json j = json::parse(v.begin(), v.end());
```

You may leave the iterators for the range [begin, end):

```cpp
374
std::vector<std::uint8_t> v = {'t', 'r', 'u', 'e'};
Niels Lohmann's avatar
Niels Lohmann committed
375
376
377
json j = json::parse(v);
```

378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
#### SAX interface

The library uses a SAX-like interface with the following functions:

```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 detail::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 `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 `bool` indicating the result of the last executed SAX event. It does not return a  `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`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/json_sax.hpp).

Niels's avatar
Niels committed
421
422
### STL-like access

423
We designed the JSON class to behave just like an STL container. In fact, it satisfies the [**ReversibleContainer**](https://en.cppreference.com/w/cpp/named_req/ReversibleContainer) requirement.
Niels's avatar
Niels committed
424
425
426
427
428
429
430
431

```cpp
// create an array using push_back
json j;
j.push_back("foo");
j.push_back(1);
j.push_back(true);

Niels Lohmann's avatar
Niels Lohmann committed
432
433
434
// also use emplace_back
j.emplace_back(1.78);

Niels's avatar
Niels committed
435
436
437
438
439
440
// iterate the array
for (json::iterator it = j.begin(); it != j.end(); ++it) {
  std::cout << *it << '\n';
}

// range-based for
Niels's avatar
Niels committed
441
for (auto& element : j) {
Niels's avatar
Niels committed
442
443
444
445
  std::cout << element << '\n';
}

// getter/setter
446
const auto tmp = j[0].get<std::string>();
Niels's avatar
Niels committed
447
448
449
j[1] = 42;
bool foo = j.at(2);

Jonathan Lee's avatar
Jonathan Lee committed
450
451
452
// comparison
j == "[\"foo\", 1, true]"_json;  // true

Niels's avatar
Niels committed
453
454
455
456
457
458
// other stuff
j.size();     // 3 entries
j.empty();    // false
j.type();     // json::value_t::array
j.clear();    // the array is empty again

Niels's avatar
Niels committed
459
460
461
462
463
464
465
466
// convenience type checkers
j.is_null();
j.is_boolean();
j.is_number();
j.is_object();
j.is_array();
j.is_string();

Niels's avatar
Niels committed
467
468
469
470
471
472
// create an object
json o;
o["foo"] = 23;
o["bar"] = false;
o["baz"] = 3.141;

Niels Lohmann's avatar
Niels Lohmann committed
473
474
475
// also use emplace
o.emplace("weather", "sunny");

Niels's avatar
Niels committed
476
477
478
479
480
// special iterator member functions for objects
for (json::iterator it = o.begin(); it != o.end(); ++it) {
  std::cout << it.key() << " : " << it.value() << "\n";
}

Niels Lohmann's avatar
Niels Lohmann committed
481
482
483
484
485
486
487
488
489
490
// the same code as range for
for (auto& el : o.items()) {
  std::cout << el.key() << " : " << el.value() << "\n";
}

// even easier with structured bindings (C++17)
for (auto& [key, value] : o.items()) {
  std::cout << key << " : " << value << "\n";
}

Niels's avatar
Niels committed
491
492
493
494
// find an entry
if (o.find("foo") != o.end()) {
  // there is an entry with key "foo"
}
Niels's avatar
Niels committed
495
496
497
498
499
500
501

// or simpler using count()
int foo_present = o.count("foo"); // 1
int fob_present = o.count("fob"); // 0

// delete an entry
o.erase("foo");
Niels's avatar
Niels committed
502
503
```

