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GoogleTest - Google Testing and Mocking Framework



Live at Head

GoogleTest now follows the Abseil Live at Head philosophy. We recommend updating to the latest commit in the main branch as often as possible. We do publish occasional semantic versions, tagged with v${major}.${minor}.${patch} (e.g. v1.15.0).

Documentation Updates

Our documentation is now live on GitHub Pages at https://google.github.io/googletest/. We recommend browsing the documentation on GitHub Pages rather than directly in the repository.

Release 1.15.0

Release 1.15.0 is now available.

The 1.15.x branch requires at least C++14.

Continuous Integration

We use Google's internal systems for continuous integration.

Coming Soon

  • We are planning to take a dependency on Abseil.

Welcome to GoogleTest, Google's C++ test framework!

This repository is a merger of the formerly separate GoogleTest and GoogleMock projects. These were so closely related that it makes sense to maintain and release them together.

Getting Started

See the GoogleTest User's Guide for documentation. We recommend starting with the GoogleTest Primer.

More information about building GoogleTest can be found at googletest/README.md.


  • xUnit test framework:
    Googletest is based on the xUnit testing framework, a popular architecture for unit testing
  • Test discovery:
    Googletest automatically discovers and runs your tests, eliminating the need to manually register your tests
  • Rich set of assertions:
    Googletest provides a variety of assertions, such as equality, inequality, exceptions, and more, making it easy to test your code
  • User-defined assertions:
    You can define your own assertions with Googletest, making it simple to write tests that are specific to your code
  • Death tests:
    Googletest supports death tests, which verify that your code exits in a certain way, making it useful for testing error-handling code
  • Fatal and non-fatal failures:
    You can specify whether a test failure should be treated as fatal or non-fatal with Googletest, allowing tests to continue running even if a failure occurs
  • Value-parameterized tests:
    Googletest supports value-parameterized tests, which run multiple times with different input values, making it useful for testing functions that take different inputs
  • Type-parameterized tests:
    Googletest also supports type-parameterized tests, which run with different data types, making it useful for testing functions that work with different data types
  • Various options for running tests:
    Googletest provides many options for running tests including running individual tests, running tests in a specific order and running tests in parallel

Supported Platforms

GoogleTest follows Google's Foundational C++ Support Policy. See this table for a list of currently supported versions of compilers, platforms, and build tools.

Who Is Using GoogleTest?

In addition to many internal projects at Google, GoogleTest is also used by the following notable projects:

Related Open Source Projects

GTest Runner is a Qt5 based automated test-runner and Graphical User Interface with powerful features for Windows and Linux platforms.

GoogleTest UI is a test runner that runs your test binary, allows you to track its progress via a progress bar, and displays a list of test failures. Clicking on one shows failure text. GoogleTest UI is written in C#.

GTest TAP Listener is an event listener for GoogleTest that implements the TAP protocol for test result output. If your test runner understands TAP, you may find it useful.

gtest-parallel is a test runner that runs tests from your binary in parallel to provide significant speed-up.

GoogleTest Adapter is a VS Code extension allowing to view GoogleTest in a tree view and run/debug your tests.

C++ TestMate is a VS Code extension allowing to view GoogleTest in a tree view and run/debug your tests.

Cornichon is a small Gherkin DSL parser that generates stub code for GoogleTest.

Contributing Changes

Please read CONTRIBUTING.md for details on how to contribute to this project.

Happy testing!

JSON for Modern C++

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'll 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 code is heavily unit-tested and covers 100% of the code, including all exceptional behavior. Furthermore, we checked with Valgrind and the Clang Sanitizers that there are no memory leaks. Google OSS-Fuzz 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.

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 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 for more information.


You can sponsor this library at GitHub Sponsors.

Priority Sponsor

Named Sponsors

Thanks everyone!


If you have a question, please check if it is already answered in the FAQ or the Q&A section. If not, please ask a new question there.

If you want to learn more about how to use the library, check out the rest of the README, have a look at code examples, or browse through the help pages.

If you want to understand the API better, check out the API Reference.

If you found a bug, please check the FAQ if it is a known issue or the result of a design decision. Please also have a look at the issue list before you create a new issue. Please provide as much information as possible to help us understand and reproduce your issue.

There is also a docset for the documentation browsers Dash, Velocity, and Zeal that contains the full documentation as offline resource.


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
Browse the standalone example files

Every API function (documented in the API Documentation) has a corresponding standalone example file. For example, the emplace() function has a matching emplace.cpp example file.

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:

#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:

{ "pi": 3.141, "happy": true}

There are various options:

// Using (raw) string literals and json::parsejson ex1 = json::parse(R"( { "pi": 3.141, "happy": true })");// Using user-defined (raw) string literalsusing namespace nlohmann::literals;json ex2 = R"( { "pi": 3.141, "happy": true })"_json;// Using initializer listsjson 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

{ "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:

// 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 boolj["happy"] = true;// add a string that is stored as std::stringj["name"] = "Niels";// add another null object by passing nullptrj["nothing"] = nullptr;// add an object inside the objectj["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() 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"}, {"value", 42.99} });

Serialization / Deserialization

To/from strings

You can create a JSON value (deserialization) by appending _json to a string literal:

// create object from string literaljson j = "{ \"happy\": true, \"pi\": 3.141 }"_json;// or even nicer with a raw string literalauto 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()).

The above example can also be expressed explicitly using json::parse():

// parse explicitlyauto j3 = json::parse(R"({"happy": true, "pi": 3.141})");

You can also get a string representation of a JSON value (serialize):

// explicit conversion to stringstd::string s = j.dump(); // {"happy":true,"pi":3.141}// serialization with pretty printing// pass in the amount of spaces to indentstd::cout << j.dump(4) << std::endl;// {// "happy": true,// "pi": 3.141// }

Note the difference between serialization and assignment:

// store a string in a JSON valuejson j_string = "this is a string";// retrieve the string valueauto 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 stringstd::cout << cpp_string << " == " << cpp_string2 << " == " << j_string.template get<std::string>() << '\n';// output of serialized valuestd::cout << j_string << " == " << serialized_string << std::endl;

.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() 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:

// deserialize from standard inputjson j;std::cin >> j;// serialize to standard outputstd::cout << j;// the setw manipulator was overloaded to set the indentation for pretty printingstd::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:

// read a JSON filestd::ifstream i("file.json");json j;i >> j;// write prettified JSON to another filestd::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>:

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):

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.

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:

// called when null is parsedbool null();// called when a boolean is parsed; value is passedbool boolean(bool val);// called when a signed or unsigned integer number is parsed; value is passedbool 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 passedbool number_float(number_float_t val, const string_t& s);// called when a string is parsed; value is passed and can be safely moved awaybool string(string_t& val);// called when a binary value is parsed; value is passed and can be safely moved awaybool 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 awaybool key(string_t& val);// called when a parse error occurs; byte position, the last token, and an exception is passedbool 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.

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_backjson j;j.push_back("foo");j.push_back(1);j.push_back(true);// also use emplace_backj.emplace_back(1.78);// iterate the arrayfor (json::iterator it = j.begin(); it != j.end(); ++it) { std::cout << *it << '\n';}// range-based forfor (auto& element : j) { std::cout << element << '\n';}// getter/setterconst auto tmp = j[0].template get<std::string>();j[1] = 42;bool foo = j.at(2);// comparisonj == R"(["foo", 1, true, 1.78])"_json; // true// other stuffj.size(); // 4 entriesj.empty(); // falsej.type(); // json::value_t::arrayj.clear(); // the array is empty again// convenience type checkersj.is_null();j.is_boolean();j.is_number();j.is_object();j.is_array();j.is_string();// create an objectjson o;o["foo"] = 23;o["bar"] = false;o["baz"] = 3.141;// also use emplaceo.emplace("weather", "sunny");// special iterator member functions for objectsfor (json::iterator it = o.begin(); it != o.end(); ++it) { std::cout << it.key() << " : " << it.value() << "\n";}// the same code as range forfor (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 entryif (o.contains("foo")) { // there is an entry with key "foo"}// or via find and an iteratorif (o.find("foo") != o.end()) { // there is an entry with key "foo"}// or simpler using count()int foo_present = o.count("foo"); // 1int fob_present = o.count("fob"); // 0// delete an entryo.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.

