Hacker News Reader

While it's not quite what you want (as the specification language is not JSON---it's more like C declarations), "protobufs" allow you to generate serializers and de-serializers in a number of languages.

http://github.com/google/protobuf

There's thrift, but they have a custom specification with types. It shouldn't be too hard to go from JSON to thrift though.

https://thrift.apache.org/

JSONedit has (experimental) C++ code generator that serializes and deserializes JSON to/from object (default name for class is "Settings" and it uses json-cpp 0.5 as parser and generator): http://tomeko.net/software/JSONedit/cpp_class_generator.php