Niels's avatar
Niels committed
504

Niels's avatar
Niels committed
505
506
### Conversion from STL containers

Niels Lohmann's avatar
Niels Lohmann committed
507
Any sequence container (`std::array`, `std::vector`, `std::deque`, `std::forward_list`, `std::list`) whose values can be used to construct JSON values (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 on how the elements are ordered in the respective STL container.
Niels's avatar
Niels committed
508
509
510
511
512
513

```cpp
std::vector<int> c_vector {1, 2, 3, 4};
json j_vec(c_vector);
// [1, 2, 3, 4]

Niels's avatar
Niels committed
514
std::deque<double> c_deque {1.2, 2.3, 3.4, 5.6};
Niels's avatar
Niels committed
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
json j_deque(c_deque);
// [1.2, 2.3, 3.4, 5.6]

std::list<bool> c_list {true, true, false, true};
json j_list(c_list);
// [true, true, false, true]

std::forward_list<int64_t> c_flist {12345678909876, 23456789098765, 34567890987654, 45678909876543};
json j_flist(c_flist);
// [12345678909876, 23456789098765, 34567890987654, 45678909876543]

std::array<unsigned long, 4> c_array {{1, 2, 3, 4}};
json j_array(c_array);
// [1, 2, 3, 4]

std::set<std::string> c_set {"one", "two", "three", "four", "one"};
json j_set(c_set); // only one entry for "one" is used
// ["four", "one", "three", "two"]

std::unordered_set<std::string> c_uset {"one", "two", "three", "four", "one"};
json j_uset(c_uset); // only one entry for "one" is used
// maybe ["two", "three", "four", "one"]

std::multiset<std::string> c_mset {"one", "two", "one", "four"};
539
540
json j_mset(c_mset); // both entries for "one" are used
// maybe ["one", "two", "one", "four"]
Niels's avatar
Niels committed
541
542
543
544
545
546

std::unordered_multiset<std::string> c_umset {"one", "two", "one", "four"};
json j_umset(c_umset); // both entries for "one" are used
// maybe ["one", "two", "one", "four"]
```

Niels Lohmann's avatar
Niels Lohmann committed
547
Likewise, any associative key-value containers (`std::map`, `std::multimap`, `std::unordered_map`, `std::unordered_multimap`) whose keys can construct an `std::string` and whose values can be used to construct JSON values (see examples above) can be used 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.
Niels's avatar
Niels committed
548
549
550
551

```cpp
std::map<std::string, int> c_map { {"one", 1}, {"two", 2}, {"three", 3} };
json j_map(c_map);
Annihil's avatar
Annihil committed
552
// {"one": 1, "three": 3, "two": 2 }
Niels's avatar
Niels committed
553

Niels's avatar
Niels committed
554
std::unordered_map<const char*, double> c_umap { {"one", 1.2}, {"two", 2.3}, {"three", 3.4} };
Niels's avatar
Niels committed
555
556
557
558
559
560
561
562
563
564
565
566
json j_umap(c_umap);
// {"one": 1.2, "two": 2.3, "three": 3.4}

std::multimap<std::string, bool> c_mmap { {"one", true}, {"two", true}, {"three", false}, {"three", true} };
json j_mmap(c_mmap); // only one entry for key "three" is used
// maybe {"one": true, "two": true, "three": true}

std::unordered_multimap<std::string, bool> c_ummap { {"one", true}, {"two", true}, {"three", false}, {"three", true} };
json j_ummap(c_ummap); // only one entry for key "three" is used
// maybe {"one": true, "two": true, "three": true}
```

Niels's avatar
Niels committed
567
568
569
570
571
572
573
574
575
576
577
578
### JSON Pointer and JSON Patch

The library supports **JSON Pointer** ([RFC 6901](https://tools.ietf.org/html/rfc6901)) as alternative means to address structured values. On top of this, **JSON Patch** ([RFC 6902](https://tools.ietf.org/html/rfc6902)) allows to describe differences between two JSON values - effectively allowing patch and diff operations known from Unix.

```cpp
// a JSON value
json j_original = R"({
  "baz": ["one", "two", "three"],
  "foo": "bar"
})"_json;

// access members with a JSON pointer (RFC 6901)
Niels's avatar
Niels committed
579
j_original["/baz/1"_json_pointer];
Niels's avatar
Niels committed
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
// "two"

// a JSON patch (RFC 6902)
json j_patch = R"([
  { "op": "replace", "path": "/baz", "value": "boo" },
  { "op": "add", "path": "/hello", "value": ["world"] },
  { "op": "remove", "path": "/foo"}
])"_json;

// apply the patch
json j_result = j_original.patch(j_patch);
// {
//    "baz": "boo",
//    "hello": ["world"]
// }

// calculate a JSON patch from two JSON values
json::diff(j_result, j_original);
// [
//   { "op":" replace", "path": "/baz", "value": ["one", "two", "three"] },
Niels's avatar
Niels committed
600
601
//   { "op": "remove","path": "/hello" },
//   { "op": "add", "path": "/foo", "value": "bar" }
Niels's avatar
Niels committed
602
603
604
// ]
```

605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
### JSON Merge Patch

The library supports **JSON Merge Patch** ([RFC 7386](https://tools.ietf.org/html/rfc7386)) as a patch format. Instead of using JSON Pointer (see above) to specify values to be manipulated, it describes the changes using a syntax that closely mimics the document being modified.

```cpp
// a JSON value
json j_document = R"({
  "a": "b",
  "c": {
    "d": "e",
    "f": "g"
  }
})"_json;

// a patch
json j_patch = R"({
  "a":"z",
  "c": {
    "f": null
  }
})"_json;

// apply the patch
Isaac Nickaein's avatar
Isaac Nickaein committed
628
j_document.merge_patch(j_patch);
629
630
631
632
633
634
635
// {
//  "a": "z",
//  "c": {
//    "d": "e"
//  }
// }
```
Niels's avatar
Niels committed
636

Niels's avatar
Niels committed
637
638
### Implicit conversions

639
640
641
Supported types can be implicitly converted to JSON values.