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.

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 describing differences between two JSON values - effectively allowing patch and diff operations known from Unix.

// a JSON valuejson 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 patchjson j_result = j_original.patch(j_patch);// {// "baz": "boo",// "hello": ["world"]// }// calculate a JSON patch from two JSON valuesjson::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) 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.

// a JSON valuejson j_document = R"({ "a": "b", "c": { "d": "e", "f": "g" }})"_json;// a patchjson j_patch = R"({ "a":"z", "c": { "f": null }})"_json;// apply the patchj_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. 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.

// stringsstd::string s1 = "Hello, world!";json js = s1;auto s2 = js.template get<std::string>();// NOT RECOMMENDEDstd::string s3 = js;std::string s4;s4 = js;// Booleansbool b1 = true;json jb = b1;auto b2 = jb.template get<bool>();// NOT RECOMMENDEDbool b3 = jb;bool b4;b4 = jb;// numbersint i = 42;json jn = i;auto f = jn.template get<double>();// NOT RECOMMENDEDdouble 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:

char ch = 'A'; // ASCII value 65json j_default = ch; // stores integer number 65json 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:

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 objectjson j;j["name"] = p.name;j["address"] = p.address;j["age"] = p.age;// ...// convert from JSON: copy each value from the JSON objectns::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:

// create a personns::person p {"Ned Flanders", "744 Evergreen Terrace", 60};// conversion: person -> jsonjson j = p;std::cout << j << std::endl;// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}// conversion: json -> personauto p2 = j.template get<ns::person>();// that's itassert(p == p2);

Basic usage

To make this work with one of your types, you only need to provide two functions:

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 for errors that may occur otherwise.
  • When using template get<your_type>(), your_typeMUST be DefaultConstructible. (There is a way to bypass this requirement described later.)
  • In function from_json, use function 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 two 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:

  • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(name, member1, member2, ...) is to be defined inside the namespace of the class/struct to create code for.
  • NLOHMANN_DEFINE_TYPE_INTRUSIVE(name, member1, member2, ...) is to be defined inside the class/struct to create code for. This macro can also access private members.

In both macros, the first parameter is the name of the class/struct, and all remaining parameters name the members.


The to_json/from_json functions for the person struct above can be created with:

namespace ns { NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)}

Here is an example with private members, where NLOHMANN_DEFINE_TYPE_INTRUSIVE is needed:

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).

It is implemented like this (simplified):

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:

// 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. You will need to specialize the adl_serializer as well, but with a special from_json overload:

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 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.

// You should use void as a second template argument// if you don't need compile-time checks on Ttemplate<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:

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! j = value; } 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! }};

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:

// example enum type declarationenum TaskState { TS_STOPPED, TS_RUNNING, TS_COMPLETED, TS_INVALID=-1,};// map TaskState values to JSON as stringsNLOHMANN_JSON_SERIALIZE_ENUM( TaskState, { {TS_INVALID, nullptr}, {TS_STOPPED, "stopped"}, {TS_RUNNING, "running"}, {TS_COMPLETED, "completed"},})

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.


// enum to JSON as stringjson j = TS_STOPPED;assert(j == "stopped");// json string to enumjson j3 = "running";assert(j3.template get<TaskState>() == TS_RUNNING);// undefined json value to enum (where the first map entry above is the default)json jPi = 3.14;assert(jPi.template get<TaskState>() == TS_INVALID );

Just as in Arbitrary Type Conversions above,

  • NLOHMANN_JSON_SERIALIZE_ENUM() MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.
  • It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.

Other Important points:

  • When using template get<ENUM_TYPE>(), undefined JSON values will default to the first pair specified in your map. Select this default pair carefully.
  • If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the map will be returned when converting to or from JSON.

Binary formats (BSON, CBOR, MessagePack, UBJSON, and BJData)

Though JSON is a ubiquitous data format, it is not a very compact format suitable for data exchange, for instance over a network. Hence, the library supports BSON (Binary JSON), CBOR (Concise Binary Object Representation), MessagePack, UBJSON (Universal Binary JSON Specification) and BJData (Binary JData) to efficiently encode JSON values to byte vectors and to decode such vectors.

// create a JSON valuejson j = R"({"compact": true, "schema": 0})"_json;// serialize to BSONstd::vector<std::uint8_t> v_bson = json::to_bson(j);// 0x1B, 0x00, 0x00, 0x00, 0x08, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0x00, 0x01, 0x10, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00// roundtripjson j_from_bson = json::from_bson(v_bson);// serialize to CBORstd::vector<std::uint8_t> v_cbor = json::to_cbor(j);// 0xA2, 0x67, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0xF5, 0x66, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x00// roundtripjson j_from_cbor = json::from_cbor(v_cbor);// serialize to MessagePackstd::vector<std::uint8_t> v_msgpack = json::to_msgpack(j);// 0x82, 0xA7, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0xC3, 0xA6, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x00// roundtripjson j_from_msgpack = json::from_msgpack(v_msgpack);// serialize to UBJSONstd::vector<std::uint8_t> v_ubjson = json::to_ubjson(j);// 0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D// roundtripjson j_from_ubjson = json::from_ubjson(v_ubjson);

The library also supports binary types from BSON, CBOR (byte strings), and MessagePack (bin, ext, fixext). They are stored by default as std::vector<std::uint8_t> to be processed outside the library.

// CBOR byte string with payload 0xCAFEstd::vector<std::uint8_t> v = {0x42, 0xCA, 0xFE};// read valuejson j = json::from_cbor(v);// the JSON value has type binaryj.is_binary(); // true// get reference to stored binary valueauto& binary = j.get_binary();// the binary value has no subtype (CBOR has no binary subtypes)binary.has_subtype(); // false// access std::vector<std::uint8_t> member functionsbinary.size(); // 2binary[0]; // 0xCAbinary[1]; // 0xFE// set subtype to 0x10binary.set_subtype(0x10);// serialize to MessagePackauto cbor = json::to_msgpack(j); // 0xD5 (fixext2), 0x10, 0xCA, 0xFE

Supported compilers

Though it's 2023 already, the support for C++11 is still a bit sparse. Currently, the following compilers are known to work:

  • GCC 4.8 - 12.0 (and possibly later)
  • Clang 3.4 - 15.0 (and possibly later)
  • Apple Clang 9.1 - 13.1 (and possibly later)
  • Intel C++ Compiler 17.0.2 (and possibly later)
  • Nvidia CUDA Compiler 11.0.221 (and possibly later)
  • Microsoft Visual C++ 2015 / Build Tools 14.0.25123.0 (and possibly later)
  • Microsoft Visual C++ 2017 / Build Tools (and possibly later)
  • Microsoft Visual C++ 2019 / Build Tools 16.3.1+1def00d3d (and possibly later)
  • Microsoft Visual C++ 2022 / Build Tools 19.30.30709.0 (and possibly later)

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

Please note:

  • GCC 4.8 has a bug 57824): multiline raw strings cannot be the arguments to macros. Don't use multiline raw strings directly in macros with this compiler.

