Functions in C++ files that should be called from Prolog must use C linkage, e.g.
extern "C" {
void myfun(SP_integer i)
{...};
};
On Windows, C++ is a first class citizen and no special steps are needed in order to mix C++ and C code.
On other platforms, to build a dynamically linked foreign resource with C++ code, you may have to explicitly include certain libraries and you may need to use an executable compiled and linked with a C++ compiler. The details are platform and C++ compiler dependent and outside the scope of this manual.