It is recommended to **NOT USE** implicit conversions **FROM** a JSON value.
kjpus's avatar
kjpus committed
642
You can find more details about this recommendation [here](https://www.github.com/nlohmann/json/issues/958). 
Niels's avatar
Niels committed
643
644

```cpp
Niels's avatar
Niels committed
645
// strings
Niels's avatar
Niels committed
646
647
std::string s1 = "Hello, world!";
json js = s1;
648
649
650
651
652
auto s2 = js.get<std::string>();
// NOT RECOMMENDED
std::string s3 = js;
std::string s4;
s4 = js;
Niels's avatar
Niels committed
653
654
655
656

// Booleans
bool b1 = true;
json jb = b1;
657
658
659
660
661
auto b2 = jb.get<bool>();
// NOT RECOMMENDED
bool b3 = jb;
bool b4;
b4 = jb;
Niels's avatar
Niels committed
662
663
664
665

// numbers
int i = 42;
json jn = i;
666
667
668
669
670
auto f = jn.get<double>();
// NOT RECOMMENDED
double f2 = jb;
double f3;
f3 = jb;
Niels's avatar
Niels committed
671
672
673

// etc.
```
Niels Lohmann's avatar
Niels Lohmann committed
674

Niels Lohmann's avatar
Niels Lohmann committed
675
676
677
678
679
680
681
682
Note that `char` types are not automatically converted to JSON strings, but to integer numbers. A conversion to a string must be specified explicitly:

```cpp
char ch = 'A';                       // ASCII value 65
json j_default = ch;                 // stores integer number 65
json j_string = std::string(1, ch);  // stores string "A"
```

Théo DELRIEU's avatar
Théo DELRIEU committed
683
684
### Arbitrary types conversions

Niels Lohmann's avatar
Niels Lohmann committed
685
Every type can be serialized in JSON, not just STL containers and scalar types. Usually, you would do something along those lines:
Théo DELRIEU's avatar
Théo DELRIEU committed
686
687
688

```cpp
namespace ns {
Niels Lohmann's avatar
Niels Lohmann committed
689
    // a simple struct to model a person
Niels Lohmann's avatar
Niels Lohmann committed
690
691
692
693
694
    struct person {
        std::string name;
        std::string address;
        int age;
    };
Théo DELRIEU's avatar
Théo DELRIEU committed
695
}
Niels Lohmann's avatar
Niels Lohmann committed
696

Niels Lohmann's avatar
Niels Lohmann committed
697
698
ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};

Niels Lohmann's avatar
Niels Lohmann committed
699
// convert to JSON: copy each value into the JSON object
Théo DELRIEU's avatar
Théo DELRIEU committed
700
701
702
703
704
705
706
json j;
j["name"] = p.name;
j["address"] = p.address;
j["age"] = p.age;

// ...

Niels Lohmann's avatar
Niels Lohmann committed
707
// convert from JSON: copy each value from the JSON object
Niels Lohmann's avatar
Niels Lohmann committed
708
709
710
711
712
ns::person p {
    j["name"].get<std::string>(),
    j["address"].get<std::string>(),
    j["age"].get<int>()
};
Théo DELRIEU's avatar
Théo DELRIEU committed
713
714
```

Niels Lohmann's avatar
Niels Lohmann committed
715
It works, but that's quite a lot of boilerplate... Fortunately, there's a better way:
Théo DELRIEU's avatar
Théo DELRIEU committed
716
717

```cpp
Niels Lohmann's avatar
Niels Lohmann committed
718
719
720
721
// create a person
ns::person p {"Ned Flanders", "744 Evergreen Terrace", 60};

// conversion: person -> json
Théo DELRIEU's avatar
Théo DELRIEU committed
722
723
json j = p;

Niels Lohmann's avatar
Niels Lohmann committed
724
725
726
727
std::cout << j << std::endl;
// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}

// conversion: json -> person
728
auto p2 = j.get<ns::person>();
Niels Lohmann's avatar
Niels Lohmann committed
729
730

// that's it
Théo DELRIEU's avatar
Théo DELRIEU committed
731
732
733
734
735
assert(p == p2);
```

