strcmp stdout, stderr and stdin .. more

[mop65715]

class File_Driver {
  FILE *op ; 
  FILE *ip;
  bool op_open ;  
  bool ip_open ; 
public : 
  File_Driver ( const char* in_file_name, const char* out_file_name )
  : op      (   0   )
  , ip      (   0   ) 
  , op_open ( false ) 
  , ip_open ( false )
  {
    if ( in_file_name ) {
      if ( strcmp ( in_file_name, "stdin" ) == 0 ) {
        op = stdin ; 
      }
    } else {
      ip = fopen ( in_file_name, "rb" ) ; 
      if ( ip ) ip_open = true; 
    }
    if ( out_file_name ) {
      if ( strcmp ( in_file_name, "stdout" ) == 0 ) {
        op = stdout ; 
      }
    } else if ( strcmp ( in_file_name, "stderr" ) == 0 ) {
      op = stderr;
    } else {
      op = fopen ( out_file_name, "wb" ) ; 
      if ( op ) op_open = true; 
    }
  }
};

I'm trying to rewrite the source code above to use C++ streams. What I'm not following however, is the string comparisons (strcmp) to stdout, stderr and stdin and assignment to the member variable op if comparisons == 0. Why is that necessary ( not sure if that's some 'Cism') or ... ?

Thanks in advance