README.md 82 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)
5
6
7
[![Ubuntu](https://github.com/nlohmann/json/workflows/Ubuntu/badge.svg)](https://github.com/nlohmann/json/actions?query=workflow%3AUbuntu)
[![macOS](https://github.com/nlohmann/json/workflows/macOS/badge.svg)](https://github.com/nlohmann/json/actions?query=workflow%3AmacOS)
[![Windows](https://github.com/nlohmann/json/workflows/Windows/badge.svg)](https://github.com/nlohmann/json/actions?query=workflow%3AWindows)
Niels Lohmann's avatar
Niels Lohmann committed
8
[![Build Status](https://circleci.com/gh/nlohmann/json.svg?style=svg)](https://circleci.com/gh/nlohmann/json)
Niels Lohmann's avatar
Niels Lohmann committed
9
[![Coverage Status](https://coveralls.io/repos/github/nlohmann/json/badge.svg?branch=develop)](https://coveralls.io/github/nlohmann/json?branch=develop)
10
[![Coverity Scan Build Status](https://scan.coverity.com/projects/5550/badge.svg)](https://scan.coverity.com/projects/nlohmann-json)
11
[![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
12
[![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)
13
[![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)
14
[![Try online](https://img.shields.io/badge/try-online-blue.svg)](https://wandbox.org/permlink/3lCHrFUZANONKv7a)
Niels's avatar
Niels committed
15
[![Documentation](https://img.shields.io/badge/docs-doxygen-blue.svg)](http://nlohmann.github.io/json)
Niels's avatar
Niels committed
16
[![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/nlohmann/json/master/LICENSE.MIT)
Niels Lohmann's avatar
Niels Lohmann committed
17
[![FOSSA Status](https://app.fossa.com/api/projects/git%2Bgithub.com%2Fnlohmann%2Fjson.svg?type=shield)](https://app.fossa.com/projects/git%2Bgithub.com%2Fnlohmann%2Fjson?ref=badge_shield)
18
[![GitHub Releases](https://img.shields.io/github/release/nlohmann/json.svg)](https://github.com/nlohmann/json/releases)
Niels Lohmann's avatar
Niels Lohmann committed
19
[![GitHub Downloads](https://img.shields.io/github/downloads/nlohmann/json/total)](https://github.com/nlohmann/json/releases)
20
[![GitHub Issues](https://img.shields.io/github/issues/nlohmann/json.svg)](http://github.com/nlohmann/json/issues)
21
[![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")
22
[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/289/badge)](https://bestpractices.coreinfrastructure.org/projects/289)
Niels Lohmann's avatar
Niels Lohmann committed
23
[![GitHub Sponsors](https://img.shields.io/badge/GitHub-Sponsors-ff69b4)](https://github.com/sponsors/nlohmann)
Carlos Gomes Martinho's avatar
Carlos Gomes Martinho committed
24

25
- [Design goals](#design-goals)
Niels Lohmann's avatar
Niels Lohmann committed
26
- [Sponsors](#sponsors)
27
- [Integration](#integration)
28
29
  - [CMake](#cmake)
  - [Package Managers](#package-managers)
30
31
32
33
34
35
- [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)
36
  - [JSON Merge Patch](#json-merge-patch)
37
  - [Implicit conversions](#implicit-conversions)
Niels Lohmann's avatar
Niels Lohmann committed
38
  - [Conversions to/from arbitrary types](#arbitrary-types-conversions)
39
  - [Specializing enum conversion](#specializing-enum-conversion)
40
  - [Binary formats (BSON, CBOR, MessagePack, and UBJSON)](#binary-formats-bson-cbor-messagepack-and-ubjson)
41
42
- [Supported compilers](#supported-compilers)
- [License](#license)
43
- [Contact](#contact)
44
- [Thanks](#thanks)
45
- [Used third-party tools](#used-third-party-tools)
46
- [Projects using JSON for Modern C++](#projects-using-json-for-modern-c)
47
48
49
- [Notes](#notes)
- [Execute unit tests](#execute-unit-tests)

Niels's avatar
Niels committed
50
51
## Design goals

Antoine Cœur's avatar
Antoine Cœur committed
52
There are myriads of [JSON](https://json.org) libraries out there, and each may even have its reason to exist. Our class had these design goals:
Niels's avatar
Niels committed
53

54
- **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
55

56
- **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
57

Manvendra Singh's avatar
Manvendra Singh committed
58
- **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
59
60
61

