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  1. 44c172f Use a source_set rather than a static_library by Adam Barth · 7 months ago master
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JSON for Modern C++

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Design goals

There are myriads of JSON 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 and you know, what I mean.

  • Trivial integration. Our whole code consists of a single header file 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.

  • Serious testing. Our class is heavily unit-tested and covers 100% of the code, including all exceptional behavior. Furthermore, we checked with Valgrind that there are no memory leaks. To maintain high quality, the project is following the Core Infrastructure Initiative (CII) best practices.

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. We currently implement the parser as naive recursive descent parser with hand coded string handling. It is fast enough, but a LALR-parser may be even faster (but would consist of more files which makes the integration harder).

See the contribution guidelines for more information.


The single required source, file json.hpp is in the src directory or released here. All you need to do is add

#include "json.hpp"

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

to the files you want to use JSON objects. That's it. Do not forget to set the necessary switches to enable C++11 (e.g., -std=c++11 for GCC and Clang).

:beer: If you are using OS X and Homebrew, 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.


Here are some examples to give you an idea how to use the class.

Assume you want to create the JSON object

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

With the JSON class, you could write:

// 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 you want to use. If you want to be explicit or express some edge cases, the functions json::array and json::object will help:

// 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"}), json::array({"value", 42.99}) };

Serialization / Deserialization

You can create an object (deserialization) by appending _json to a string literal:

// 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

// or explicitly
auto j3 = json::parse("{ \"happy\": true, \"pi\": 3.141 }");

You can also get a string representation (serialize):

// 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
// }

You can also use streams to serialize and deserialize:

// 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.

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.

STL-like access

We designed the JSON class to behave just like an STL container. In fact, it satisfies the ReversibleContainer requirement.

// create an array using push_back
json j;

// 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 std::string tmp = j[0];
j[1] = 42;
bool foo =;

// other stuff
j.size();     // 3 entries
j.empty();    // false
j.type();     // json::value_t::array
j.clear();    // the array is empty again

// convenience type checkers

// comparison
j == "[\"foo\", 1, true]"_json;  // true

// create an object
json o;
o["foo"] = 23;
o["bar"] = false;
o["baz"] = 3.141;

// special iterator member functions for objects
for (json::iterator it = o.begin(); it != o.end(); ++it) {
  std::cout << it.key() << " : " << it.value() << "\n";

// find an entry
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

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 types (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 how the elements are ordered in the respective STL container.

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); // only one entry for "one" is used
// maybe ["one", "two", "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 types (see examples above) can be used to 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.

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) as alternative means to address structured values. On top of this, JSON Patch (RFC 6902) allows to describe differences between two JSON values - effectively allowing patch and diff operations known from Unix.

// a JSON value
json j_original = R"({
  "baz": ["one", "two", "three"],
  "foo": "bar"

// access members with a JSON pointer (RFC 6901)
// "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"}

// 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" }
// ]

Implicit conversions

The type of the JSON object is determined automatically by the expression to store. Likewise, the stored value is implicitly converted.

// strings
std::string s1 = "Hello, world!";
json js = s1;
std::string s2 = js;

// Booleans
bool b1 = true;
json jb = b1;
bool b2 = jb;

// numbers
int i = 42;
json jn = i;
double f = jn;

// etc.

You can also explicitly ask for the value:

std::string vs = js.get<std::string>();
bool vb = jb.get<bool>();
int vi = jn.get<int>();

// etc.

Supported compilers

Though it's 2016 already, the support for C++11 is still a bit sparse. Currently, the following compilers are known to work:

  • GCC 4.9 - 6.0 (and possibly later)
  • Clang 3.4 - 3.9 (and possibly later)
  • Microsoft Visual C++ 2015 / Build Tools 14.0.25123.0 (and possibly later)

I would be happy to learn about other compilers/versions.

