Dynamic data library for C++

I am writing an application in C++ and need a way to build and represent dynamic data like it would be contained in JSON or MsgPack data.

It should support values like null, bool, strings, integers, floating point numbers, lists and objects (hashmaps).

It obviously would not be hard to cook something up myself, but I'd prefer a tested and most importantly efficient(!) library.

I've looked into just using the implementation of some JSON parser library, but I'd prefer a dedicated library for just this purpose.

I haven't been able to find anything appropriate, though. Any recommendations?