Calling C++ from Fortran 90

Calling Fortran 90 from C++: not too terribly difficult although character strings are a pain.

Calling C++ from Fortran 90: Rather more complicated if you're not going to use pre-written interface-generating software. Best summary of the information you'll need I've found so far is this conference proceeding by Wang et al (2005), see also their ppt presentation.

Yang Wang, Raghurama Reddy, Roberto Gomez, Junwoo Lim, Sergiu Sanielevici, Jaideep Ray, James Sutherland5and Jackie Chen, 2005, "A General Approach to Creating Fortran Interface for C++ Application Libraries", in Current Trends in High Performance Computing and Its Applications, (Springer Berlin Heidelberg), 145-154, DOI 10.1007/3-540-27912-1_14