Please note:

  • GCC 4.8 does not work because of two bugs (55817 and 57824) in the C++11 support. Note there is a pull request to fix some of the issues.

  • Android defaults to using very old compilers and C++ libraries. To fix this, add the following to your This will switch to the LLVM C++ library, the Clang compiler, and enable C++11 and other features disabled by default.

    APP_STL := c++_shared
    APP_CPPFLAGS += -frtti -fexceptions

    The code compiles successfully with Android NDK, Revision 9 - 11 (and possibly later) and CrystaX's Android NDK version 10.

  • For GCC running on MinGW or Android SDK, the error 'to_string' is not a member of 'std' (or similarly, for strtod) may occur. Note this is not an issue with the code, but rather with the compiler itself. On Android, see above to build with a newer environment. For MinGW, please refer to this site and this discussion for information on how to fix this bug. For Android NDK using APP_STL := gnustl_static, please refer to this discussion.

The following compilers are currently used in continuous integration at Travis and AppVeyor:

CompilerOperating SystemVersion String
GCC 4.9.3Ubuntu 14.04.4 LTSg++-4.9 (Ubuntu 4.9.3-8ubuntu2~14.04) 4.9.3
GCC 5.3.0Ubuntu 14.04.4 LTSg++-5 (Ubuntu 5.3.0-3ubuntu1~14.04) 5.3.0 20151204
GCC 6.1.1Ubuntu 14.04.4 LTSg++-6 (Ubuntu 6.1.1-3ubuntu11~14.04.1) 6.1.1 20160511
Clang 3.6.0Ubuntu 14.04.4 LTSclang version 3.6.0 (tags/RELEASE_360/final)
Clang 3.6.1Ubuntu 14.04.4 LTSclang version 3.6.1 (tags/RELEASE_361/final)
Clang 3.6.2Ubuntu 14.04.4 LTSclang version 3.6.2 (tags/RELEASE_362/final)
Clang 3.7.0Ubuntu 14.04.4 LTSclang version 3.7.0 (tags/RELEASE_370/final)
Clang 3.7.1Ubuntu 14.04.4 LTSclang version 3.7.1 (tags/RELEASE_371/final)
Clang 3.8.0Ubuntu 14.04.4 LTSclang version 3.8.0 (tags/RELEASE_380/final)
Clang 3.8.1Ubuntu 14.04.4 LTSclang version 3.8.1 (tags/RELEASE_381/final)
Clang Xcode 6.1Darwin Kernel Version 13.4.0 (OSX 10.9.5)Apple LLVM version 6.0 (clang-600.0.54) (based on LLVM 3.5svn)
Clang Xcode 6.2Darwin Kernel Version 13.4.0 (OSX 10.9.5)Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
Clang Xcode 6.3Darwin Kernel Version 14.3.0 (OSX 10.10.3)Apple LLVM version 6.1.0 (clang-602.0.49) (based on LLVM 3.6.0svn)
Clang Xcode 6.4Darwin Kernel Version 14.3.0 (OSX 10.10.3)Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Clang Xcode 7.1Darwin Kernel Version 14.5.0 (OSX 10.10.5)Apple LLVM version 7.0.0 (clang-700.1.76)
Clang Xcode 7.2Darwin Kernel Version 15.0.0 (OSX 10.10.5)Apple LLVM version 7.0.2 (clang-700.1.81)
Clang Xcode 7.3Darwin Kernel Version 15.0.0 (OSX 10.10.5)Apple LLVM version 7.3.0 (clang-703.0.29)
Clang Xcode 8.0Darwin Kernel Version 15.5.0 (OSX 10.11.5)Apple LLVM version 8.0.0 (clang-800.0.24.1)
Visual Studio 14 2015Windows Server 2012 R2 (x64)Microsoft (R) Build Engine version 14.0.25123.0


The class is licensed under the MIT License:

Copyright © 2013-2016 Niels Lohmann

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.



I deeply appreciate the help of the following people.

