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README.md
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[](https://github.com/nlohmann/json/releases)
[](https://ci.appveyor.com/project/nlohmann/json)
[](https://github.com/nlohmann/json/actions?query=workflow%3AUbuntu)
[](https://github.com/nlohmann/json/actions?query=workflow%3AmacOS)
[](https://github.com/nlohmann/json/actions?query=workflow%3AWindows)
[](https://coveralls.io/github/nlohmann/json?branch=develop)
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[](https://cirrus-ci.com/github/nlohmann/json)
[](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:json)
[](https://wandbox.org/permlink/1mp10JbaANo6FUc7)
[](https://json.nlohmann.me)
[](https://raw.githubusercontent.com/nlohmann/json/master/LICENSE.MIT)
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[](https://github.com/nlohmann/json/releases)
[](https://github.com/nlohmann/json/issues)
[](https://isitmaintained.com/project/nlohmann/json "Average time to resolve an issue")
[](https://bestpractices.coreinfrastructure.org/projects/289)
[](https://scorecard.dev/viewer/?uri=github.com/nlohmann/json)
[](https://github.com/sponsors/nlohmann)
[](https://api.reuse.software/info/github.com/nlohmann/json)
[](https://discord.gg/6mrGXKvX7y)
- [Design goals](#design-goals)
- [Sponsors](#sponsors)
- [Support](#support) ([documentation](https://json.nlohmann.me), [FAQ](https://json.nlohmann.me/home/faq/), [discussions](https://github.com/nlohmann/json/discussions), [API](https://json.nlohmann.me/api/basic_json/), [bug issues](https://github.com/nlohmann/json/issues))
- [Quick reference](#quick-reference)
- [Examples](#examples)
- [Read JSON from a file](#read-json-from-a-file)
- [Creating `json` objects from JSON literals](#creating-json-objects-from-json-literals)
- [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)
- [JSON Merge Patch](#json-merge-patch)
- [Implicit conversions](#implicit-conversions)
- [Conversions to/from arbitrary types](#arbitrary-types-conversions)
- [Specializing enum conversion](#specializing-enum-conversion)
- [Binary formats (BSON, CBOR, MessagePack, UBJSON, and BJData)](#binary-formats-bson-cbor-messagepack-ubjson-and-bjdata)
- [Customers](#customers)
- [Supported compilers](#supported-compilers)
- [Integration](#integration)
- [CMake](#cmake)
- [Package Managers](#package-managers)
- [Pkg-config](#pkg-config)
- [License](#license)
- [Contact](#contact)
- [Thanks](#thanks)
- [Used third-party tools](#used-third-party-tools)
- [Notes](#notes)
- [Execute unit tests](#execute-unit-tests)
## Design goals
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:
- **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.
- **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. The library is also included in all popular [package managers](https://json.nlohmann.me/integration/package_managers/).
- **Serious testing**. Our code is heavily [unit-tested](https://github.com/nlohmann/json/tree/develop/tests/src) and covers [100%](https://coveralls.io/r/nlohmann/json) of the code, including all exceptional behavior. Furthermore, we checked with [Valgrind](https://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). See the [quality assurance](https://json.nlohmann.me/community/quality_assurance) overview documentation.
Other aspects were not so important to us:
- **Memory efficiency**. Each JSON object has an overhead of one pointer (the maximal size of a union) and one enumeration element (1 byte). 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.
- **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.
See the [contribution guidelines](https://github.com/nlohmann/json/blob/master/.github/CONTRIBUTING.md#please-dont) for more information.
## Sponsors
You can sponsor this library at [GitHub Sponsors](https://github.com/sponsors/nlohmann).
