C++ interface to a Fortran90 data structure

Let us consider a C++/Fortran90 interface for the SEPlib90 data structure. The IGF90Space class provides an encapsulation of the SEPlib90 header information. The following lines of code show part of the IGF90Space class definition:

  typedef void* HANDLE;
  #define OPAQUE_SIZE 1024;

  extern "C" sep90_reed_(const char * , int *, HANDLE);
  class IGF90Space: public HCL_VectorSpace {
     protected:
        HANDLE opaque_f90_handle;   // pointer to Fortran90 data structure;

     public:
        IGF90Space();
        IGF90Space(char *tag);
        ~IGF90Space();

        ...    // some other constructors and virtual member functions
  
        HANDLE GetOpaque();
  };

  IGF90Space::IGF90Space(){
     opaque_f90_handle = new char[OPAQUE_SIZE];
  }

  IGF90Space::IGF90Space(char *tag){
     opaque_f90_handle = new char[OPAQUE_SIZE];
     int length = strlen(tag);
     sep90_reed_( tag, &length , opaque_f90_handle );
  }

  IGF90Space::~IGF90Space(){ 
     delete opaque_f90_handle;
  }

  HANDLE IGF90Space::GetOpaque() const { 
     return(opaque_f90_handle);
  }

The IGF90Space class is composed of an opaque_f90_handle pointer to the Fortran90 data structure. The opaque_f90_handle pointer is used to pass the Fortran90 data structure in the C++ world. The constructor IGFSpace90(char *tag) initializes an IGF90Space object by reading the header information of the SEPlib90 data structure from disk, which is done by the sep90_reed Fortran90 subroutine. Because of class encapsulation, access to the opaque_f90_handle pointer is obtained only through the GetOpaque() member function.

The Fortran90 subroutine sep90_reed provides a wrapping layer to the sep_reed _mod module. The Fortran90 sep_reed_mod module is described in this report by Clapp and Crawley 1996. Some Fortran90 compilers use the ``$'' sign to name subroutines defined within modules. The direct call of Fortran90 subroutines that are within a module will require the inclusion of the ``$'' sign in the C++ extern call. Some compilers will only allow the use of the ``$'' sign if one uses special compiler flags. We opted not to use these flags, but used another layer of Fortran90 wrapping in order to be completely portable. The next lines of code show the wrapper file for the Fortran90 sep_reed_mod module:

  subroutine sep90_reed(tag,length,sepdata)
  use sep_reed_mod

  integer length
  character*(length) tag
  type (sep_90), pointer     ::sepdata

  character(len=length)      :: newtag
  newtag = tag

  call init_vars(sepdata)
  !read in only the headers
  call sep_reed(newtag,sepdata,header_only=.true.) 

  return
  end subroutine