  • Teemperor implemented CMake support and lcov integration, realized escape and Unicode handling in the string parser, and fixed the JSON serialization.
  • elliotgoodrich fixed an issue with double deletion in the iterator classes.
  • kirkshoop made the iterators of the class composable to other libraries.
  • wancw fixed a bug that hindered the class to compile with Clang.
  • Tomas Åblad found a bug in the iterator implementation.
  • Joshua C. Randall fixed a bug in the floating-point serialization.
  • Aaron Burghardt implemented code to parse streams incrementally. Furthermore, he greatly improved the parser class by allowing the definition of a filter function to discard undesired elements while parsing.
  • Daniel Kopeček fixed a bug in the compilation with GCC 5.0.
  • Florian Weber fixed a bug in and improved the performance of the comparison operators.
  • Eric Cornelius pointed out a bug in the handling with NaN and infinity values. He also improved the performance of the string escaping.
  • 易思龙 implemented a conversion from anonymous enums.
  • kepkin patiently pushed forward the support for Microsoft Visual studio.
  • gregmarr simplified the implementation of reverse iterators and helped with numerous hints and improvements.
  • Caio Luppi fixed a bug in the Unicode handling.
  • dariomt fixed some typos in the examples.
  • Daniel Frey cleaned up some pointers and implemented exception-safe memory allocation.
  • Colin Hirsch took care of a small namespace issue.
  • Huu Nguyen correct a variable name in the documentation.
  • Silverweed overloaded parse() to accept an rvalue reference.
  • dariomt fixed a subtlety in MSVC type support and implemented the get_ref() function to get a reference to stored values.
  • ZahlGraf added a workaround that allows compilation using Android NDK.
  • whackashoe replaced a function that was marked as unsafe by Visual Studio.
  • 406345 fixed two small warnings.
  • Glen Fernandes noted a potential portability problem in the has_mapped_type function.
  • Corbin Hughes fixed some typos in the contribution guidelines.
  • twelsby fixed the array subscript operator, an issue that failed the MSVC build, and floating-point parsing/dumping. He further added support for unsigned integer numbers and implemented better roundtrip support for parsed numbers.
  • Volker Diels-Grabsch fixed a link in the README file.
  • msm- added support for american fuzzy lop.
  • Annihil fixed an example in the README file.
  • Themercee noted a wrong URL in the README file.
  • Lv Zheng fixed a namespace issue with int64_t and uint64_t.
  • abc100m analyzed the issues with GCC 4.8 and proposed a partial solution.
  • zewt added useful notes to the README file about Android.
  • Róbert Márki added a fix to use move iterators and improved the integration via CMake.
  • Chris Kitching cleaned up the CMake files.
  • Tom Needham fixed a subtle bug with MSVC 2015 which was also proposed by Michael K..
  • Mário Feroldi fixed a small typo.
  • duncanwerner found a really embarrassing performance regression in the 2.0.0 release.
  • Damien fixed one of the last conversion warnings.

Thanks a lot for helping out!


  • The code contains numerous debug assertions which can be switched off by defining the preprocessor macro NDEBUG, see the documentation of assert. In particular, note operator[] implements unchecked access for const objects: If the given key is not present, the behavior is undefined (think of a dereferenced null pointer) and yields an assertion failure if assertions are switched on. If you are not sure whether an element in an object exists, use checked access with the at() function.
  • As the exact type of a number is not defined in the JSON specification, this library tries to choose the best fitting C++ number type automatically. As a result, the type double may be used to store numbers which may yield floating-point exceptions in certain rare situations if floating-point exceptions have been unmasked in the calling code. These exceptions are not caused by the library and need to be fixed in the calling code, such as by re-masking the exceptions prior to calling library functions.

Execute unit tests

To compile and run the tests, you need to execute

$ make check

All tests passed (8905099 assertions in 32 test cases)

For more information, have a look at the file .travis.yml.