Pluma seems to be one of the easiest and most straight-forward C++ Plugin frameworks. I used it in one of my projects and will use it in future projects again.
Pluma-framework works perfect.
Easy to use, small and cross platform. There are still some flaws, like the lacks of support for exceptions ("fprintf(stderr,message);" is used instead, which does not allow easy error management), some C-style cast where C++ reinterpret_cast could be used, and it may give easier access to few things, but it is globally a good tool.