Files to change

There are currently two files which need to be edited:

  1. src/json.hpp.re2c (note the .re2c suffix) - This file contains a comment section which describes the JSON lexic. This section is translated by re2c into file src/json.hpp which is plain “vanilla” C++11 code. (In fact, the generated lexer consists of some hundred lines of gotos, which is a hint you never want to edit this file...).

    If you only edit file src/json.hpp (without the .re2c suffix), your changes will be overwritten as soon as the lexer is touched again. To generate the src/json.hpp file which is actually used during compilation of the tests and all other code, please execute

    make re2c

    To run re2c and generate/overwrite file src/json.hpp with your changes in file src/json.hpp.re2c.

  2. test/src/unit.cpp - This contains the Catch unit tests which currently cover 100 % of the library's code.

    If you add or change a feature, please also add a unit test to this file. The unit tests can be compiled with


    and can be executed with


    The test cases are also executed with several different compilers on Travis once you open a pull request.

Please understand that I cannot accept pull requests changing only file src/json.hpp.


  • If you open a pull request, the code will be automatically tested with Valgrind's Memcheck tool to detect memory leaks. Please be aware that the execution with Valgrind may in rare cases yield different behavior than running the code directly. This can result in failing unit tests which run successfully without Valgrind.

Please don't

  • Only make changes to file src/json.hpp -- please read the paragraph above and understand why src/json.hpp.re2c exists.
  • The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.8 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
  • Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for these kind of bugs). I understand that even in 2016, complete C++11 support isn‘t there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft’s sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
  • Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
  • Please do not open pull requests that address multiple issues.