Quick start

Introduction

JSON (JavaScript Object Notation) is a lightweight data-interchange format.

Here is an example of JSON data:

{
    "encoding" : "UTF-8",
    "plug-ins" : [
        "python",
        "c++",
        "ruby"
        ],
    "indent" : { "length" : 3, "use_space": true }
}

JsonCpp supports comments as meta-data:

// Configuration options
{
    // Default encoding for text
    "encoding" : "UTF-8",
 
    // Plug-ins loaded at start-up
    "plug-ins" : [
        "python",
        "c++",  // trailing comment
        "ruby"
        ],
 
    // Tab indent size
    // (multi-line comment)
    "indent" : { /*embedded comment*/ "length" : 3, "use_space": true }
}

Features

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

Notes: Comments used to be supported in JSON but were 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;   // 'root' will contain the root value after parsing.
std::cin >> root;
 
// You can also read into a particular sub-value.
std::cin >> root["subtree"];
 
// Get the value of the member of root named 'encoding',
// and 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 'plug-ins'; return a 'null' value if
// there is no such member.
const Json::Value plugins = root["plug-ins"];
 
// Iterate over the sequence elements.
for ( int index = 0; index < plugins.size(); ++index )
   loadPlugIn( plugins[index].asString() );
 
// Try other datatypes. Some are auto-convertible to others.
foo::setIndentLength( root["indent"].get("length", 3).asInt() );
foo::setIndentUseSpace( root["indent"].get("use_space", true).asBool() );
 
// Since Json::Value has an implicit constructor for all value types, it is not
// necessary to explicitly construct the Json::Value object.
root["encoding"] = foo::getCurrentEncoding();
root["indent"]["length"] = foo::getCurrentIndentLength();
root["indent"]["use_space"] = foo::getCurrentIndentUseSpace();
 
// If you like the defaults, you can insert directly into a stream.
std::cout << root;
// Of course, you can write to `std::ostringstream` if you prefer.
 
// If desired, remember to add a linefeed and flush.
std::cout << std::endl;

Advanced usage

Configure builders to create readers and writers. For configuration, we use our own Json::Value (rather than standard setters/getters) so that we can add features without losing binary-compatibility.

// For convenience, use `writeString()` with a specialized builder.
Json::StreamWriterBuilder wbuilder;
wbuilder["indentation"] = "\t";
std::string document = Json::writeString(wbuilder, root);
 
// Here, using a specialized Builder, we discard comments and
// record errors as we parse.
Json::CharReaderBuilder rbuilder;
rbuilder["collectComments"] = false;
std::string errs;
bool ok = Json::parseFromStream(rbuilder, std::cin, &root, &errs);

Yes, compile-time configuration-checking would be helpful, but Json::Value lets you write and read the builder configuration, which is better! In other words, you can configure your JSON parser using JSON.

CharReaders and StreamWriters are not thread-safe, but they are re-usable.

Json::CharReaderBuilder rbuilder;
cfg >> rbuilder.settings_;
std::unique_ptr<Json::CharReader> const reader(rbuilder.newCharReader());
reader->parse(start, stop, &value1, &errs);
// ...
reader->parse(start, stop, &value2, &errs);
// etc.

See Json::CharReaderBuilder::settings_ and Json::StreamWriterBuilder::settings_ for the full list of settings.

Versioning

We make strong guarantees about binary-compatibility, consistent with the Apache versioning scheme.

See also

version.h

Related links

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