Other aspects were not so important to us:

Niels's avatar
Niels committed
62
- **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
63

Niels Lohmann's avatar
Niels Lohmann committed
64
- **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
65

Niels's avatar
cleanup    
Niels committed
66
See the [contribution guidelines](https://github.com/nlohmann/json/blob/master/.github/CONTRIBUTING.md#please-dont) for more information.
Niels's avatar
Niels committed
67

Niels's avatar
Niels committed
68

Niels Lohmann's avatar
Niels Lohmann committed
69
70
71
72
73
74
75
## Sponsors

You can sponsor this library at [GitHub Sponsors](https://github.com/sponsors/nlohmann).

### :label: Named Sponsors

- [Michael Hartmann](https://github.com/reFX-Mike)
Niels Lohmann's avatar
Niels Lohmann committed
76
- [Stefan Hagen](https://github.com/sthagen)
Niels Lohmann's avatar
Niels Lohmann committed
77
- [Steve Sperandeo](https://github.com/homer6)
Niels Lohmann's avatar
Niels Lohmann committed
78
79
80
81

Thanks everyone!


Niels's avatar
Niels committed
82
83
## Integration

Niels Lohmann's avatar
Niels Lohmann committed
84
[`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
85
86

```cpp
Patrik Huber's avatar
Patrik Huber committed
87
#include <nlohmann/json.hpp>
Niels's avatar
Niels committed
88
89
90
91
92

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

Wilson's avatar
Wilson committed
93
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
94

Wilson's avatar
Wilson committed
95
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`.
96

97
98
99
100
101
102
103
### 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
104

105
106
```cmake
# CMakeLists.txt
107
find_package(nlohmann_json 3.2.0 REQUIRED)
108
109
110
111
112
...
add_library(foo ...)
...
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
```
Niels Lohmann's avatar
Niels Lohmann committed
113

114
115
The package configuration file, `nlohmann_jsonConfig.cmake`, can be used either from an install tree or directly out of the build tree.

116
117
118
#### 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
119

120
121
122
123
124
```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 "")

125
126
127
128
# 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 "")

129
# Don't use include(nlohmann_json/CMakeLists.txt) since that carries with it
Mark Beckwith's avatar
Mark Beckwith committed
130
# unintended consequences that will break the build.  It's generally
131
132
133
134
135
136
137
138
139
# 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)
```

140
##### Embedded (FetchContent)
141

142
143
Since CMake v3.11,
[FetchContent](https://cmake.org/cmake/help/v3.11/module/FetchContent.html) can
144
be used to automatically download the repository as a dependency at configure type.
145
146

Example:
147
```cmake
148
149
150
151
152
153
154
155
156
157
158
159
160
include(FetchContent)

FetchContent_Declare(json
  GIT_REPOSITORY https://github.com/nlohmann/json
  GIT_TAG v3.7.3)

FetchContent_GetProperties(json)
if(NOT json_POPULATED)
  FetchContent_Populate(json)
  add_subdirectory(${json_SOURCE_DIR} ${json_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()

target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
ArthurSonzogni's avatar
ArthurSonzogni committed
161
```
162
163
164

**Note**: The repository https://github.com/nlohmann/json download size is huge.
It contains all the dataset used for the benchmarks. You might want to depend on
165
a smaller repository. For instance, you might want to replace the URL above by
166
167
https://github.com/ArthurSonzogni/nlohmann_json_cmake_fetchcontent

168
#### Supporting Both
Niels Lohmann's avatar
Niels Lohmann committed
169

170
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
171

172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
``` 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
197

198
`thirdparty/nlohmann_json` is then a complete copy of this source tree.
199

200
201
### Package Managers

202
: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
203

204
205
206
If you are using the [Meson Build System](http://mesonbuild.com), add this source tree as a [meson subproject](https://mesonbuild.com/Subprojects.html#using-a-subproject). You may also use the `include.zip` published in this project's [Releases](https://github.com/nlohmann/json/releases) to reduce the size of the vendored source tree. Alternatively, 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`. Please see the meson project for any issues regarding the packaging.