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

    APP_STL := c++_sharedNDK_TOOLCHAIN_VERSION := clang3.6APP_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 or strtof) 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.

  • Unsupported versions of GCC and Clang are rejected by #error directives. This can be switched off by defining JSON_SKIP_UNSUPPORTED_COMPILER_CHECK. Note that you can expect no support in this case.

The following compilers are currently used in continuous integration at AppVeyor, Cirrus CI, and GitHub Actions:

CompilerOperating SystemCI Provider
Apple Clang 11.0.3 (clang-1103.0.32.62); Xcode 11.7macOS 11.7.1GitHub Actions
Apple Clang 12.0.0 (clang-1200.0.32.29); Xcode 12.4macOS 11.7.1GitHub Actions
Apple Clang 12.0.5 (clang-1205.0.22.11); Xcode 12.5.1macOS 11.7.1GitHub Actions
Apple Clang 13.0.0 (clang-1300.0.29.3); Xcode 13.0macOS 11.7.1GitHub Actions
Apple Clang 13.0.0 (clang-1300.0.29.3); Xcode 13.1macOS 12.6.1GitHub Actions
Apple Clang 13.0.0 (clang-1300.0.29.30); Xcode 13.2.1macOS 12.6.1GitHub Actions
Apple Clang 13.1.6 (clang-1316.; Xcode 13.3.1macOS 12.6.1GitHub Actions
Apple Clang 13.1.6 (clang-1316.; Xcode 13.4.1macOS 12.6.1GitHub Actions
Apple Clang 14.0.0 (clang-1400.0.29.102); Xcode 14.0macOS 12.6.1GitHub Actions
Apple Clang 14.0.0 (clang-1400.0.29.102); Xcode 14.0.1macOS 12.6.1GitHub Actions
Apple Clang 14.0.0 (clang-1400.0.29.202); Xcode 14.1macOS 12.6.1GitHub Actions
Clang 3.5.2Ubuntu 20.04.3 LTSGitHub Actions
Clang 3.6.2Ubuntu 20.04.3 LTSGitHub Actions
Clang 3.7.1Ubuntu 20.04.3 LTSGitHub Actions
Clang 3.8.1Ubuntu 20.04.3 LTSGitHub Actions
Clang 3.9.1Ubuntu 20.04.3 LTSGitHub Actions
Clang 4.0.1Ubuntu 20.04.3 LTSGitHub Actions
Clang 5.0.2Ubuntu 20.04.3 LTSGitHub Actions
Clang 6.0.1Ubuntu 20.04.3 LTSGitHub Actions
Clang 7.0.1Ubuntu 20.04.3 LTSGitHub Actions
Clang 8.0.0Ubuntu 20.04.3 LTSGitHub Actions
Clang 9.0.0Ubuntu 20.04.3 LTSGitHub Actions
Clang 10.0.0Ubuntu 20.04.3 LTSGitHub Actions
Clang 10.0.0 with GNU-like command-lineWindows-10.0.17763GitHub Actions
Clang 11.0.0 with GNU-like command-lineWindows-10.0.17763GitHub Actions
Clang 11.0.0 with MSVC-like command-lineWindows-10.0.17763GitHub Actions
Clang 11.0.0Ubuntu 20.04.3 LTSGitHub Actions
Clang 12.0.0Ubuntu 20.04.3 LTSGitHub Actions
Clang 12.0.0 with GNU-like command-lineWindows-10.0.17763GitHub Actions
Clang 13.0.0Ubuntu 20.04.3 LTSGitHub Actions
Clang 13.0.0 with GNU-like command-lineWindows-10.0.17763GitHub Actions
Clang 14.0.0Ubuntu 20.04.3 LTSGitHub Actions
Clang 14.0.0 with GNU-like command-lineWindows-10.0.17763GitHub Actions
Clang 15.0.0 with GNU-like command-lineWindows-10.0.17763GitHub Actions
Clang 15.0.4Ubuntu 20.04.3 LTSGitHub Actions
Clang 16.0.0 (16.0.0-++20221031071727+500876226c60-1exp120221031071831.439)Ubuntu 20.04.3 LTSGitHub Actions
GCC 4.8.5 (Ubuntu 4.8.5-4ubuntu2)Ubuntu 20.04.3 LTSGitHub Actions
GCC 4.9.4Ubuntu 20.04.3 LTSGitHub Actions
GCC 5.5.0Ubuntu 20.04.3 LTSGitHub Actions
GCC 6.5.0Ubuntu 20.04.3 LTSGitHub Actions
GCC 7.5.0Ubuntu 20.04.3 LTSGitHub Actions
GCC 8.1.0 (i686-posix-dwarf-rev0, Built by MinGW-W64 project)Windows-10.0.17763GitHub Actions
GCC 8.1.0 (x86_64-posix-seh-rev0, Built by MinGW-W64 project)Windows-10.0.17763GitHub Actions
GCC 8.5.0Ubuntu 20.04.3 LTSGitHub Actions
GCC 9.5.0Ubuntu 20.04.3 LTSGitHub Actions
GCC 10.4.0Ubuntu 20.04.3 LTSGitHub Actions
GCC 11.1.0Ubuntu (aarch64)Cirrus CI
GCC 11.3.0Ubuntu 20.04.3 LTSGitHub Actions
GCC 12.2.0Ubuntu 20.04.3 LTSGitHub Actions
GCC 13.0.0 20220605 (experimental)Ubuntu 20.04.3 LTSGitHub Actions
Intel C++ Compiler 2021.5.0.20211109Ubuntu 20.04.3 LTSGitHub Actions
NVCC 11.0.221Ubuntu 20.04.3 LTSGitHub Actions
Visual Studio 14 2015 MSVC 19.0.24241.7 (Build Engine version 14.0.25420.1)Windows-6.3.9600AppVeyor
Visual Studio 15 2017 MSVC 19.16.27035.0 (Build Engine version 15.9.21+g9802d43bc3 for .NET Framework)Windows-10.0.14393AppVeyor
Visual Studio 16 2019 MSVC 19.28.29912.0 (Build Engine version 16.9.0+57a23d249 for .NET Framework)Windows-10.0.17763GitHub Actions
Visual Studio 16 2019 MSVC 19.28.29912.0 (Build Engine version 16.9.0+57a23d249 for .NET Framework)Windows-10.0.17763AppVeyor
Visual Studio 17 2022 MSVC 19.30.30709.0 (Build Engine version 17.0.31804.368 for .NET Framework)Windows-10.0.20348GitHub Actions


json.hpp is the single required file in single_include/nlohmann or released here. You need to add

#include <nlohmann/json.hpp>// for convenienceusing json = nlohmann::json;

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).

You can further use file 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.


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.


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:

# CMakeLists.txtfind_package(nlohmann_json 3.2.0 REQUIRED)...add_library(foo ...)...target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)

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


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:

# 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 "")# 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 "")# Don't use include(nlohmann_json/CMakeLists.txt) since that carries with it# unintended consequences that will break the build. It's generally# 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)
Embedded (FetchContent)

Since CMake v3.11, FetchContent can be used to automatically download a release as a dependency at configure time.


include(FetchContent)FetchContent_Declare(json URL https://github.com/nlohmann/json/releases/download/v3.11.3/json.tar.xz)FetchContent_MakeAvailable(json)target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)

Note: It is recommended to use the URL approach described above which is supported as of version 3.10.0. See https://json.nlohmann.me/integration/cmake/#fetchcontent for more information.