### :raising_hand: Priority Sponsor
- [Martti Laine](https://github.com/codeclown)
- [Paul Harrington](https://github.com/phrrngtn)
### :label: Named Sponsors
- [Michael Hartmann](https://github.com/reFX-Mike)
- [Stefan Hagen](https://github.com/sthagen)
- [Steve Sperandeo](https://github.com/homer6)
- [Robert Jefe Lindstädt](https://github.com/eljefedelrodeodeljefe)
- [Steve Wagner](https://github.com/ciroque)
- [Lion Yang](https://github.com/LionNatsu)
### Further support
The development of the library is further supported by JetBrains by providing free access to their IDE tools.
[](https://jb.gg/OpenSourceSupport)
Thanks everyone!
## Support
:question: If you have a **question**, please check if it is already answered in the [**FAQ**](https://json.nlohmann.me/home/faq/) or the [**Q&A**](https://github.com/nlohmann/json/discussions/categories/q-a) section. If not, please [**ask a new question**](https://github.com/nlohmann/json/discussions/new) there.
:books: If you want to **learn more** about how to use the library, check out the rest of the [**README**](#examples), have a look at [**code examples**](https://github.com/nlohmann/json/tree/develop/docs/mkdocs/docs/examples), or browse through the [**help pages**](https://json.nlohmann.me).
:construction: If you want to understand the **API** better, check out the [**API Reference**](https://json.nlohmann.me/api/basic_json/) or have a look at the [quick reference](#quick-reference) below.
:bug: If you found a **bug**, please check the [**FAQ**](https://json.nlohmann.me/home/faq/) if it is a known issue or the result of a design decision. Please also have a look at the [**issue list**](https://github.com/nlohmann/json/issues) before you [**create a new issue**](https://github.com/nlohmann/json/issues/new/choose). Please provide as much information as possible to help us understand and reproduce your issue.
There is also a [**docset**](https://github.com/Kapeli/Dash-User-Contributions/tree/master/docsets/JSON_for_Modern_C%2B%2B) for the documentation browsers [Dash](https://kapeli.com/dash), [Velocity](https://velocity.silverlakesoftware.com), and [Zeal](https://zealdocs.org) that contains the full [documentation](https://json.nlohmann.me) as offline resource.
## Quick reference
- **Constructors** [basic_json](https://json.nlohmann.me/api/basic_json/basic_json), [array](https://json.nlohmann.me/api/basic_json/array), [binary](https://json.nlohmann.me/api/basic_json/binary), [object](https://json.nlohmann.me/api/basic_json/object)
- **Object inspection**: [type](https://json.nlohmann.me/api/basic_json/type), [operator value_t](https://json.nlohmann.me/api/basic_json/operator_value_t), [type_name](https://json.nlohmann.me/api/basic_json/type_name), [is_primitive](https://json.nlohmann.me/api/basic_json/is_primitive), [is_structured](https://json.nlohmann.me/api/basic_json/is_structured), [is_null](https://json.nlohmann.me/api/basic_json/is_null), [is_boolean](https://json.nlohmann.me/api/basic_json/is_boolean), [is_number](https://json.nlohmann.me/api/basic_json/is_number), [is_number_integer](https://json.nlohmann.me/api/basic_json/is_number_integer), [is_number_unsigned](https://json.nlohmann.me/api/basic_json/is_number_unsigned), [is_number_float](https://json.nlohmann.me/api/basic_json/is_number_float), [is_object](https://json.nlohmann.me/api/basic_json/is_object), [is_array](https://json.nlohmann.me/api/basic_json/is_array), [is_string](https://json.nlohmann.me/api/basic_json/is_string), [is_binary](https://json.nlohmann.me/api/basic_json/is_binary), [is_discarded](https://json.nlohmann.me/api/basic_json/is_discarded)
