INFO: Syntax for a Function Returning a Pointer to a Function

• Microsoft C compiler for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax
• Microsoft C/C++ compiler for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0, 1.5
• Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

The syntax for a function returning a pointer to another function is somewhat complicated due to the precedence rules involved. The syntax can be greatly simplified with the use of typedefs, but typedefs are not required. The following discussion applies to ANSI C and to C++.

MORE INFORMATION

The simplest way to define such a function is by using a typedef as an alias for a pointer to a function of the desired type. This is the recommended method. For example, consider the following statements:

   typedef void (*FPN)( int );
   FPN return_func_ptr( void );

To declare such a function without using a typedef, the syntax is rather different:

   void (* (return_func_ptr(void)) )( int )
   {
      return a_func;  /* where a_func is a defined function */
   }

The syntax for calling a function through a pointer is shown in the following example:

   void (*func_ptr)( int );     /* Define a pointer to a function
                                   taking an int and returning void */
   func_ptr = return_func_ptr();
   (*func_ptr)( 3 );            /* Dereference it to call function  */

Sample Code

   /* Compile options needed: none
   */

   #include <stdio.h>

   void afunc( char *cp, int ivar )
   {
      printf( "%s : int=%d\n", cp, ivar );
   }

   /* Typedef FPN as a pointer to a function taking a char pointer and
      an int and returning nothing.
   */
   typedef void (*FPN)( char *, int );

   /* Define a function with a void parameter list returning a pointer to
      a function taking a character pointer and an int and returning
      nothing.
   */

   void (* (retfunc1(void)) )( char *, int )
   {
      return afunc;
   }

   /* Define a function exactly the same as retfunc1, but using the
      FPN typedef previously defined to simplify the code.
   */

   FPN retfunc2( void )
   {
      return afunc;
   }

   void main( void )
   {
      void (*fp1)(char *, int);   /* fp1 and fp2 are both pointers to */
      FPN fp2;                    /* functions taking a char pointer  */
                                  /* and an int and returning nothing */

     fp1 = retfunc1();      /* Examples of setting function pointers  */
     (*fp1)( "Test1", 0 );  /* and calling the functions through them */

     fp1 = retfunc2();
     (*fp1)( "Test2", 1 );

     fp2 = retfunc1();
     (*fp2)( "Test3", 2 );

     fp2 = retfunc2();
     (*fp2)( "Test4", 3 );
   }

Keywords          : CLngIss kbcode kbfasttip
Version           : MS-DOS:5.1,6.0,6.00a,6.00ax,7.0; WINDOWS:1.0,1.5; WINDOWS  NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbinfo