Thank you all for your replies. I am sorry @kshegunov but I did not get the side question. But the way I understood I say yes, mixing the code. C++ is used to create the interface and Fortran holds all the advanced mathematical calculations.
Also, I got a step further. Was stupid and used MSVC and the gfortran, which is included just in the MinGW compiler. Got part of it compiled but at the same time getting plenty of errors. Guess that this is caused by the set flags.
To make the picture complete I developed the app in the Visual Studio before and I have the intel fortran in my PC. Can I include that somehow as I know it optimizes the code more?
And hopefully one of my last questions. When I use either gfortran or ifort in my code and I'll try to do the cross compilation. Will the fortran part of the code run flawlessly on Mac or Linux?
Thank you a lot