- **Value access**; [get](https://json.nlohmann.me/api/basic_json/get), [get_to](https://json.nlohmann.me/api/basic_json/get_to), [get_ptr](https://json.nlohmann.me/api/basic_json/get_ptr), [get_ref](https://json.nlohmann.me/api/basic_json/get_ref), [operator ValueType](https://json.nlohmann.me/api/basic_json/operator_ValueType), [get_binary](https://json.nlohmann.me/api/basic_json/get_binary)
- **Element access**: [at](https://json.nlohmann.me/api/basic_json/at), [operator[]](https://json.nlohmann.me/api/basic_json/operator[]), [value](https://json.nlohmann.me/api/basic_json/value), [front](https://json.nlohmann.me/api/basic_json/front), [back](https://json.nlohmann.me/api/basic_json/back)
- **Lookup**: [find](https://json.nlohmann.me/api/basic_json/find), [count](https://json.nlohmann.me/api/basic_json/count), [contains](https://json.nlohmann.me/api/basic_json/contains)
- **Iterators**: [begin](https://json.nlohmann.me/api/basic_json/begin), [cbegin](https://json.nlohmann.me/api/basic_json/cbegin), [end](https://json.nlohmann.me/api/basic_json/end), [cend](https://json.nlohmann.me/api/basic_json/cend), [rbegin](https://json.nlohmann.me/api/basic_json/rbegin), [rend](https://json.nlohmann.me/api/basic_json/rend), [crbegin](https://json.nlohmann.me/api/basic_json/crbegin), [crend](https://json.nlohmann.me/api/basic_json/crend), [items](https://json.nlohmann.me/api/basic_json/items)
- **Capacity**: [empty](https://json.nlohmann.me/api/basic_json/empty), [size](https://json.nlohmann.me/api/basic_json/size), [max_size](https://json.nlohmann.me/api/basic_json/max_size)
- **Modifiers**: [clear](https://json.nlohmann.me/api/basic_json/clear), [push_back](https://json.nlohmann.me/api/basic_json/push_back), [operator+=](https://json.nlohmann.me/api/basic_json/operator+=), [emplace_back](https://json.nlohmann.me/api/basic_json/emplace_back), [emplace](https://json.nlohmann.me/api/basic_json/emplace), [erase](https://json.nlohmann.me/api/basic_json/erase), [insert](https://json.nlohmann.me/api/basic_json/insert), [update](https://json.nlohmann.me/api/basic_json/update), [swap](https://json.nlohmann.me/api/basic_json/swap)
- **Lexicographical comparison operators**: [operator==](https://json.nlohmann.me/api/basic_json/operator_eq), [operator!=](https://json.nlohmann.me/api/basic_json/operator_ne), [operator<](https://json.nlohmann.me/api/basic_json/operator_lt), [operator>](https://json.nlohmann.me/api/basic_json/operator_gt), [operator<=](https://json.nlohmann.me/api/basic_json/operator_le), [operator>=](https://json.nlohmann.me/api/basic_json/operator_ge), [operator<=>](https://json.nlohmann.me/api/basic_json/operator_spaceship)
- **Serialization / Dumping**: [dump](https://json.nlohmann.me/api/basic_json/dump)
- **Deserialization / Parsing**: [parse](https://json.nlohmann.me/api/basic_json/parse), [accept](https://json.nlohmann.me/api/basic_json/accept), [sax_parse](https://json.nlohmann.me/api/basic_json/sax_parse)
- **JSON Pointer functions**: [flatten](https://json.nlohmann.me/api/basic_json/flatten), [unflatten](https://json.nlohmann.me/api/basic_json/unflatten)
- **JSON Patch functions**: [patch](https://json.nlohmann.me/api/basic_json/patch), [patch_inplace](https://json.nlohmann.me/api/basic_json/patch_inplace), [diff](https://json.nlohmann.me/api/basic_json/diff), [merge_patch](https://json.nlohmann.me/api/basic_json/merge_patch)
- **Static functions**: [meta](https://json.nlohmann.me/api/basic_json/meta), [get_allocator](https://json.nlohmann.me/api/basic_json/get_allocator)