The provided meson.build can also be used as an alternative to cmake for installing `nlohmann_json` system-wide in which case a pkg-config file is installed. To use it, simply have your build system require the `nlohmann_json` pkg-config dependency. In Meson, it is preferred to use the [`dependency()`](https://mesonbuild.com/Reference-manual.html#dependency) object with a subproject fallback, rather than using the subproject directly.
Jay Sistar's avatar
Jay Sistar committed
207

208
If you are using [Conan](https://www.conan.io/) to manage your dependencies, merely add `nlohmann_json/x.y.z` to your `conanfile`'s requires, where `x.y.z` is the release version you want to use. Please file issues [here](https://github.com/conan-io/conan-center-index/issues) if you experience problems with the packages.
209

Niels Lohmann's avatar
Niels Lohmann committed
210
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
211

jothepro's avatar
jothepro committed
212
If you are using [hunter](https://github.com/cpp-pm/hunter) on your project for external dependencies, then you can use the [nlohmann_json package](https://hunter.readthedocs.io/en/latest/packages/pkg/nlohmann_json.html). Please see the hunter project for any issues regarding the packaging.
gregmarr's avatar
gregmarr committed
213

njlr's avatar
njlr committed
214
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).
215

gregmarr's avatar
gregmarr committed
216
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.
217

Wilson's avatar
Wilson committed
218
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`).
219

220
221
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).

222
223
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).

224
225
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).

226
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.
227

Joël Lamotte's avatar
Joël Lamotte committed
228
If you are using [`build2`](https://build2.org), you can use the [`nlohmann-json`](https://cppget.org/nlohmann-json) package from the public repository http://cppget.org or directly from the [package's sources repository](https://github.com/build2-packaging/nlohmann-json). In your project's `manifest` file, just add `depends: nlohmann-json` (probably with some [version constraints](https://build2.org/build2-toolchain/doc/build2-toolchain-intro.xhtml#guide-add-remove-deps)). If you are not familiar with using dependencies in `build2`, [please read this introduction](https://build2.org/build2-toolchain/doc/build2-toolchain-intro.xhtml).
229
230
Please file issues [here](https://github.com/build2-packaging/nlohmann-json) if you experience problems with the packages.

231
232
If you are using [`wsjcpp`](http://wsjcpp.org), you can use the command `wsjcpp install "https://github.com/nlohmann/json:develop"` to get the latest version. Note you can change the branch ":develop" to an existing tag or another branch.

Niels's avatar
Niels committed
233
234
## Examples

235
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)).
236

237
238
### JSON as first-class data type

Niels's avatar
Niels committed
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
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
260
With this library, you could write:
Niels's avatar
Niels committed
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303

```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}
  }}
};
```

304
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
305
306

```cpp
Niels's avatar
Niels committed
307
// a way to express the empty array []
Niels's avatar
Niels committed
308
309
json empty_array_explicit = json::array();

Niels's avatar
Niels committed
310
311
// ways to express the empty object {}
json empty_object_implicit = json({});
Niels's avatar
Niels committed
312
313
314
json empty_object_explicit = json::object();

// a way to express an _array_ of key/value pairs [["currency", "USD"], ["value", 42.99]]
315
json array_not_object = json::array({ {"currency", "USD"}, {"value", 42.99} });
Niels's avatar
Niels committed
316
317
318
319
```

### Serialization / Deserialization

Niels Lohmann's avatar
Niels Lohmann committed
320
321
#### To/from strings

Niels Lohmann's avatar
Niels Lohmann committed
322
You can create a JSON value (deserialization) by appending `_json` to a string literal:
Niels's avatar
Niels committed
323
324
325
326
327

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

Niels's avatar
Niels committed
328
// or even nicer with a raw string literal
Niels's avatar
Niels committed
329
330
331
332
333
334
auto j2 = R"(
  {
    "happy": true,
    "pi": 3.141
  }
)"_json;
335
336
```

Niels Lohmann's avatar
Niels Lohmann committed
337
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
338

339
The above example can also be expressed explicitly using [`json::parse()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a265a473e939184aa42655c9ccdf34e58.html#a265a473e939184aa42655c9ccdf34e58):
340
341
342

```cpp
// parse explicitly
Niels's avatar
Niels committed
343
344
345
auto j3 = json::parse("{ \"happy\": true, \"pi\": 3.141 }");
```

Niels Lohmann's avatar
Niels Lohmann committed
346
You can also get a string representation of a JSON value (serialize):
Niels's avatar
Niels committed
347
348
349

```cpp
// explicit conversion to string
350
std::string s = j.dump();    // {"happy":true,"pi":3.141}
Niels's avatar
Niels committed
351
352
353
354
355
356
357
358
359
360

