C++ Yacc CMake Lex
Latest commit 217ac19 Sep 24, 2015 @tunnuz Merge pull request #7 from verlinked/master
Pedantic warn level and CMAKE_CXX_FLAGS preservation.



JSON++ is a self contained Flex/Bison JSON parser for C++11. It parses strings and files in JSON format, and builds an in-memory tree representing the JSON structure. JSON objects are mapped to std::maps, arrays to std::vectors, JSON native types are mapped onto C++ native types. The library also includes printing on streams. Classes exploit move semantics to avoid copying parsed structures around. It doesn't require any additional library (not even libfl).


July 20, 2014 - In the past few months I have merged various pull requests, which I considered good additions to the code (e.g., someone fixed memory leaks, someone improved the CMake build system, someone started to add support for unicode escaped characters). These were all very good contributions, and I'd like to thank anyone that has filed a pull request for JSON++. However, I have sometimes overlooked the code that I was merging and, sometimes, pull requests with early code were merged in the main distribution. For this reason, a few days ago I have rolled back to a previous version of the code, from the end of June. This is my first open source projects that gains a bit of traction (the repository has about 1500 visits per month), and I'll be more careful in merging pull requests from now on.

Git repository

A version of this repository (regularly mirrored) is available on GitHub.


JSON++ is not a personal project anymore, people is constantly writing to me, and sending pull requests to improve it and make it better. I'd like to thank these people by adding them to this Contributors section (in order of contribution).

Thanks for your effort fellas.


#include <iostream>
#include "json.hh"

using namespace std;
using namespace JSON;

int main(int argc, char** argv)
    // Read JSON from a string
    Value v = parse_string(<your_json_string>);
    cout << v << endl;

    // Read JSON from a file
    v = parse_file("<your_json_file>.json");
    cout << v << endl;

    // Or build the object manually
    Object obj;

    obj["foo"] = true;
    obj["bar"] = 3;

    Object o;
    o["given_name"] = "John";
    o["family_name"] = "Boags";

    obj["baz"] = o;

    Array a;

    obj["test"] = a;

    cout << o << endl;

    return 0;

How to build JSON++

The project includes a CMakeLists.txt files which allows you to generate build files for most build systems. Just run

cmake .    

and then


The project generates

  • json.tab.hh,
  • json.tab.cc, and
  • lex.yy.cc

files from json.l and json.y, then compiles them (and a few other files) into a libjson library, which is finally used to link the test executable. You can use the library in your projects, or use the Flex/Bison files straight away.

How to build with unit tests

If you have the cppunit framework (http://sourceforge.net/projects/cppunit/) installed on your system, you can make a build with unit tests as follows:

mkdir build
cd build
ctest -V

The usage of an out of source build is strongly advised, since even more files are generated by the CTest testing tool.

How to build for measuring code coverage

Specify the Coverage build type as follows:

cd build
cmake .. -DCMAKE_BUILD_TYPE=Coverage

You can get a code coverage report with gcovr (http://gcovr.com):

cd build
gcovr --xml --root .. --exclude "ut/.*" --exclude "test.cc" > coverage.xml

This produces a report in the XML file format, which can be visualized with tools such as the Cobertura plugin for the jenkins continuous integration server.

How to generate API documentation

If you have doxygen (http://www.stack.nl/~dimitri/doxygen/) installed on your system, an API documentation will be generated automatically as part of make. You can also request its generation explicitly:

make doc

You will find the documentation in your build directory at ./html/index.html.

Flex/Bison quirks when using C++ classes

This section is for the ones who got here because they're trying to build stuff with Flex/Bison and C++. This was my first Flex/Bison parser (the main motivation behind its development being that I didn't find a parser for JSON in C++ which didn't require a number of extra libraries, plus I wanted to learn Flex/Bison).

So, for the ones venturing in this world, here's a few things I wish I knew when I set off to write the parser.

  1. Every rule of the Bison grammar has a left-hand side, to which the parsed objects (no matter their type), must be assigned. To do this, a union is used. Bison uses the %union { ... } rule to declare the types inside the union, which must only contain native C types or pointers to C++ classes,
  2. in case pointers to C++ classes are used in %union, classes extending std containers won't work, so you'll need to wrap std stuff in your own classes,
  3. always put a starting rule in the grammar to assign the result of the overall parse to a variable, e.g., json: value { $$ = $1; },
  4. as a general rule, functions requiring Flex functions, e.g., yy_scan_string, etc., should be defined in the .l file, and their prototypes put in the .y file as well, so that they can be called from the parser's functions,
  5. ... (to be continued as I find out more).


This code is distributed under the very permissive MIT License but, if you use it, you might consider referring to the repository.