- **Binary formats**: [from_bjdata](https://json.nlohmann.me/api/basic_json/from_bjdata), [from_bson](https://json.nlohmann.me/api/basic_json/from_bson), [from_cbor](https://json.nlohmann.me/api/basic_json/from_cbor), [from_msgpack](https://json.nlohmann.me/api/basic_json/from_msgpack), [from_ubjson](https://json.nlohmann.me/api/basic_json/from_ubjson), [to_bjdata](https://json.nlohmann.me/api/basic_json/to_bjdata), [to_bson](https://json.nlohmann.me/api/basic_json/to_bson), [to_cbor](https://json.nlohmann.me/api/basic_json/to_cbor), [to_msgpack](https://json.nlohmann.me/api/basic_json/to_msgpack), [to_ubjson](https://json.nlohmann.me/api/basic_json/to_ubjson)
- **Non-member functions**: [operator<<](https://json.nlohmann.me/api/operator_ltlt/), [operator>>](https://json.nlohmann.me/api/operator_gtgt/), [to_string](https://json.nlohmann.me/api/basic_json/to_string)
- **Literals**: [operator""_json](https://json.nlohmann.me/api/operator_literal_json)
- **Helper classes**: [std::hash<basic_json>](https://json.nlohmann.me/api/basic_json/std_hash), [std::swap<basic_json>](https://json.nlohmann.me/api/basic_json/std_swap)
[**Full API documentation**](https://json.nlohmann.me/api/basic_json/)
## Examples
Here are some examples to give you an idea how to use the class.
Beside the examples below, you may want to:
→ Check the [documentation](https://json.nlohmann.me/)\
→ Browse the [standalone example files](https://github.com/nlohmann/json/tree/develop/docs/mkdocs/docs/examples)\
→ Read the full [API Documentation](https://json.nlohmann.me/api/basic_json/) with self-contained examples for every function
### Read JSON from a file
The `json` class provides an API for manipulating a JSON value. To create a `json` object by reading a JSON file:
```cpp
#include <fstream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
// ...
std::ifstream f("example.json");
json data = json::parse(f);
```
### Creating `json` objects from JSON literals
Assume you want to create hard-code this literal JSON value in a file, as a `json` object:
```json
{
"pi": 3.141,
"happy": true
}
```
There are various options:
```cpp
// Using (raw) string literals and json::parse
json ex1 = json::parse(R"(
{
"pi": 3.141,
"happy": true
}
)");
// Using user-defined (raw) string literals
using namespace nlohmann::literals;
json ex2 = R"(
{
"pi": 3.141,
"happy": true
}
)"_json;
// Using initializer lists
json ex3 = {
{"happy", true},
{"pi", 3.141},
};
```
### JSON as first-class data type
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
}
}
```
With this library, you could write:
```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}
}}
};
```
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://json.nlohmann.me/api/basic_json/array/) and [`json::object()`](https://json.nlohmann.me/api/basic_json/object/) will help:
```cpp
// a way to express the empty array []
json empty_array_explicit = json::array();
// ways to express the empty object {}
json empty_object_implicit = json({});
json empty_object_explicit = json::object();
// a way to express an _array_ of key/value pairs [["currency", "USD"], ["value", 42.99]]
json array_not_object = json::array({ {"currency", "USD"}, {"value", 42.99} });
```
### Serialization / Deserialization
#### To/from strings
You can create a JSON value (deserialization) by appending `_json` to a string literal:
```cpp
// create object from string literal
json j = "{ \"happy\": true, \"pi\": 3.141 }"_json;
// or even nicer with a raw string literal
auto j2 = R"(
{
"happy": true,
"pi": 3.141
}
)"_json;
```
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.
The string literal should be brought into scope with `using namespace nlohmann::literals;`
(see [`json::parse()`](https://json.nlohmann.me/api/operator_literal_json/)).