Supporting Both

To allow your project to support either an externally supplied or an embedded JSON library, you can use a pattern akin to the following:

# Top level CMakeLists.txtproject(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 methodtarget_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)
# 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()...

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

Package Managers

If you are using OS X and Homebrew, just type 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. See nlohmann-json for more information.

If you are using the Meson Build System, add this source tree as a meson subproject. You may also use the include.zip published in this project's Releases to reduce the size of the vendored source tree. Alternatively, you can get a wrap file by downloading it from Meson WrapDB, 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() object with a subproject fallback, rather than using the subproject directly.

If you are using Bazel you can simply reference this repository using http_archive or git_repository and depend on @nlohmann_json//:json.

If you are using Conan 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 if you experience problems with the packages.

If you are using Spack to manage your dependencies, you can use the nlohmann-json package. Please see the spack project for any issues regarding the packaging.

If you are using hunter on your project for external dependencies, then you can use the nlohmann_json package. Please see the hunter project for any issues regarding the packaging.

If you are using Buckaroo, you can install this library's module with buckaroo add github.com/buckaroo-pm/nlohmann-json. Please file issues here. There is a demo repo here.

If you are using vcpkg on your project for external dependencies, then you can install the nlohmann-json package with vcpkg install nlohmann-json and follow the then displayed descriptions. Please see the vcpkg project for any issues regarding the packaging.

If you are using cget, you can install the latest development version with cget install nlohmann/json. A specific version can be installed with cget install nlohmann/[email protected]. Also, the multiple header version can be installed by adding the -DJSON_MultipleHeaders=ON flag (i.e., cget install nlohmann/json -DJSON_MultipleHeaders=ON).

If you are using CocoaPods, you can use the library by adding pod "nlohmann_json", '~>3.1.2' to your podfile (see an example). Please file issues here.

If you are using Swift Package Manager, you can use the library by adding a package dependency to this repository. And target dependency as .product(name: "nlohmann-json", package: "json").

If you are using NuGet, you can use the package nlohmann.json. Please check this extensive description on how to use the package. Please file issues here.

If you are using conda, you can use the package nlohmann_json from conda-forge executing conda install -c conda-forge nlohmann_json. Please file issues here.

If you are using MSYS2, you can use the 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 if you experience problems with the packages.

If you are using MacPorts, execute sudo port install nlohmann-json to install the nlohmann-json package.

If you are using build2, you can use the nlohmann-json package from the public repository https://cppget.org or directly from the package's sources repository. In your project's manifest file, just add depends: nlohmann-json (probably with some version constraints). If you are not familiar with using dependencies in build2, please read this introduction. Please file issues here if you experience problems with the packages.

If you are using wsjcpp, 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.

If you are using CPM.cmake, you can check this example. After adding CPM script to your project, implement the following snippet to your CMake:

CPMAddPackage( NAME nlohmann_json GITHUB_REPOSITORY nlohmann/json VERSION 3.9.1)


If you are using bare Makefiles, you can use pkg-config to generate the include flags that point to where the library is installed:

pkg-config nlohmann_json --cflags

Users of the Meson build system will also be able to use a system-wide library, which will be found by pkg-config:

json = dependency('nlohmann_json', required: true)


The class is licensed under the MIT License:

Copyright © 2013-2022 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.


The class contains the UTF-8 Decoder from Bjoern Hoehrmann which is licensed under the MIT License (see above). Copyright © 2008-2009 Björn Hoehrmann[email protected]

The class contains a slightly modified version of the Grisu2 algorithm from Florian Loitsch which is licensed under the MIT License (see above). Copyright © 2009 Florian Loitsch

The class contains a copy of Hedley from Evan Nemerson which is licensed as CC0-1.0.

The class contains parts of Google Abseil which is licensed under the Apache 2.0 License.


If you have questions regarding the library, I would like to invite you to open an issue at GitHub. Please describe your request, problem, or question as detailed as possible, and also mention the version of the library you are using as well as the version of your compiler and operating system. Opening an issue at GitHub allows other users and contributors to this library to collaborate. For instance, I have little experience with MSVC, and most issues in this regard have been solved by a growing community. If you have a look at the closed issues, you will see that we react quite timely in most cases.

Only if your request would contain confidential information, please send me an email. For encrypted messages, please use this key.


Commits by Niels Lohmann and releases are signed with this PGP Key.


I deeply appreciate the help of the following people.

