If you want to call class methods from C++ from within C, use a class wrapper.
One advantage of this method is that the C++ class remains unmodified, and can
even exist in a library.

First, let's define the C++ class, "Circle". For simplicity, we'll do everything
in a .h file, but it works just as well for classes declared in a .h and defined
in a .cpp file.
Code:
// Circle.h - a C++ class
...