The above example can also be expressed explicitly using [`json::parse()`](https://json.nlohmann.me/api/basic_json/parse/):
```cpp
// parse explicitly
auto j3 = json::parse(R"({"happy": true, "pi": 3.141})");
```
You can also get a string representation of a JSON value (serialize):
```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
// }
```
Note the difference between serialization and assignment:
```cpp
// store a string in a JSON value
json j_string = "this is a string";
// retrieve the string value
auto cpp_string = j_string.template get<std::string>();
// retrieve the string value (alternative when a variable already exists)
std::string cpp_string2;
j_string.get_to(cpp_string2);
// retrieve the serialized value (explicit JSON serialization)
std::string serialized_string = j_string.dump();
// output of original string
std::cout << cpp_string << " == " << cpp_string2 << " == " << j_string.template get<std::string>() << '\n';
// output of serialized value
std::cout << j_string << " == " << serialized_string << std::endl;
```
[`.dump()`](https://json.nlohmann.me/api/basic_json/dump/) returns the originally stored string value.
Note the library only supports UTF-8. When you store strings with different encodings in the library, calling [`dump()`](https://json.nlohmann.me/api/basic_json/dump/) may throw an exception unless `json::error_handler_t::replace` or `json::error_handler_t::ignore` are used as error handlers.
#### To/from streams (e.g. files, string streams)
You can also use streams to serialize and deserialize:
```cpp
// deserialize from standard input
json j;
std::cin >> j;
// 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;
```
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;
```
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.
#### Read from iterator range
You can also parse JSON from an iterator range; that is, from any container accessible by iterators whose `value_type` is an integral type of 1, 2 or 4 bytes, which will be interpreted as UTF-8, UTF-16 and UTF-32 respectively. For instance, a `std::vector<std::uint8_t>`, or a `std::list<std::uint16_t>`:
```cpp
std::vector<std::uint8_t> v = {'t', 'r', 'u', 'e'};
json j = json::parse(v.begin(), v.end());
```
You may leave the iterators for the range [begin, end):
```cpp
std::vector<std::uint8_t> v = {'t', 'r', 'u', 'e'};
json j = json::parse(v);
```
#### Custom data source
Since the parse function accepts arbitrary iterator ranges, you can provide your own data sources by implementing the `LegacyInputIterator` concept.
```cpp
struct MyContainer {
void advance();
const char& get_current();
};
struct MyIterator {
using difference_type = std::ptrdiff_t;
using value_type = char;
using pointer = const char*;
using reference = const char&;
using iterator_category = std::input_iterator_tag;
MyIterator& operator++() {
target->advance();
return *this;
}
bool operator!=(const MyIterator& rhs) const {
return rhs.target != target;
}
reference operator*() const {
return target->get_current();
}
MyContainer* target = nullptr;
};
MyIterator begin(MyContainer& tgt) {
return MyIterator{&tgt};
}
MyIterator end(const MyContainer&) {
return {};
}
void foo() {
MyContainer c;
json j = json::parse(c);
}
```
#### 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 a binary value is parsed; value is passed and can be safely moved away
bool binary(binary_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).
### STL-like access
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.
```cpp
// create an array using push_back
json j;
j.push_back("foo");
j.push_back(1);
j.push_back(true);
// also use emplace_back
j.emplace_back(1.78);
// iterate the array
for (json::iterator it = j.begin(); it != j.end(); ++it) {
std::cout << *it << '\n';
}
// range-based for
for (auto& element : j) {
std::cout << element << '\n';
}
// getter/setter
const auto tmp = j[0].template get<std::string>();
j[1] = 42;
bool foo = j.at(2);
// comparison
j == R"(["foo", 1, true, 1.78])"_json; // true
// other stuff
j.size(); // 4 entries
j.empty(); // false
j.type(); // json::value_t::array
j.clear(); // the array is empty again
// convenience type checkers
j.is_null();
j.is_boolean();
j.is_number();
j.is_object();
j.is_array();
j.is_string();
// create an object
json o;
o["foo"] = 23;
o["bar"] = false;
o["baz"] = 3.141;
// also use emplace
o.emplace("weather", "sunny");
// special iterator member functions for objects
for (json::iterator it = o.begin(); it != o.end(); ++it) {
std::cout << it.key() << " : " << it.value() << "\n";
}
// 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";
}
// find an entry
if (o.contains("foo")) {
// there is an entry with key "foo"
}
// or via find and an iterator
if (o.find("foo") != o.end()) {
// there is an entry with key "foo"
}
// or simpler using count()
int foo_present = o.count("foo"); // 1
int fob_present = o.count("fob"); // 0
// delete an entry
o.erase("foo");
```
### Conversion from STL containers
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.