  1. Teemperor implemented CMake support and lcov integration, realized escape and Unicode handling in the string parser, and fixed the JSON serialization.
  2. elliotgoodrich fixed an issue with double deletion in the iterator classes.
  3. kirkshoop made the iterators of the class composable to other libraries.
  4. wancw fixed a bug that hindered the class to compile with Clang.
  5. Tomas Åblad found a bug in the iterator implementation.
  6. Joshua C. Randall fixed a bug in the floating-point serialization.
  7. 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.
  8. Daniel Kopeek fixed a bug in the compilation with GCC 5.0.
  9. Florian Weber fixed a bug in and improved the performance of the comparison operators.
  10. Eric Cornelius pointed out a bug in the handling with NaN and infinity values. He also improved the performance of the string escaping.
  11. implemented a conversion from anonymous enums.
  12. kepkin patiently pushed forward the support for Microsoft Visual studio.
  13. gregmarr simplified the implementation of reverse iterators and helped with numerous hints and improvements. In particular, he pushed forward the implementation of user-defined types.
  14. Caio Luppi fixed a bug in the Unicode handling.
  15. dariomt fixed some typos in the examples.
  16. Daniel Frey cleaned up some pointers and implemented exception-safe memory allocation.
  17. Colin Hirsch took care of a small namespace issue.
  18. Huu Nguyen correct a variable name in the documentation.
  19. Silverweed overloaded parse() to accept an rvalue reference.
  20. dariomt fixed a subtlety in MSVC type support and implemented the get_ref() function to get a reference to stored values.
  21. ZahlGraf added a workaround that allows compilation using Android NDK.
  22. whackashoe replaced a function that was marked as unsafe by Visual Studio.
  23. 406345 fixed two small warnings.
  24. Glen Fernandes noted a potential portability problem in the has_mapped_type function.
  25. Corbin Hughes fixed some typos in the contribution guidelines.
  26. 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.
  27. Volker Diels-Grabsch fixed a link in the README file.
  28. msm- added support for American Fuzzy Lop.
  29. Annihil fixed an example in the README file.
  30. Themercee noted a wrong URL in the README file.
  31. Lv Zheng fixed a namespace issue with int64_t and uint64_t.
  32. abc100m analyzed the issues with GCC 4.8 and proposed a partial solution.
  33. zewt added useful notes to the README file about Android.
  34. Róbert Márki added a fix to use move iterators and improved the integration via CMake.
  35. Chris Kitching cleaned up the CMake files.
  36. Tom Needham fixed a subtle bug with MSVC 2015 which was also proposed by Michael K..
  37. Mário Feroldi fixed a small typo.
  38. duncanwerner found a really embarrassing performance regression in the 2.0.0 release.
  39. Damien fixed one of the last conversion warnings.
  40. Thomas Braun fixed a warning in a test case and adjusted MSVC calls in the CI.
  41. Théo DELRIEU patiently and constructively oversaw the long way toward iterator-range parsing. He also implemented the magic behind the serialization/deserialization of user-defined types and split the single header file into smaller chunks.
  42. Stefan fixed a minor issue in the documentation.
  43. Vasil Dimov fixed the documentation regarding conversions from std::multiset.
  44. ChristophJud overworked the CMake files to ease project inclusion.
  45. Vladimir Petrigo made a SFINAE hack more readable and added Visual Studio 17 to the build matrix.
  46. Denis Andrejew fixed a grammar issue in the README file.
  47. Pierre-Antoine Lacaze found a subtle bug in the dump() function.
  48. TurpentineDistillery pointed to std::locale::classic() to avoid too much locale joggling, found some nice performance improvements in the parser, improved the benchmarking code, and realized locale-independent number parsing and printing.
  49. cgzones had an idea how to fix the Coverity scan.
  50. Jared Grubb silenced a nasty documentation warning.
  51. Yixin Zhang fixed an integer overflow check.
  52. Bosswestfalen merged two iterator classes into a smaller one.
  53. Daniel599 helped to get Travis execute the tests with Clang's sanitizers.
  54. Jonathan Lee fixed an example in the README file.
  55. gnzlbg supported the implementation of user-defined types.
  56. Alexej Harm helped to get the user-defined types working with Visual Studio.
  57. Jared Grubb supported the implementation of user-defined types.
  58. EnricoBilla noted a typo in an example.
  59. Martin Hoeovský found a way for a 2x speedup for the compilation time of the test suite.
  60. ukhegg found proposed an improvement for the examples section.
  61. rswanson-ihi noted a typo in the README.
  62. Mihai Stan fixed a bug in the comparison with nullptrs.
  63. Tushar Maheshwari added cotire support to speed up the compilation.
  64. TedLyngmo noted a typo in the README, removed unnecessary bit arithmetic, and fixed some -Weffc++ warnings.
  65. Krzysztof Wo made exceptions more visible.
  66. ftillier fixed a compiler warning.
  67. tinloaf made sure all pushed warnings are properly popped.
  68. Fytch found a bug in the documentation.
  69. Jay Sistar implemented a Meson build description.
  70. Henry Lee fixed a warning in ICC and improved the iterator implementation.
  71. Vincent Thiery maintains a package for the Conan package manager.
  72. Steffen fixed a potential issue with MSVC and std::min.
  73. Mike Tzou fixed some typos.
  74. amrcode noted a misleading documentation about comparison of floats.
  75. Oleg Endo reduced the memory consumption by replacing <iostream> with <iosfwd>.
  76. dan-42 cleaned up the CMake files to simplify including/reusing of the library.
  77. Nikita Ofitserov allowed for moving values from initializer lists.
  78. Greg Hurrell fixed a typo.
  79. Dmitry Kukovinets fixed a typo.
  80. kbthomp1 fixed an issue related to the Intel OSX compiler.
  81. Markus Werle fixed a typo.
  82. WebProdPP fixed a subtle error in a precondition check.
  83. Alex noted an error in a code sample.
  84. Tom de Geus reported some warnings with ICC and helped to fix them.
  85. Perry Kundert simplified reading from input streams.
  86. Sonu Lohani fixed a small compilation error.
  87. Jamie Seward fixed all MSVC warnings.
  88. Nate Vargas added a Doxygen tag file.
  89. pvleuven helped to fix a warning in ICC.
  90. Pavel helped to fix some warnings in MSVC.
  91. Jamie Seward avoided unnecessary string copies in find() and count().
  92. Mitja fixed some typos.
  93. Jorrit Wronski updated the Hunter package links.
  94. Matthias Möller added a .natvis for the MSVC debug view.
  95. bogemic fixed some C++17 deprecation warnings.
  96. Eren Okka fixed some MSVC warnings.
  97. abolz integrated the Grisu2 algorithm for proper floating-point formatting, allowing more roundtrip checks to succeed.
  98. Vadim Evard fixed a Markdown issue in the README.
  99. zerodefect fixed a compiler warning.
  100. Kert allowed to template the string type in the serialization and added the possibility to override the exceptional behavior.
  101. mark-99 helped fixing an ICC error.
  102. Patrik Huber fixed links in the README file.
  103. johnfb found a bug in the implementation of CBOR's indefinite length strings.
  104. Paul Fultz II added a note on the cget package manager.
  105. Wilson Lin made the integration section of the README more concise.
  106. RalfBielig detected and fixed a memory leak in the parser callback.
  107. agrianius allowed to dump JSON to an alternative string type.
  108. Kevin Tonon overworked the C++11 compiler checks in CMake.
  109. Axel Huebl simplified a CMake check and added support for the Spack package manager.
  110. Carlos O'Ryan fixed a typo.
  111. James Upjohn fixed a version number in the compilers section.
  112. Chuck Atkins adjusted the CMake files to the CMake packaging guidelines and provided documentation for the CMake integration.
  113. Jan Schöppach fixed a typo.
  114. martin-mfg fixed a typo.
  115. Matthias Möller removed the dependency from std::stringstream.
  116. agrianius added code to use alternative string implementations.
  117. Daniel599 allowed to use more algorithms with the items() function.
  118. Julius Rakow fixed the Meson include directory and fixed the links to cppreference.com.
  119. Sonu Lohani fixed the compilation with MSVC 2015 in debug mode.
  120. grembo fixed the test suite and re-enabled several test cases.
  121. Hyeon Kim introduced the macro JSON_INTERNAL_CATCH to control the exception handling inside the library.
  122. thyu fixed a compiler warning.
  123. David Guthrie fixed a subtle compilation error with Clang 3.4.2.
  124. Dennis Fischer allowed to call find_package without installing the library.
  125. Hyeon Kim fixed an issue with a double macro definition.
  126. Ben Berman made some error messages more understandable.
  127. zakalibit fixed a compilation problem with the Intel C++ compiler.
  128. mandreyel fixed a compilation problem.
  129. Kostiantyn Ponomarenko added version and license information to the Meson build file.
  130. Henry Schreiner added support for GCC 4.8.
  131. knilch made sure the test suite does not stall when run in the wrong directory.
  132. Antonio Borondo fixed an MSVC 2017 warning.
  133. Dan Gendreau implemented the NLOHMANN_JSON_SERIALIZE_ENUM macro to quickly define an enum/JSON mapping.
  134. efp added line and column information to parse errors.
  135. julian-becker added BSON support.
  136. Pratik Chowdhury added support for structured bindings.
  137. David Avedissian added support for Clang 5.0.1 (PS4 version).
  138. Jonathan Dumaresq implemented an input adapter to read from FILE*.
  139. kjpus fixed a link in the documentation.
  140. Manvendra Singh fixed a typo in the documentation.
  141. ziggurat29 fixed an MSVC warning.
  142. Sylvain Corlay added code to avoid an issue with MSVC.
  143. mefyl fixed a bug when JSON was parsed from an input stream.
  144. Millian Poquet allowed to install the library via Meson.
  145. Michael Behrns-Miller found an issue with a missing namespace.
  146. Nasztanovics Ferenc fixed a compilation issue with libc 2.12.
  147. Andreas Schwab fixed the endian conversion.
  148. Mark-Dunning fixed a warning in MSVC.
  149. Gareth Sylvester-Bradley added operator/ for JSON Pointers.
  150. John-Mark noted a missing header.
  151. Vitaly Zaitsev fixed compilation with GCC 9.0.
  152. Laurent Stacul fixed compilation with GCC 9.0.
  153. Ivor Wanders helped to reduce the CMake requirement to version 3.1.
  154. njlr updated the Buckaroo instructions.
  155. Lion fixed a compilation issue with GCC 7 on CentOS.
  156. Isaac Nickaein improved the integer serialization performance and implemented the contains() function.
  157. past-due suppressed an unfixable warning.
  158. Elvis Oric improved Meson support.
  159. Matj Plch fixed an example in the README.
