Including C++ functions in Fortran

Fortran makes it quite easy to add subroutines/functions from other languages like C/C++. Below is a short example on how to rewrite the Hello world example in C++ then run it from a Fortran script. See documentation at the bottom of the page to learn how to do it the other way around, implementing Fortran functions in C++ code

First we write the function hello() in C++. Create a .cpp file and name it "sub" (short for subroutine) . Below is  an example of the code to put in sub.cpp. Feel free to modify the function declaration.

#include <iostream>

extern "C" void hello(); #This line is needed for the main function to recognize the hello() function 

void hello()
{
  std::cout << "Hello World" << std::endl; 
  return;
}

Now we may create the main file. Start by creating a blank f.90 file and name it "main" Add the following code

program main
  implicit none
  ! Declare C/C++ function.
  interface
    subroutine hello() bind (c)
    end subroutine
  end interface
  ! Call C/C++ function.
  call hello()
end program

Now try running the project. Try changing the hello() function and play around with the interface in main. To check your understanding . 

For further reading on how to implement C++ in Fortran (and vice versa) Check this

pdf from Lawrence Livermore National Laboratory