```cpp
std::vector<int> c_vector {1, 2, 3, 4};
json j_vec(c_vector);
// [1, 2, 3, 4]
std::deque<double> c_deque {1.2, 2.3, 3.4, 5.6};
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"};
json j_mset(c_mset); // both entries for "one" are used
// maybe ["one", "two", "one", "four"]
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"]
```
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.
```cpp
std::map<std::string, int> c_map { {"one", 1}, {"two", 2}, {"three", 3} };
json j_map(c_map);
// {"one": 1, "three": 3, "two": 2 }
std::unordered_map<const char*, double> c_umap { {"one", 1.2}, {"two", 2.3}, {"three", 3.4} };
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}
```
### 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 describing 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)
j_original["/baz/1"_json_pointer];
// "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"] },
// { "op": "remove","path": "/hello" },
// { "op": "add", "path": "/foo", "value": "bar" }
// ]
```
### 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
j_document.merge_patch(j_patch);
// {
// "a": "z",
// "c": {
// "d": "e"
// }
// }
```
### Implicit conversions
Supported types can be implicitly converted to JSON values.
It is recommended to **NOT USE** implicit conversions **FROM** a JSON value.
You can find more details about this recommendation [here](https://www.github.com/nlohmann/json/issues/958).
You can switch off implicit conversions by defining `JSON_USE_IMPLICIT_CONVERSIONS` to `0` before including the `json.hpp` header. When using CMake, you can also achieve this by setting the option `JSON_ImplicitConversions` to `OFF`.
```cpp
// strings
std::string s1 = "Hello, world!";
json js = s1;
auto s2 = js.template get<std::string>();
// NOT RECOMMENDED
std::string s3 = js;
std::string s4;
s4 = js;
// Booleans
bool b1 = true;
json jb = b1;
auto b2 = jb.template get<bool>();
// NOT RECOMMENDED
bool b3 = jb;
bool b4;
b4 = jb;
// numbers
int i = 42;
json jn = i;
auto f = jn.template get<double>();
// NOT RECOMMENDED
double f2 = jb;
double f3;
f3 = jb;
// etc.
```
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"
```
### Arbitrary types conversions
Every type can be serialized in JSON, not just STL containers and scalar types. Usually, you would do something along those lines:
```cpp
namespace ns {
// a simple struct to model a person
struct person {
std::string name;
std::string address;
int age;
};
}
ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};
// convert to JSON: copy each value into the JSON object
json j;
j["name"] = p.name;
j["address"] = p.address;
j["age"] = p.age;
// ...
// convert from JSON: copy each value from the JSON object
ns::person p {
j["name"].template get<std::string>(),
j["address"].template get<std::string>(),
j["age"].template get<int>()
};
```
It works, but that's quite a lot of boilerplate... Fortunately, there's a better way:
```cpp
// create a person
ns::person p {"Ned Flanders", "744 Evergreen Terrace", 60};
// conversion: person -> json
json j = p;
std::cout << j << std::endl;
// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}
// conversion: json -> person
auto p2 = j.template get<ns::person>();
// that's it
assert(p == p2);
```
#### Basic usage
To make this work with one of your types, you only need to provide two functions:
```cpp
using json = nlohmann::json;
namespace ns {
void to_json(json& j, const person& p) {
j = json{{"name", p.name}, {"address", p.address}, {"age", p.age}};
}
void from_json(const json& j, person& p) {
j.at("name").get_to(p.name);
j.at("address").get_to(p.address);
j.at("age").get_to(p.age);
}
} // namespace ns
```
That's all! When calling the `json` constructor with your type, your custom `to_json` method will be automatically called.
