User's Guide

Mixing Fortran and C++

Most of the information in this chapter applies to Fortran, C, and Pascal -- languages with similar data types and naming schemes. However, to mix Fortran and C++ in the same program, you must add an extra level of indirection and pass the interlanguage calls through C "wrapper" functions.

Because the C++ compiler "mangles" the names of some C++ objects, you must use the xlC command to link the final program and include -L and -l options for the XL Fortran library directories and libraries as shown in Linking 32-Bit Non-SMP Object Files Using the ld Command.

Figure 1. Main Fortran Program That Calls C++ (main1.f)

       program main
 
       integer idim,idim1
 
       idim = 35
       idim1= 45
 
       write(6,*) 'Inside Fortran calling first C function'
       call cfun(idim)
       write(6,*) 'Inside Fortran calling second C function'
       call cfun1(idim1)
       write(6,*) 'Exiting the Fortran program'
       end

Figure 2. C Wrapper Functions for Calling C++ (cfun.C)

 #include <stdio.h>
 #include "cplus.h"
 
extern "C" void cfun(int *idim);
extern "C" void cfun1(int *idim1);
 
 void cfun(int *idim){
   printf("%%%Inside C function before creating C++ Object\n");
   int i = *idim;
   junk<int>* jj= new junk<int>(10,30);
   jj->store(idim);
   jj->print();
   printf("%%%Inside C function after creating C++ Object\n");
   delete jj;
   return;
 }
 
 void cfun1(int *idim1) {
   printf("%%%Inside C function cfun1 before creating C++ Object\n");
   int i = *idim1;
   temp<double> *tmp = new temp<double>(40, 50.54);
   tmp->print();
   printf("%%%Inside C function after creating C++ temp object\n");
   delete tmp;
   return;
 }

Figure 3. C++ Code Called from Fortran (cplus.h)

 #include <iostream.h>
 
 template<class T> class junk {
 
   private:
    int inter;
    T   templ_mem;
    T   stor_val;
 
   public:
    junk(int i,T j): inter(i),templ_mem(j)
                     {cout <<"***Inside C++ constructor" << endl;}
 
     ~junk()          {cout <<"***Inside C++ Destructor"  <<	endl;}
 
    void store(T *val){ stor_val = *val;}
 
    void print(void) {cout << inter << "\t" << templ_mem ;
                      cout <<"\t" << stor_val << endl; }};
 
 template<class T> class temp {
 
   private:
     int internal;
     T temp_var;
 
   public:
     temp(int i, T j): internal(i),temp_var(j)
                      {cout <<"***Inside C++ temp Constructor" <<endl;}
 
      ~temp()          {cout <<"***Inside C++ temp destructor"  <<endl;}
 
     void print(void) {cout << internal << "\t" << temp_var << endl;}};

Compiling this program, linking it with the xlC command, and running it produces this output:

 Inside Fortran calling first C function
%Inside C function before creating C++ Object
***Inside C++ constructor
10      30      35
%Inside C function after creating C++ Object
***Inside C++ Destructor
 Inside Fortran calling second C function
%Inside C function cfun1 before creating C++ Object
***Inside C++ temp Constructor
40      50.54
%Inside C function after creating C++ temp object
***Inside C++ temp destructor
 Exiting the Fortran program