A C++ library for interacting with JSON.
C++ Python CMake Other
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JSON is a lightweight data-interchange format. It can represent numbers, strings, ordered sequences of values, and collections of name/value pairs.

JsonCpp is a C++ library that allows manipulating JSON values, including serialization and deserialization to and from strings. It can also preserve existing comment in unserialization/serialization steps, making it a convenient format to store user input files.

A note on backward-compatibility

  • 1.y.z is built with C++11.
  • 0.y.z can be used with older compilers.
  • Major versions maintain binary-compatibility.

Using JsonCpp in your project

The recommended approach to integrating JsonCpp in your project is to include the amalgamated source (a single .cpp file and two .h files) in your project, and compile and build as you would any other source file. This ensures consistency of compilation flags and ABI compatibility, issues which arise when building shared or static libraries. See the next section for instructions.

The include/ should be added to your compiler include path. Jsoncpp headers should be included as follow:

#include <json/json.h>

If JsonCpp was built as a dynamic library on Windows, then your project needs to define the macro JSON_DLL.

Generating amalgamated source and header

JsonCpp is provided with a script to generate a single header and a single source file to ease inclusion into an existing project. The amalgamated source can be generated at any time by running the following command from the top-directory (this requires Python 2.6):

python amalgamate.py

It is possible to specify header name. See the -h option for detail.

By default, the following files are generated:

  • dist/jsoncpp.cpp: source file that needs to be added to your project.
  • dist/json/json.h: corresponding header file for use in your project. It is equivalent to including json/json.h in non-amalgamated source. This header only depends on standard headers.
  • dist/json/json-forwards.h: header that provides forward declaration of all JsonCpp types.

The amalgamated sources are generated by concatenating JsonCpp source in the correct order and defining the macro JSON_IS_AMALGAMATION to prevent inclusion of other headers.

Contributing to JsonCpp

Building and testing with CMake

CMake is a C++ Makefiles/Solution generator. It is usually available on most Linux system as package. On Ubuntu:

sudo apt-get install cmake

Note that Python is also required to run the JSON reader/writer tests. If missing, the build will skip running those tests.

When running CMake, a few parameters are required:

  • a build directory where the makefiles/solution are generated. It is also used to store objects, libraries and executables files.
  • the generator to use: makefiles or Visual Studio solution? What version or Visual Studio, 32 or 64 bits solution?

Steps for generating solution/makefiles using cmake-gui:

  • Make "source code" point to the source directory.
  • Make "where to build the binary" point to the directory to use for the build.
  • Click on the "Grouped" check box.
  • Review JsonCpp build options (tick BUILD_SHARED_LIBS to build as a dynamic library).
  • Click the configure button at the bottom, then the generate button.
  • The generated solution/makefiles can be found in the binary directory.

Alternatively, from the command-line on Unix in the source directory:

mkdir -p build/debug
cd build/debug

For a good pkg-config file, add:


Running cmake -h will display the list of available generators (passed using the -G option).

By default CMake hides compilation commands. This can be modified by specifying -DCMAKE_VERBOSE_MAKEFILE=true when generating makefiles.

Building and testing with SCons

Note: The SCons-based build system is deprecated. Please use CMake; see the section above.

JsonCpp can use Scons as a build system. Note that SCons requires Python to be installed.

Invoke SCons as follows:

scons platform=$PLATFORM [TARGET]

where $PLATFORM may be one of:

  • suncc: Sun C++ (Solaris)
  • vacpp: Visual Age C++ (AIX)
  • mingw
  • msvc6: Microsoft Visual Studio 6 service pack 5-6
  • msvc70: Microsoft Visual Studio 2002
  • msvc71: Microsoft Visual Studio 2003
  • msvc80: Microsoft Visual Studio 2005
  • msvc90: Microsoft Visual Studio 2008
  • linux-gcc: Gnu C++ (linux, also reported to work for Mac OS X)

If you are building with Microsoft Visual Studio 2008, you need to set up the environment by running vcvars32.bat (e.g. MSVC 2008 command prompt) before running SCons.

Running the tests manually

You need to run tests manually only if you are troubleshooting an issue.

In the instructions below, replace path/to/jsontest with the path of the jsontest executable that was compiled on your platform.

cd test
# This will run the Reader/Writer tests
python runjsontests.py path/to/jsontest

# This will run the Reader/Writer tests, using JSONChecker test suite
# (http://www.json.org/JSON_checker/).
# Notes: not all tests pass: JsonCpp is too lenient (for example,
# it allows an integer to start with '0'). The goal is to improve
# strict mode parsing to get all tests to pass.
python runjsontests.py --with-json-checker path/to/jsontest

# This will run the unit tests (mostly Value)
python rununittests.py path/to/test_lib_json

# You can run the tests using valgrind:
python rununittests.py --valgrind path/to/test_lib_json

Running the tests using scons

Note that tests can be run using SCons using the check target:

scons platform=$PLATFORM check

Building the documentation

Run the Python script doxybuild.py from the top directory:

python doxybuild.py --doxygen=$(which doxygen) --open --with-dot

See doxybuild.py --help for options.

Adding a reader/writer test

To add a test, you need to create two files in test/data:

  • a TESTNAME.json file, that contains the input document in JSON format.
  • a TESTNAME.expected file, that contains a flatened representation of the input document.

The TESTNAME.expected file format is as follows:

  • each line represents a JSON element of the element tree represented by the input document.
  • each line has two parts: the path to access the element separated from the element value by =. Array and object values are always empty (i.e. represented by either [] or {}).
  • element path: . represents the root element, and is used to separate object members. [N] is used to specify the value of an array element at index N.

See the examples test_complex_01.json and test_complex_01.expected to better understand element paths.

Understanding reader/writer test output

When a test is run, output files are generated beside the input test files. Below is a short description of the content of each file:

  • test_complex_01.json: input JSON document.
  • test_complex_01.expected: flattened JSON element tree used to check if parsing was corrected.
  • test_complex_01.actual: flattened JSON element tree produced by jsontest from reading test_complex_01.json.
  • test_complex_01.rewrite: JSON document written by jsontest using the Json::Value parsed from test_complex_01.json and serialized using Json::StyledWritter.
  • test_complex_01.actual-rewrite: flattened JSON element tree produced by jsontest from reading test_complex_01.rewrite.
  • test_complex_01.process-output: jsontest output, typically useful for understanding parsing errors.


See the LICENSE file for details. In summary, JsonCpp is licensed under the MIT license, or public domain if desired and recognized in your jurisdiction.