#### Basic usage

Niels Lohmann's avatar
Niels Lohmann committed
736
To make this work with one of your types, you only need to provide two functions:
Théo DELRIEU's avatar
Théo DELRIEU committed
737
738
739
740
741

```cpp
using nlohmann::json;

namespace ns {
Niels Lohmann's avatar
Niels Lohmann committed
742
    void to_json(json& j, const person& p) {
Niels Lohmann's avatar
Niels Lohmann committed
743
744
745
        j = json{{"name", p.name}, {"address", p.address}, {"age", p.age}};
    }

Niels Lohmann's avatar
Niels Lohmann committed
746
    void from_json(const json& j, person& p) {
747
748
749
        j.at("name").get_to(p.name);
        j.at("address").get_to(p.address);
        j.at("age").get_to(p.age);
Niels Lohmann's avatar
Niels Lohmann committed
750
    }
Théo DELRIEU's avatar
Théo DELRIEU committed
751
752
753
} // namespace ns
```

Niels Lohmann's avatar
Niels Lohmann committed
754
That's all! When calling the `json` constructor with your type, your custom `to_json` method will be automatically called.
755
Likewise, when calling `get<your_type>()` or `get_to(your_type&)`, the `from_json` method will be called.
Théo DELRIEU's avatar
Théo DELRIEU committed
756
757
758

Some important things:

Niels Lohmann's avatar
Niels Lohmann committed
759
* 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).
Mark Beckwith's avatar
Mark Beckwith committed
760
* 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.
761
* 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.)
Itja's avatar
Itja committed
762
* 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.
Niels Lohmann's avatar
Niels Lohmann committed
763
764
* You do not need to add serializers or deserializers for STL types like `std::vector`: the library already implements these.

Théo DELRIEU's avatar
Théo DELRIEU committed
765
766
767

#### How do I convert third-party types?

Niels Lohmann's avatar
Niels Lohmann committed
768
This requires a bit more advanced technique. But first, let's see how this conversion mechanism works:
Théo DELRIEU's avatar
Théo DELRIEU committed
769
770

The library uses **JSON Serializers** to convert types to json.
771
The default serializer for `nlohmann::json` is `nlohmann::adl_serializer` (ADL means [Argument-Dependent Lookup](https://en.cppreference.com/w/cpp/language/adl)).
Théo DELRIEU's avatar
Théo DELRIEU committed
772
773
774
775
776

It is implemented like this (simplified):

```cpp
template <typename T>
Niels Lohmann's avatar
Niels Lohmann committed
777
778
779
780
781
782
783
784
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
    }
Théo DELRIEU's avatar
Théo DELRIEU committed
785
786
787
};
```

Niels Lohmann's avatar
Niels Lohmann committed
788
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`...
Théo DELRIEU's avatar
Théo DELRIEU committed
789
790
791
792
793
794

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 {
Niels Lohmann's avatar
Niels Lohmann committed
795
796
797
798
799
800
    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 {
Niels Lohmann's avatar
Niels Lohmann committed
801
802
              j = *opt; // this will call adl_serializer<T>::to_json which will
                        // find the free function to_json in T's namespace!
Niels Lohmann's avatar
Niels Lohmann committed
803
804
805
806
            }
        }

        static void from_json(const json& j, boost::optional<T>& opt) {
807
808
809
            if (j.is_null()) {
                opt = boost::none;
            } else {
Wilson's avatar
Wilson committed
810
                opt = j.get<T>(); // same as above, but with
Niels Lohmann's avatar
Niels Lohmann committed
811
812
813
814
                                  // adl_serializer<T>::from_json
            }
        }
    };
Théo DELRIEU's avatar
Théo DELRIEU committed
815
816
817
818
819
}
```

#### How can I use `get()` for non-default constructible/non-copyable types?

820
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:
Théo DELRIEU's avatar
Théo DELRIEU committed
821
822
823

```cpp
struct move_only_type {
Niels Lohmann's avatar
Niels Lohmann committed
824
825
826
827
    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;
Wilson's avatar
Wilson committed
828

Niels Lohmann's avatar
Niels Lohmann committed
829
    int i;
Théo DELRIEU's avatar
Théo DELRIEU committed
830
831
832
};

namespace nlohmann {
Niels Lohmann's avatar
Niels Lohmann committed
833
834
    template <>
    struct adl_serializer<move_only_type> {
Niels Lohmann's avatar
Niels Lohmann committed
835
836
        // note: the return type is no longer 'void', and the method only takes
        // one argument
Niels Lohmann's avatar
Niels Lohmann committed
837
838
839
        static move_only_type from_json(const json& j) {
            return {j.get<int>()};
        }
Wilson's avatar
Wilson committed
840