Likewise, when calling `template get<your_type>()` or `get_to(your_type&)`, the `from_json` method will be called.
Some important things:
- 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).
- 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.
- When using `template 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.)
- In function `from_json`, use function [`at()`](https://json.nlohmann.me/api/basic_json/at/) 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.
- You do not need to add serializers or deserializers for STL types like `std::vector`: the library already implements these.
#### Simplify your life with macros
If you just want to serialize/deserialize some structs, the `to_json`/`from_json` functions can be a lot of boilerplate. There are [**several macros**](https://json.nlohmann.me/features/arbitrary_types/#simplify-your-life-with-macros) to make your life easier as long as you (1) want to use a JSON object as serialization and (2) want to use the member variable names as object keys in that object.
Which macro to choose depends on whether private member variables need to be accessed, a deserialization is needed, missing values should yield an error or should be replaced by default values, and if derived classes are used. See [this overview to choose the right one for your use case](https://json.nlohmann.me/api/macros/#serializationdeserialization-macros).
##### Example usage of macros
The `to_json`/`from_json` functions for the `person` struct above can be created with [`NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE`](https://json.nlohmann.me/api/macros/nlohmann_define_type_non_intrusive/). In all macros, the first parameter is the name of the class/struct, and all remaining parameters name the members.
```cpp
namespace ns {
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)
}
```
Here is another example with private members, where [`NLOHMANN_DEFINE_TYPE_INTRUSIVE`](https://json.nlohmann.me/api/macros/nlohmann_define_type_intrusive/) is needed:
```cpp
namespace ns {
class address {
private:
std::string street;
int housenumber;
int postcode;
public:
NLOHMANN_DEFINE_TYPE_INTRUSIVE(address, street, housenumber, postcode)
};
}
```
#### How do I convert third-party types?
This requires a bit more advanced technique. But first, let's see how this conversion mechanism works:
The library uses **JSON Serializers** to convert types to json.
The default serializer for `nlohmann::json` is `nlohmann::adl_serializer` (ADL means [Argument-Dependent Lookup](https://en.cppreference.com/w/cpp/language/adl)).
It is implemented like this (simplified):
```cpp
template <typename T>
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
}
};
```
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`...
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 {
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 {
j = *opt; // this will call adl_serializer<T>::to_json which will
// find the free function to_json in T's namespace!
}
}
static void from_json(const json& j, boost::optional<T>& opt) {
if (j.is_null()) {
opt = boost::none;
} else {
opt = j.template get<T>(); // same as above, but with
// adl_serializer<T>::from_json
}
}
};
}
```
#### How can I use `get()` for non-default constructible/non-copyable types?
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:
```cpp
struct move_only_type {
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;
int i;
};
namespace nlohmann {
template <>
struct adl_serializer<move_only_type> {
// note: the return type is no longer 'void', and the method only takes
// one argument
static move_only_type from_json(const json& j) {
return {j.template get<int>()};
}
// 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
static void to_json(json& j, move_only_type t) {
j = t.i;
}
};
}
```
#### Can I write my own serializer? (Advanced use)
Yes. You might want to take a look at [`unit-udt.cpp`](https://github.com/nlohmann/json/blob/develop/tests/src/unit-udt.cpp) in the test suite, to see a few examples.
If you write your own serializer, you'll need to do a few things:
- 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
Here is an example, without simplifications, that only accepts types with a size <= 32, and uses ADL.
```cpp
// 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);
}
template <typename BasicJsonType>
static void from_json(const BasicJsonType& j, T& value) {
// same thing here
using nlohmann::from_json;
from_json(j, value);
}
};
```
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
{
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!