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JsonCpp Documentation


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

Here is an example of JSON data:

    "encoding" : "UTF-8",
    "plug-ins" : [
    "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" : [
"c++", // trailing comment
// Tab indent size
// (multi-line comment)
"indent" : { /*embedded comment*/ "length" : 3, "use_space": true }


  • 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;
Represents a JSON value.
Definition: value.h:193
Value get(ArrayIndex index, const Value &defaultValue) const
If the array contains at least index+1 elements, returns the element value, otherwise returns default...
Definition: json_value.cpp:1074
ArrayIndex size() const
Number of values in array or object.
Definition: json_value.cpp:859
String asString() const
Embedded zeroes are possible.
Definition: json_value.cpp:628

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.
wbuilder["indentation"] = "\t";
std::string document = Json::writeString(wbuilder, root);
// Here, using a specialized Builder, we discard comments and
// record errors as we parse.
rbuilder["collectComments"] = false;
std::string errs;
bool ok = Json::parseFromStream(rbuilder, std::cin, &root, &errs);
Build a CharReader implementation.
Definition: reader.h:289
Build a StreamWriter implementation.
Definition: writer.h:89
String writeString(StreamWriter::Factory const &factory, Value const &root)
Write into stringstream, then return string, for convenience.
Definition: json_writer.cpp:1246
bool parseFromStream(CharReader::Factory const &, IStream &, Value *root, String *errs)
Consume entire stream and use its begin/end.
Definition: json_reader.cpp:1971

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.

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.
CharReader * newCharReader() const override
Allocate a CharReader via operator new().
Definition: json_reader.cpp:1883
Json::Value settings_
Configuration of this builder.
Definition: reader.h:332

Build instructions

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

The latest version of the source is available in the project's GitHub repository: jsoncpp

What's New?

The description of latest changes can be found in the NEWS wiki .

Related links

Old project links


See file LICENSE in the top-directory of the project.

Basically JsonCpp is licensed under MIT license, or public domain if desired and recognized in your jurisdiction.

Baptiste Lepilleur blep@.nosp@m.user.nosp@m.s.sou.nosp@m.rcef.nosp@m.orge..nosp@m.net (originator)
Christopher Dunn cdunn.nosp@m.2001.nosp@m.@gmai.nosp@m.l.co.nosp@m.m (primary maintainer)
We make strong guarantees about binary-compatibility, consistent with the Apache versioning scheme.
See also