Niels Lohmann's avatar
Niels Lohmann committed
841
842
843
        // 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
Niels Lohmann's avatar
Niels Lohmann committed
844
845
846
847
        static void to_json(json& j, move_only_type t) {
            j = t.i;
        }
    };
Théo DELRIEU's avatar
Théo DELRIEU committed
848
849
850
851
852
}
```

#### Can I write my own serializer? (Advanced use)

Niels Lohmann's avatar
Niels Lohmann committed
853
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.
Théo DELRIEU's avatar
Théo DELRIEU committed
854
855
856

If you write your own serializer, you'll need to do a few things:

Niels Lohmann's avatar
Niels Lohmann committed
857
858
859
- 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
Théo DELRIEU's avatar
Théo DELRIEU committed
860
861
862
863

Here is an example, without simplifications, that only accepts types with a size <= 32, and uses ADL.

```cpp
Niels Lohmann's avatar
Niels Lohmann committed
864
865
866
867
868
869
870
871
872
873
874
// 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);
    }
Wilson's avatar
Wilson committed
875

Niels Lohmann's avatar
Niels Lohmann committed
876
877
878
879
880
881
    template <typename BasicJsonType>
    static void from_json(const BasicJsonType& j, T& value) {
        // same thing here
        using nlohmann::from_json;
        from_json(j, value);
    }
Théo DELRIEU's avatar
Théo DELRIEU committed
882
883
884
885
886
887
888
889
890
};
```

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
{
Niels Lohmann's avatar
Niels Lohmann committed
891
892
893
894
895
896
    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;
    }
Wilson's avatar
Wilson committed
897

Niels Lohmann's avatar
Niels Lohmann committed
898
899
900
901
902
903
    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!
    }
Théo DELRIEU's avatar
Théo DELRIEU committed
904
905
};
```
Niels's avatar
Niels committed
906

907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
### 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"},
Matěj Plch's avatar
Matěj Plch committed
928
})
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
```

The `NLOHMANN_JSON_SERIALIZE_ENUM()` macro declares a set of `to_json()` / `from_json()` functions for type `TaskState` while avoiding repetition and boilerplate serilization 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 );
```

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.

Niels Lohmann's avatar
Niels Lohmann committed
957
### Binary formats (BSON, CBOR, MessagePack, and UBJSON)
958

Niels Lohmann's avatar
Niels Lohmann committed
959
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](http://bsonspec.org) (Binary JSON), [CBOR](http://cbor.io) (Concise Binary Object Representation), [MessagePack](http://msgpack.org), and [UBJSON](http://ubjson.org) (Universal Binary JSON Specification) to efficiently encode JSON values to byte vectors and to decode such vectors.
960
961
962
963
964

```cpp
// create a JSON value
json j = R"({"compact": true, "schema": 0})"_json;

Niels Lohmann's avatar
Niels Lohmann committed
965
966
967
968
969
970
971
972
// serialize to BSON
std::vector<std::uint8_t> v_bson = json::to_bson(j);

// 0x1B, 0x00, 0x00, 0x00, 0x08, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0x00, 0x01, 0x10, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00

// roundtrip
json j_from_bson = json::from_bson(v_bson);

973
// serialize to CBOR
974
std::vector<std::uint8_t> v_cbor = json::to_cbor(j);
975

Niels Lohmann's avatar
Niels Lohmann committed
976
// 0xA2, 0x67, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0xF5, 0x66, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x00
977
978
979
980
981

// roundtrip
json j_from_cbor = json::from_cbor(v_cbor);

// serialize to MessagePack
982
std::vector<std::uint8_t> v_msgpack = json::to_msgpack(j);
983

Niels Lohmann's avatar
Niels Lohmann committed
984
// 0x82, 0xA7, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0xC3, 0xA6, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x00
985
986
987

// roundtrip
json j_from_msgpack = json::from_msgpack(v_msgpack);
Niels Lohmann's avatar
Niels Lohmann committed
988
989
990
991
992
993
994
995

// serialize to UBJSON
std::vector<std::uint8_t> v_ubjson = json::to_ubjson(j);

// 0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D

// roundtrip
json j_from_ubjson = json::from_ubjson(v_ubjson);
996
997
```

Niels's avatar
Niels committed
998
999
1000

## Supported compilers