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

361
362
363
364
365
366
Note the difference between serialization and assignment:

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

367
368
369
370
371
// 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);
372
373
374
375
376

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

// output of original string
377
std::cout << cpp_string << " == " << cpp_string2 << " == " << j_string.get<std::string>() << '\n';
378
379
380
381
// output of serialized value
std::cout << j_string << " == " << serialized_string << std::endl;
```

382
[`.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.
383

384
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.
385

Niels Lohmann's avatar
Niels Lohmann committed
386
387
#### To/from streams (e.g. files, string streams)

Niels's avatar
Niels committed
388
389
390
391
392
You can also use streams to serialize and deserialize:

```cpp
// deserialize from standard input
json j;
Niels's avatar
Niels committed
393
std::cin >> j;
Niels's avatar
Niels committed
394
395
396
397
398
399
400
401

// 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
402
403
404
405
406
407
408
409
410
411
412
413
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
414

Kevin Dixon's avatar
Kevin Dixon committed
415
416
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
417
418
#### Read from iterator range

Niels Lohmann's avatar
Niels Lohmann committed
419
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
420
421

```cpp
422
std::vector<std::uint8_t> v = {'t', 'r', 'u', 'e'};
Niels Lohmann's avatar
Niels Lohmann committed
423
424
425
426
427
428
json j = json::parse(v.begin(), v.end());
```

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

```cpp
429
std::vector<std::uint8_t> v = {'t', 'r', 'u', 'e'};
Niels Lohmann's avatar
Niels Lohmann committed
430
431
432
json j = json::parse(v);
```

433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
#### 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
476
477
### STL-like access

478
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
479
480
481
482
483
484
485
486

```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
487
488
489
// also use emplace_back
j.emplace_back(1.78);

Niels's avatar
Niels committed
490
491
492
493
494
495
// 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
496
for (auto& element : j) {
Niels's avatar
Niels committed
497
498
499
500
  std::cout << element << '\n';
}

// getter/setter
501
const auto tmp = j[0].get<std::string>();
Niels's avatar
Niels committed
502
503
504
j[1] = 42;
bool foo = j.at(2);

Jonathan Lee's avatar
Jonathan Lee committed
505
// comparison
Paul Jurczak's avatar
Paul Jurczak committed
506
j == "[\"foo\", 42, true]"_json;  // true
Jonathan Lee's avatar
Jonathan Lee committed
507

Niels's avatar
Niels committed
508
509
510
511
512
513
// 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
514
515
516
517
518
519
520
521
// 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
522
523
524
525
526
527
// create an object
json o;
o["foo"] = 23;
o["bar"] = false;
o["baz"] = 3.141;

Niels Lohmann's avatar
Niels Lohmann committed
528
529
530
// also use emplace
o.emplace("weather", "sunny");

Niels's avatar
Niels committed
531
532
533
534
535
// 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
536
537
538
539
540
541
542
543
544
545
// 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
546
// find an entry
547
548
549
550
551
if (o.contains("foo")) {
  // there is an entry with key "foo"
}

// or via find and an iterator
Niels's avatar
Niels committed
552
553
554
if (o.find("foo") != o.end()) {
  // there is an entry with key "foo"
}
Niels's avatar
Niels committed
555
556
557
558
559
560
561

// 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
562
563
```

Niels's avatar
Niels committed
564

Niels's avatar
Niels committed
565
566
### Conversion from STL containers

Niels Lohmann's avatar
Niels Lohmann committed
567
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
568
569
570
571
572
573

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

Niels's avatar
Niels committed
574
std::deque<double> c_deque {1.2, 2.3, 3.4, 5.6};
Niels's avatar
Niels committed
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
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"};
599
600
json j_mset(c_mset); // both entries for "one" are used
// maybe ["one", "two", "one", "four"]
Niels's avatar
Niels committed
601
602
603
604
605
606

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
607
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
608
609
610
611

```cpp
std::map<std::string, int> c_map { {"one", 1}, {"two", 2}, {"three", 3} };
json j_map(c_map);
Annihil's avatar
Annihil committed
612
// {"one": 1, "three": 3, "two": 2 }
Niels's avatar
Niels committed
613

Niels's avatar
Niels committed
614
std::unordered_map<const char*, double> c_umap { {"one", 1.2}, {"two", 2.3}, {"three", 3.4} };
Niels's avatar
Niels committed
615
616
617
618
619
620
621
622
623
624
625
626
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
627
628
629
630
631
632
633
634
635
636
637
638
### 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
639
j_original["/baz/1"_json_pointer];
Niels's avatar
Niels committed
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
// "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
660
661
//   { "op": "remove","path": "/hello" },
//   { "op": "add", "path": "/foo", "value": "bar" }
Niels's avatar
Niels committed
662
663
664
// ]
```

