It's a property tree library that happen to have functions to create JSON-compatible or XML compatible output

or read from one.

See: 

 - http://lists.boost.org/boost-users/2013/02/77471.php

 - https://groups.google.com/forum/#!topic/boostusers/IjASBJvwhVY

There are apparently work in progress for an Boost JSON library but I don't have the details right now.

For the time being, maybe you can use JSonCPP or JSon Spirit libraries.