  160. Mark Beckwith fixed a typo.
  161. scinart fixed bug in the serializer.
  162. Patrick Boettcher implemented push_back() and pop_back() for JSON Pointers.
  163. Bruno Oliveira added support for Conda.
  164. Michele Caini fixed links in the README.
  165. Hani documented how to install the library with NuGet.
  166. Mark Beckwith fixed a typo.
  167. yann-morin-1998 helped to reduce the CMake requirement to version 3.1.
  168. Konstantin Podsvirov maintains a package for the MSYS2 software distro.
  169. remyabel added GNUInstallDirs to the CMake files.
  170. Taylor Howard fixed a unit test.
  171. Gabe Ron implemented the to_string method.
  172. Watal M. Iwasaki fixed a Clang warning.
  173. Viktor Kirilov switched the unit tests from Catch to doctest
  174. Juncheng E fixed a typo.
  175. tete17 fixed a bug in the contains function.
  176. Xav83 fixed some cppcheck warnings.
  177. 0xflotus fixed some typos.
  178. Christian Deneke added a const version of json_pointer::back.
  179. Julien Hamaide made the items() function work with custom string types.
  180. Evan Nemerson updated fixed a bug in Hedley and updated this library accordingly.
  181. Florian Pigorsch fixed a lot of typos.
  182. Camille Bégué fixed an issue in the conversion from std::pair and std::tuple to json.
  183. Anthony VH fixed a compile error in an enum deserialization.
  184. Yuriy Vountesmery noted a subtle bug in a preprocessor check.
  185. Chen fixed numerous issues in the library.
  186. Antony Kellermann added a CI step for GCC 10.1.
  187. Alex fixed an MSVC warning.
  188. Rainer proposed an improvement in the floating-point serialization in CBOR.
  189. Francois Chabot made performance improvements in the input adapters.
  190. Arthur Sonzogni documented how the library can be included via FetchContent.
  191. Rimas Miseviius fixed an error message.
  192. Alexander Myasnikov fixed some examples and a link in the README.
  193. Hubert Chathi made CMake's version config file architecture-independent.
  194. OmnipotentEntity implemented the binary values for CBOR, MessagePack, BSON, and UBJSON.
  195. ArtemSarmini fixed a compilation issue with GCC 10 and fixed a leak.
  196. Evgenii Sopov integrated the library to the wsjcpp package manager.
  197. Sergey Linev fixed a compiler warning.
  198. Miguel Magalhães fixed the year in the copyright.
  199. Gareth Sylvester-Bradley fixed a compilation issue with MSVC.
  200. Alexander weej Jones fixed an example in the README.
  201. Antoine Cur fixed some typos in the documentation.
  202. jothepro updated links to the Hunter package.
  203. Dave Lee fixed link in the README.
  204. Joël Lamotte added instruction for using Build2's package manager.
  205. Paul Jurczak fixed an example in the README.
  206. Sonu Lohani fixed a warning.
  207. Carlos Gomes Martinho updated the Conan package source.
  208. Konstantin Podsvirov fixed the MSYS2 package documentation.
  209. Tridacnid improved the CMake tests.
  210. Michael fixed MSVC warnings.
  211. Quentin Barbarat fixed an example in the documentation.
  212. XyFreak fixed a compiler warning.
  213. TotalCaesar659 fixed links in the README.
  214. Tanuj Garg improved the fuzzer coverage for UBSAN input.
  215. AODQ fixed a compiler warning.
  216. jwittbrodt made NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE inline.
  217. pfeatherstone improved the upper bound of arguments of the NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE/NLOHMANN_DEFINE_TYPE_INTRUSIVE macros.
  218. Jan Procházka fixed a bug in the CBOR parser for binary and string values.
  219. T0b1-iOS fixed a bug in the new hash implementation.
  220. Matthew Bauer adjusted the CBOR writer to create tags for binary subtypes.
  221. gatopeich implemented an ordered map container for nlohmann::ordered_json.
  222. Érico Nogueira Rolim added support for pkg-config.
  223. KonanM proposed an implementation for the NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE/NLOHMANN_DEFINE_TYPE_INTRUSIVE macros.
  224. Guillaume Racicot implemented string_view support and allowed C++20 support.
  225. Alex Reinking improved CMake support for FetchContent.
  226. Hannes Domani provided a GDB pretty printer.
  227. Lars Wirzenius reviewed the README file.
  228. Jun Jie fixed a compiler path in the CMake scripts.
  229. Ronak Buch fixed typos in the documentation.
  230. Alexander Karzhenkov fixed a move constructor and the Travis builds.
  231. Leonardo Lima added CPM.Cmake support.
  232. Joseph Blackman fixed a warning.
  233. Yaroslav updated doctest and implemented unit tests.
  234. Martin Stump fixed a bug in the CMake files.
  235. Jaakko Moisio fixed a bug in the input adapters.
  236. bl-ue fixed some Markdown issues in the README file.
  237. William A. Wieselquist fixed an example from the README.
  238. abbaswasim fixed an example from the README.
  239. Remy Jette fixed a warning.
  240. Fraser fixed the documentation.
  241. Ben Beasley updated doctest.
  242. Doron Behar fixed pkg-config.pc.
  243. raduteo fixed a warning.
  244. David Pfahler added the possibility to compile the library without I/O support.
  245. Morten Fyhn Amundsen fixed a typo.
  246. jpl-mac allowed to treat the library as a system header in CMake.
  247. Jason Dsouza fixed the indentation of the CMake file.
  248. offa added a link to Conan Center to the documentation.
  249. TotalCaesar659 updated the links in the documentation to use HTTPS.
  250. Rafail Giavrimis fixed the Google Benchmark default branch.
  251. Louis Dionne fixed a conversion operator.
  252. justanotheranonymoususer made the examples in the README more consistent.
  253. Finkman suppressed some -Wfloat-equal warnings.
  254. Ferry Huberts fixed -Wswitch-enum warnings.
  255. Arseniy Terekhin made the GDB pretty-printer robust against unset variable names.
  256. Amir Masoud Abdol updated the Homebrew command as nlohmann/json is now in homebrew-core.
  257. Hallot fixed some -Wextra-semi-stmt warnings.
  258. Giovanni Cerretani fixed -Wunused warnings on JSON_DIAGNOSTICS.
  259. Bogdan Popescu hosts the docset for offline documentation viewers.
  260. Carl Smedstad fixed an assertion error when using JSON_DIAGNOSTICS.
  261. miikka75 provided an important fix to compile C++17 code with Clang 9.
  262. Maarten Becker fixed a warning for shadowed variables.
  263. Cristi Vîjdea fixed typos in the operator[] documentation.
  264. Alex Beregszaszi fixed spelling mistakes in comments.
  265. Dirk Stolle fixed typos in documentation.
  266. Daniel Albuschat corrected the parameter name in the parse documentation.
  267. Prince Mendiratta fixed a link to the FAQ.
  268. Florian Albrechtskirchinger implemented std::string_view support for object keys and made dozens of other improvements.
  269. Qianqian Fang implemented the Binary JData (BJData) format.
  271. DarkZeros adjusted to code to not clash with Arduino defines.
  272. flagarde fixed the output of meta() for MSVC.
  273. Giovanni Cerretani fixed a check for std::filesystem.
  274. Dimitris Apostolou fixed a typo.
  275. Ferry Huberts fixed a typo.
  276. Michael Nosthoff fixed a typo.
  277. JungHoon Lee fixed a typo.
  278. Faruk D. fixed the CITATION.CFF file.
  279. Andrea Cocito added a clarification on macro usage to the documentation.
  280. Krzysiek Karbowiak refactored the tests to use CHECK_THROWS_WITH_AS.
  281. Chaoqi Zhang fixed a typo.
  282. ivanovmp fixed a whitespace error.
  283. KsaNL fixed a build error when including <windows.h>.
  284. Andrea Pappacoda moved .pc and .cmake files to share directory.
  285. Wolf Vollprecht added the patch_inplace function.
  286. Jake Zimmerman highlighted common usage patterns in the README file.
  287. NN added the Visual Studio output directory to .gitignore.
  288. Romain Reignier improved the performance the vector output adapter.
  289. Mike fixed the std::iterator_traits.
  290. Richard Hozák added macro JSON_NO_ENUM to disable default enum conversions.
  291. vakokako fixed tests when compiling with C++20.
  292. Alexander weej Jones fixed an example in the README.
  293. Eli Schwartz added more files to the include.zip archive.
  294. Kevin Lu fixed a compilation issue when typedefs with certain names were present.
  295. Trevor Hickey improved the description of an example.
  296. Jef LeCompte updated the year in the README file.
  297. Alexandre Hamez fixed a warning.
  298. Maninderpal Badhan fixed a typo.
  299. kevin-- added a note to an example in the README file.
  300. I fixed a typo.
  301. Gregorio Litenstein fixed the Clang detection.
  302. Andreas Smas added a Doozer badge.
  303. WanCW fixed the string conversion with Clang.
  304. zhaohuaxishi fixed a Doxygen error.
  305. emvivre removed an invalid parameter from CMake.
  306. Tobias Hermann fixed a link in the README file.
  307. Michael fixed a warning.
  308. Ryan Mulder added ensure_ascii to the dump function.
  309. Muri Nicanor fixed the sed discovery in the Makefile.
  310. David Avedissian implemented SFINAE-friendly iterator_traits.
  311. AQNOUCH Mohammed fixed a typo in the README.
  312. Gareth Sylvester-Bradley added operator/= and operator/ to construct JSON pointers.
  313. Michael Macnair added support for afl-fuzz testing.
  314. Berkus Decker fixed a typo in the README.
  315. Illia Polishchuk improved the CMake testing.
  316. Ikko Ashimine fixed a typo.
  317. Raphael Grimm added the possibility to define a custom base class.
  318. tocic fixed typos in the documentation.
  319. Vertexwahn added Bazel build support.
  320. Dirk Stolle fixed typos in the documentation.
  321. DavidKorczynski added a CIFuzz CI GitHub action.
  322. Finkman fixed the debug pretty-printer.
  323. Florian Segginger bumped the years in the README.
  324. haadfida cleaned up the badges of used services.
  325. Arsen Arsenovi fixed a build error.
  326. theevilone45 fixed a typo in a CMake file.
  327. Sergei Trofimovich fixed the custom allocator support.
  328. Joyce fixed some security issues in the GitHub workflows.
  329. Nicolas Jakob add vcpkg version badge.
  330. Tomerkm added tests.
  331. No. fixed the use of get<> calls.
  332. taro fixed a typo in the CODEOWNERS file.
  333. Ikko Eltociear Ashimine fixed a typo.
  334. Felix Yan fixed a typo in the README.
  335. HO-COOH fixed a parentheses in the documentation.
  336. Ivor Wanders fixed the examples to catch exception by const&.
  337. miny1233 fixed a parentheses in the documentation.
  338. tomalakgeretkal fixed a compilation error.
  339. alferov fixed a compilation error.
  340. Craig Scott fixed a deprecation warning in CMake.
  341. Vyacheslav Zhdanovskiy added macros for serialization-only types.
  342. Mathieu Westphal fixed typos.
  343. scribam fixed the MinGW workflow.
  344. Aleksei Sapitskii added support for Apple's Swift Package Manager.
  345. Benjamin Buch fixed the installation path in CMake.
  346. Colby Haskell clarified the parse error message in case a file cannot be opened.