665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
### 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
688
j_document.merge_patch(j_patch);
689
690
691
692
693
694
695
// {
//  "a": "z",
//  "c": {
//    "d": "e"
//  }
// }
```
Niels's avatar
Niels committed
696

Niels's avatar
Niels committed
697
698
### Implicit conversions

699
700
701
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
702
You can find more details about this recommendation [here](https://www.github.com/nlohmann/json/issues/958). 
Niels's avatar
Niels committed
703
704

```cpp
Niels's avatar
Niels committed
705
// strings
Niels's avatar
Niels committed
706
707
std::string s1 = "Hello, world!";
json js = s1;
708
709
710
711
712
auto s2 = js.get<std::string>();
// NOT RECOMMENDED
std::string s3 = js;
std::string s4;
s4 = js;
Niels's avatar
Niels committed
713
714
715
716

// Booleans
bool b1 = true;
json jb = b1;
717
718
719
720
721
auto b2 = jb.get<bool>();
// NOT RECOMMENDED
bool b3 = jb;
bool b4;
b4 = jb;
Niels's avatar
Niels committed
722
723
724
725

// numbers
int i = 42;
json jn = i;
726
727
728
729
730
auto f = jn.get<double>();
// NOT RECOMMENDED
double f2 = jb;
double f3;
f3 = jb;
Niels's avatar
Niels committed
731
732
733

// etc.
```
Niels Lohmann's avatar
Niels Lohmann committed
734

Niels Lohmann's avatar
Niels Lohmann committed
735
736
737
738
739
740
741
742
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
743
744
### Arbitrary types conversions

Niels Lohmann's avatar
Niels Lohmann committed
745
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
746
747
748

```cpp
namespace ns {
Niels Lohmann's avatar
Niels Lohmann committed
749
    // a simple struct to model a person
Niels Lohmann's avatar
Niels Lohmann committed
750
751
752
753
754
    struct person {
        std::string name;
        std::string address;
        int age;
    };
Théo DELRIEU's avatar
Théo DELRIEU committed
755
}
Niels Lohmann's avatar
Niels Lohmann committed
756

Niels Lohmann's avatar
Niels Lohmann committed
757
758
ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};

Niels Lohmann's avatar
Niels Lohmann committed
759
// convert to JSON: copy each value into the JSON object
Théo DELRIEU's avatar
Théo DELRIEU committed
760
761
762
763
764
765
766
json j;
j["name"] = p.name;
j["address"] = p.address;
j["age"] = p.age;

// ...

Niels Lohmann's avatar
Niels Lohmann committed
767
// convert from JSON: copy each value from the JSON object
Niels Lohmann's avatar
Niels Lohmann committed
768
769
770
771
772
ns::person p {
    j["name"].get<std::string>(),
    j["address"].get<std::string>(),
    j["age"].get<int>()
};
Théo DELRIEU's avatar
Théo DELRIEU committed
773
774
```

Niels Lohmann's avatar
Niels Lohmann committed
775
It works, but that's quite a lot of boilerplate... Fortunately, there's a better way:
Théo DELRIEU's avatar
Théo DELRIEU committed
776
777

```cpp
Niels Lohmann's avatar
Niels Lohmann committed
778
779
780
781
// create a person
ns::person p {"Ned Flanders", "744 Evergreen Terrace", 60};

// conversion: person -> json
Théo DELRIEU's avatar
Théo DELRIEU committed
782
783
json j = p;

Niels Lohmann's avatar
Niels Lohmann committed
784
785
786
787
std::cout << j << std::endl;
// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}

// conversion: json -> person
788
auto p2 = j.get<ns::person>();
Niels Lohmann's avatar
Niels Lohmann committed
789
790

// that's it
Théo DELRIEU's avatar
Théo DELRIEU committed
791
792
793
794
795
assert(p == p2);
```

#### Basic usage

Niels Lohmann's avatar
Niels Lohmann committed
796
To make this work with one of your types, you only need to provide two functions:
Théo DELRIEU's avatar
Théo DELRIEU committed
797
798
799
800
801

