A header-only library and a code generator to automagically translate between JSON and C++ types.


JSON is a popular format for data exchange. Reading and writing JSON in C++, however, is nontrivial. Even with the help of libraries, one still needs to write lots of boilerplate code, and it is extremely hard to guard against all possible errors, since JSON is dynamically typed while C++ employs static typing.

More importantly, manually writing such code is a violation of DRY principle. When manually written, the class definition, parsing and serialization code can easily become out of sync, leading to brittle code and subtle bugs.

autojsoncxx is an attempt to solve this problem by automating such process.



  • The parsing/serializing code are automagically generated.
  • Detailed error message. Unlike ordinary JSON libraries, autojsoncxx will detect not only invalid JSON errors, but also mismatch between JSON value and C++ type specification.
  • Ease of use. A single function call is enough for most use cases. The library has no complicated build setup, since it's header only.
  • Fast. Based on the streaming API of rapidjson and C++ templates, this library is efficient both in space and time.
  • Flexible. Whenever the support of a certain class Foo is added to the library, all the combination of types, like std::vector<Foo>, std::shared_ptr<Foo>, std::deque<std::map<std::string, Foo>>> is automatically supported as well, thanks to the use of template specialization.
  • Liberal license. Both the library and its dependency are licensed liberally (MIT or BSD-like). Anyone is free to copy, distribute, modify or include in their own projects, be it open source or commercial.


Build Status

To build the test, you need a sufficiently new compiler because the goal is to test all the type support, including many ones only introduced in c++11.

First clone the repository, and pull the dependency

git clone --recursive

UNIX/Linux/Mac users:

cmake .
make test

Windows users:

Generate the test/userdef.hpp file from the definition examples/userdef.json. Then open the solution file under test/mscvXX_test/ to build and run the test.

If too many tests fail, make sure your work directory points to the test directory.

Currently tested compilers

  • Clang 3.4/3.5 on Mac OS X (11.9)
  • GCC 4.9 (Homebrew) on Mac OS X (11.9)
  • Clang 3.0 on Ubuntu 12.04 (x64)
  • GCC 4.8 on Ubuntu 14.04.1 (x86/x64)
  • MSVC 10 (x86) on Windows 7
  • MSVC 11/12 (x86/x64) on Windows 7

Quick start

The code generator reads a JSON file that defines the class structure. An example definition is like this

    "name": "Person",
        ["unsigned long long", "ID", {"required": true}],
        ["std::string", "name", {"default": "anonymous"}],
        ["double", "height"],
        ["double", "weight"],
        ["std::vector<unsigned long long>", "known_associates"]

Run the script (requires Python 2.7+, including version 3+) on this definition file, and a header file will be generated. It includes a definition for Person as well as some helper classes.

python --input=persondef.json --output=person.hpp


Person p;
autojsoncxx::to_pretty_json_file("person.json", p);
autojsoncxx::to_json_file(stdout, p);


autojsoncxx::ParsingResult result;
Person p;
if (!autojsoncxx::from_json_file("person.json", p, result)) {
    std::cerr << result << '\n';
    return -1;

Error handling

If the JSON file is malformed, any decent JSON library will detect it and tell you what goes wrong. But what if the JSON value is perfectly valid, but not laid out the way you expected? Usually you have to manually check the DOM tree against your specification, but this library will automatically generates the necessary code.

Here is valid JSON file

    "name": "Mike",
    "ID": 8940220481904,
    "height": 1.77,
    "weight": 70.0,
    "known_associates": [
        "Jack", "Mary"

Running through the parsing code, and you will get an error output:

Parsing failed at offset 127 with error code 16:
Terminate parsing due to Handler error.

Trace back (last call first):
(*) Type mismatch between expected type "uint64_t" and actual type "string"
(*) Error at array element with index 0
(*) Error at object member with name "known_associates"

One can also query the errors programmingly.


Read more here.


This project is inspired by google protobuf. It adopts the same approach in protobuf (compiler + definition file), and borrows various parse options from it.