Thanks a lot for helping out! Please let me know if I forgot someone.

Used third-party tools

The library itself consists of a single header file licensed under the MIT license. However, it is built, tested, documented, and whatnot using a lot of third-party tools and services. Thanks a lot!

Projects using JSON for Modern C++

The library is currently used in Apple macOS Sierra-Monterey and iOS 10-15. I am not sure what they are using the library for, but I am happy that it runs on so many devices.


Character encoding

The library supports Unicode input as follows:

  • Only UTF-8 encoded input is supported which is the default encoding for JSON according to RFC 8259.
  • std::u16string and std::u32string can be parsed, assuming UTF-16 and UTF-32 encoding, respectively. These encodings are not supported when reading from files or other input containers.
  • Other encodings such as Latin-1 or ISO 8859-1 are not supported and will yield parse or serialization errors.
  • Unicode noncharacters will not be replaced by the library.
  • Invalid surrogates (e.g., incomplete pairs such as \uDEAD) will yield parse errors.
  • The strings stored in the library are UTF-8 encoded. When using the default string type (std::string), note that its length/size functions return the number of stored bytes rather than the number of characters or glyphs.
  • When you store strings with different encodings in the library, calling dump() may throw an exception unless json::error_handler_t::replace or json::error_handler_t::ignore are used as error handlers.
  • To store wide strings (e.g., std::wstring), you need to convert them to a UTF-8 encoded std::string before, see an example.

Comments in JSON

This library does not support comments by default. It does so for three reasons:

  1. Comments are not part of the JSON specification. You may argue that // or /* */ are allowed in JavaScript, but JSON is not JavaScript.

  2. This was not an oversight: Douglas Crockford wrote on this in May 2012:

    I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.

    Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.

  3. It is dangerous for interoperability if some libraries would add comment support while others don't. Please check The Harmful Consequences of the Robustness Principle on this.

However, you can pass set parameter ignore_comments to true in the parse function to ignore // or /* */ comments. Comments will then be treated as whitespace.

Order of object keys

By default, the library does not preserve the insertion order of object elements. This is standards-compliant, as the JSON standard defines objects as "an unordered collection of zero or more name/value pairs".

If you do want to preserve the insertion order, you can try the type nlohmann::ordered_json. Alternatively, you can use a more sophisticated ordered map like tsl::ordered_map (integration) or nlohmann::fifo_map (integration).

Memory Release

We checked with Valgrind and the Address Sanitizer (ASAN) that there are no memory leaks.

If you find that a parsing program with this library does not release memory, please consider the following case, and it may be unrelated to this library.

Your program is compiled with glibc. There is a tunable threshold that glibc uses to decide whether to actually return memory to the system or whether to cache it for later reuse. If in your program you make lots of small allocations and those small allocations are not a contiguous block and are presumably below the threshold, then they will not get returned to the OS. Here is a related issue #1924.

Further notes

  • 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. Furthermore, you can define JSON_ASSERT(x) to replace calls to assert(x).
  • As the exact number type 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.
  • The code can be compiled without C++ runtime type identification features; that is, you can use the -fno-rtti compiler flag.
  • Exceptions are used widely within the library. They can, however, be switched off with either using the compiler flag -fno-exceptions or by defining the symbol JSON_NOEXCEPTION. In this case, exceptions are replaced by abort() calls. You can further control this behavior by defining JSON_THROW_USER (overriding throw), JSON_TRY_USER (overriding try), and JSON_CATCH_USER (overriding catch). Note that JSON_THROW_USER should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield undefined behavior. Note the explanatory what() string of exceptions is not available for MSVC if exceptions are disabled, see #2824.

Execute unit tests

To compile and run the tests, you need to execute

$ mkdir build$ cd build$ cmake .. -DJSON_BuildTests=On$ cmake --build .$ ctest --output-on-failure

Note that during the ctest stage, several JSON test files are downloaded from an external repository. If policies forbid downloading artifacts during testing, you can download the files yourself and pass the directory with the test files via -DJSON_TestDataDirectory=path to CMake. Then, no Internet connectivity is required. See issue #2189 for more information.