```cpp
using nlohmann::json;

namespace ns {
Niels Lohmann's avatar
Niels Lohmann committed
802
    void to_json(json& j, const person& p) {
Niels Lohmann's avatar
Niels Lohmann committed
803
804
805
        j = json{{"name", p.name}, {"address", p.address}, {"age", p.age}};
    }

Niels Lohmann's avatar
Niels Lohmann committed
806
    void from_json(const json& j, person& p) {
807
808
809
        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
810
    }
Théo DELRIEU's avatar
Théo DELRIEU committed
811
812
813
} // namespace ns
```

Niels Lohmann's avatar
Niels Lohmann committed
814
That's all! When calling the `json` constructor with your type, your custom `to_json` method will be automatically called.
815
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
816
817
818

Some important things:

Niels Lohmann's avatar
Niels Lohmann committed
819
* 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
820
* 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.
821
* 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
822
* 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
823
824
* 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
825
826
827

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

Niels Lohmann's avatar
Niels Lohmann committed
828
This requires a bit more advanced technique. But first, let's see how this conversion mechanism works:
Théo DELRIEU's avatar
Théo DELRIEU committed
829
830

The library uses **JSON Serializers** to convert types to json.
831
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
832
833
834
835
836

It is implemented like this (simplified):

```cpp
template <typename T>
Niels Lohmann's avatar
Niels Lohmann committed
837
838
839
840
841
842
843
844
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
845
846
847
};
```

Niels Lohmann's avatar
Niels Lohmann committed
848
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
849
850
851
852
853
854

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
855
856
857
858
859
860
    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
861
862
              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
863
864
865
866
            }
        }

        static void from_json(const json& j, boost::optional<T>& opt) {
867
868
869
            if (j.is_null()) {
                opt = boost::none;
            } else {
Wilson's avatar
Wilson committed
870
                opt = j.get<T>(); // same as above, but with
Niels Lohmann's avatar
Niels Lohmann committed
871
872
873
874
                                  // adl_serializer<T>::from_json
            }
        }
    };
Théo DELRIEU's avatar
Théo DELRIEU committed
875
876
877
878
879
}
```

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

880
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
881
882
883

```cpp
struct move_only_type {
Niels Lohmann's avatar
Niels Lohmann committed
884
885
886
887
    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
888

Niels Lohmann's avatar
Niels Lohmann committed
889
    int i;
Théo DELRIEU's avatar
Théo DELRIEU committed
890
891
892
};

namespace nlohmann {
Niels Lohmann's avatar
Niels Lohmann committed
893
894
    template <>
    struct adl_serializer<move_only_type> {
Niels Lohmann's avatar
Niels Lohmann committed
895
896
        // note: the return type is no longer 'void', and the method only takes
        // one argument
Niels Lohmann's avatar
Niels Lohmann committed
897
898
899
        static move_only_type from_json(const json& j) {
            return {j.get<int>()};
        }
Wilson's avatar
Wilson committed
900

Niels Lohmann's avatar
Niels Lohmann committed
901
902
903
        // 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
904
905
906
907
        static void to_json(json& j, move_only_type t) {
            j = t.i;
        }
    };
Théo DELRIEU's avatar
Théo DELRIEU committed
908
909
910
911
912
}
```

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

Niels Lohmann's avatar
Niels Lohmann committed
913
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
914
915
916

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

Niels Lohmann's avatar
Niels Lohmann committed
917
918
919
- 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
920
921
922
923

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
924
925
926
927
928
929
930
931
932
933
934
// 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
935

Niels Lohmann's avatar
Niels Lohmann committed
936
937
938
939
940
941
    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
942
943
944
945
946
947
948
949
950
};
```

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
951
952
953
954
955
956
    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
957

Niels Lohmann's avatar
Niels Lohmann committed
958
959
960
961
962
963
    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
964
965
};
```
Niels's avatar
Niels committed
966

967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
### 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
988
})
989
990
```

0xflotus's avatar
0xflotus committed
991
The `NLOHMANN_JSON_SERIALIZE_ENUM()` macro declares a set of `to_json()` / `from_json()` functions for type `TaskState` while avoiding repetition and boilerplate serialization code.
992
993
994
995
996
997
998
999
1000

**Usage:**

```cpp
// enum to JSON as string
json j = TS_STOPPED;
assert(j == "stopped");

// json string to enum