JsonCpp Documentation

Introduction

JSON (JavaScript Object Notation) is a lightweight data-interchange format. It can represent integer, real number, string, an ordered sequence of value, and a collection of name/value pairs.

Here is an example of JSON data:

// Configuration options
{
    // Default encoding for text
    "encoding" : "UTF-8",
    
    // Plug-ins loaded at start-up
    "plug-ins" : [
        "python",
        "c++",
        "ruby"
        ],
        
    // Tab indent size
    "indent" : { "length" : 3, "use_space" = true }
}

Features

• read and write JSON document
• attach C and C++ style comments to element during parsing
• rewrite JSON document preserving original comments

Notes: Comments used to be supported in JSON but where removed for portability (C like comments are not supported in Python). Since comments are useful in configuration/input file, this feature was preserved.

Code example

Json::Value root;   // will contains the root value after parsing.
Json::Reader reader;
bool parsingSuccessful = reader.parse( config_doc, root );
if ( !parsingSuccessful )
{
    // report to the user the failure and their locations in the document.
    std::cout  << "Failed to parse configuration\n"
               << reader.getFormatedErrorMessages();
    return;
}

// Get the value of the member of root named 'encoding', return 'UTF-8' if there is no
// such member.
std::string encoding = root.get("encoding", "UTF-8" ).asString();
// Get the value of the member of root named 'encoding', return a 'null' value if
// there is no such member.
const Json::Value plugins = root["plug-ins"];
for ( int index = 0; index < plugins.size(); ++index )  // Iterates over the sequence elements.
   loadPlugIn( plugins[index].asString() );
   
setIndentLength( root["indent"].get("length", 3).asInt() );
setIndentUseSpace( root["indent"].get("use_space", true).asBool() );

// ...
// At application shutdown to make the new configuration document:
// Since Json::Value has implicit constructor for all value types, it is not
// necessary to explicitly construct the Json::Value object:
root["encoding"] = getCurrentEncoding();
root["indent"]["length"] = getCurrentIndentLength();
root["indent"]["use_space"] = getCurrentIndentUseSpace();

Json::StyledWriter writer;
// Make a new JSON document for the configuration. Preserve original comments.
std::string outputConfig = writer.write( root );

// You can also use streams.  This will put the contents of any JSON
// stream at a particular sub-value, if you'd like.
std::cin >> root["subtree"];

// And you can write to a stream, using the StyledWriter automatically.
std::cout << root;

Project links

The build instructions are located in the file README.txt in the top-directory of the project.

Permanent link to the latest revision of the file in subversion: latest README.txt

Download

The sources can be downloaded from SourceForge download page.

The latest version of the source is available in the project's subversion repository: http://jsoncpp.svn.sourceforge.net/svnroot/jsoncpp/trunk/

To checkout the source, see the following instructions.

Project links

• json-cpp home
• json-cpp sourceforge project

Related links

• JSON Specification and alternate language implementations.
• YAML A data format designed for human readability.
• UTF-8 and Unicode FAQ.

License

The json-cpp library and this documentation are in Public Domain.

Author

Baptiste Lepilleur blep@.nosp@m.user.nosp@m.s.sou.nosp@m.rcef.nosp@m.orge..nosp@m.net