If the test suite is not found, several test suites will fail like this:

===============================================================================json/tests/src/make_test_data_available.hpp:21:TEST CASE: check test suite is downloadedjson/tests/src/make_test_data_available.hpp:23: FATAL ERROR: REQUIRE( utils::check_testsuite_downloaded() ) is NOT correct! values: REQUIRE( false ) logged: Test data not found in 'json/cmake-build-debug/json_test_data'. Please execute target 'download_test_data' before running this test suite. See <https://github.com/nlohmann/json#execute-unit-tests> for more information.===============================================================================

In case you have downloaded the library rather than checked out the code via Git, test cmake_fetch_content_configure will fail. Please execute ctest -LE git_required to skip these tests. See issue #2189 for more information.

Some tests change the installed files and hence make the whole process not reproducible. Please execute ctest -LE not_reproducible to skip these tests. See issue #2324 for more information.

Note you need to call cmake -LE "not_reproducible|git_required" to exclude both labels. See issue #2596 for more information.

As Intel compilers use unsafe floating point optimization by default, the unit tests may fail. Use flag /fp:precise then.

Open Source Routing Machine - C++ backend

Open Source Routing Machine

osrm-backend CICodecovDiscord

High performance routing engine written in C++ designed to run on OpenStreetMap data.

The following services are available via HTTP API, C++ library interface and NodeJs wrapper:

  • Nearest - Snaps coordinates to the street network and returns the nearest matches
  • Route - Finds the fastest route between coordinates
  • Table - Computes the duration or distances of the fastest route between all pairs of supplied coordinates
  • Match - Snaps noisy GPS traces to the road network in the most plausible way
  • Trip - Solves the Traveling Salesman Problem using a greedy heuristic
  • Tile - Generates Mapbox Vector Tiles with internal routing metadata

To quickly try OSRM use our demo server which comes with both the backend and a frontend on top.

For a quick introduction about how the road network is represented in OpenStreetMap and how to map specific road network features have a look at the OSM wiki on routing or this guide about mapping for navigation.

Related Project-OSRM repositories:


Full documentation


  • Discord: join
  • IRC: irc.oftc.net, channel: #osrm (Webchat)
  • Mailinglist: https://lists.openstreetmap.org/listinfo/osrm-talk

Quick Start

The easiest and quickest way to setup your own routing engine is to use Docker images we provide.

There are two pre-processing pipelines available:

  • Contraction Hierarchies (CH)
  • Multi-Level Dijkstra (MLD)

we recommend using MLD by default except for special use-cases such as very large distance matrices where CH is still a better fit for the time being. In the following we explain the MLD pipeline. If you want to use the CH pipeline instead replace osrm-partition and osrm-customize with a single osrm-contract and change the algorithm option for osrm-routed to --algorithm ch.

Using Docker

We base our Docker images (backend, frontend) on Debian and make sure they are as lightweight as possible. Older backend versions can be found on Docker Hub.

Download OpenStreetMap extracts for example from Geofabrik

wget http://download.geofabrik.de/europe/germany/berlin-latest.osm.pbf

Pre-process the extract with the car profile and start a routing engine HTTP server on port 5000

docker run -t -v "${PWD}:/data" ghcr.io/project-osrm/osrm-backend osrm-extract -p /opt/car.lua /data/berlin-latest.osm.pbf || echo "osrm-extract failed"

The flag -v "${PWD}:/data" creates the directory /data inside the docker container and makes the current working directory "${PWD}" available there. The file /data/berlin-latest.osm.pbf inside the container is referring to "${PWD}/berlin-latest.osm.pbf" on the host.

docker run -t -v "${PWD}:/data" ghcr.io/project-osrm/osrm-backend osrm-partition /data/berlin-latest.osrm || echo "osrm-partition failed"docker run -t -v "${PWD}:/data" ghcr.io/project-osrm/osrm-backend osrm-customize /data/berlin-latest.osrm || echo "osrm-customize failed"

Note there is no berlin-latest.osrm file, but multiple berlin-latest.osrm.* files, i.e. berlin-latest.osrm is not file path, but "base" path referring to set of files and there is an option to omit this .osrm suffix completely(e.g. osrm-partition /data/berlin-latest).

docker run -t -i -p 5000:5000 -v "${PWD}:/data" ghcr.io/project-osrm/osrm-backend osrm-routed --algorithm mld /data/berlin-latest.osrm

Make requests against the HTTP server

curl ",52.517037;13.385983,52.496891?steps=true"

Optionally start a user-friendly frontend on port 9966, and open it up in your browser

docker run -p 9966:9966 osrm/osrm-frontendxdg-open ''

In case Docker complains about not being able to connect to the Docker daemon make sure you are in the docker group.

sudo usermod -aG docker $USER

After adding yourself to the docker group make sure to log out and back in again with your terminal.

We support the following images in the Container Registry:

latestmaster compiled with release flag
latest-assertionsmaster compiled with with release flag, assertions enabled and debug symbols
latest-debugmaster compiled with debug flag
<tag>specific tag compiled with release flag
<tag>-debugspecific tag compiled with debug flag

Building from Source

The following targets Ubuntu 22.04. For instructions how to build on different distributions, macOS or Windows see our Wiki.

Install dependencies

sudo apt install build-essential git cmake pkg-config \libbz2-dev libxml2-dev libzip-dev libboost-all-dev \lua5.2 liblua5.2-dev libtbb-dev

Compile and install OSRM binaries

mkdir -p buildcd buildcmake ..cmake --build .sudo cmake --build . --target install

Request Against the Demo Server

Read the API usage policy.

Simple query with instructions and alternatives on Berlin:

curl "https://router.project-osrm.org/route/v1/driving/13.388860,52.517037;13.385983,52.496891?steps=true&alternatives=true"

Using the Node.js Bindings

The Node.js bindings provide read-only access to the routing engine. We provide API documentation and examples here.

You will need a modern libstdc++ toolchain (>= GLIBCXX_3.4.26) for binary compatibility if you want to use the pre-built binaries. For older Ubuntu systems you can upgrade your standard library for example with:

sudo add-apt-repository ppa:ubuntu-toolchain-r/testsudo apt-get update -ysudo apt-get install -y libstdc++-9-dev

You can install the Node.js bindings via npm install @project-osrm/osrm or from this repository either via

npm install

which will check and use pre-built binaries if they're available for this release and your Node version, or via

npm install --build-from-source

to always force building the Node.js bindings from source.

Unscoped packages

Prior to v5.27.0, the osrm Node package was unscoped. If you are upgrading from an old package, you will need to do the following:

npm uninstall osrm --savenpm install @project-osrm/osrm --save

Package docs

For usage details have a look these API docs.

An exemplary implementation by a 3rd party with Docker and Node.js can be found here.

References in publications

When using the code in a (scientific) publication, please cite

@inproceedings{luxen-vetter-2011, author = {Luxen, Dennis and Vetter, Christian}, title = {Real-time routing with OpenStreetMap data}, booktitle = {Proceedings of the 19th ACM SIGSPATIAL International Conference on Advances in Geographic Information Systems}, series = {GIS '11}, year = {2011}, isbn = {978-1-4503-1031-4}, location = {Chicago, Illinois}, pages = {513--516}, numpages = {4}, url = {http://doi.acm.org/10.1145/2093973.2094062}, doi = {10.1145/2093973.2094062}, acmid = {2094062}, publisher = {ACM}, address = {New York, NY, USA},}