Pointers to functions

[Dave Schaumann]

In article <AJ3U.91May14140642 at wilbury.cs.virginia.edu> aj3u at wilbury.cs.virginia.edu (Asim Jalis) writes:
>What is the difference between these two (pf is a pointer to a
>function, and hello is a function):
>
>pf = hello;	
>
>and 
>
>pf = &hello;
>
>The definitions for pf and hello are as follows:
>
>void (*pf)();	// pointer to a function
>
>void hello()
>{
>  printf("Hello World\n");
>}
>
>I used the two different forms of assignments and got the same output.
>Is there a difference between them?

You have found one of the weirdnesses of ANSI-C.  When you have a pointer
to a function, you (normally) invoke the function by saying

	(*funptr)()

However, ANSI, in it's infinite wisdom, decided that you should be able to
say

	funptr()

in the same context, with similar results.

---- Possible rationalization to follow ... press 'n' if you don't care ----

Actually, I believe this is probably due to an influence from C++, where you
have constructs called "classes" (which are similar in some ways to structs).
Suppose you have a class "foo", with an function called "foo_fn".  Now, you
declare v to be a variable of class foo:

  foo v ;

You now can invoke the function foo_fn on v in the following manner:

  v.foo_fn()

(A pointer to v is an implicit variable in the function call)

This functionality can be mimiced in C in the following manner:

typedef struct FOO {
  ...
  void (*foo_fn) ( struct FOO * ) c;
  ...
  } foo ;

foo v ; /* assume that v.foo_fn is initialized appropriately */

Now, according to the traditional interpretation, you must say

  (*v.foo_fn)(&v) ; /* A ptr to v must be explicitly passed */

To invoke foo_fn on v.  The new interpretation allows you to say

  v.foo_fn(&v) ;

Almost like C++ (almost since you still have to explicitly pass a
pointer to the invoking "variable" (known as "self" in C++)).  Thus,
this feature allows you some measureof "poor man's C++" in ANSI C.
Of course, many of the nice features of C++ must be done by hand...

-- 
Dave Schaumann      | There is no cause so right that one cannot find a fool
dave at cs.arizona.edu | following it.	- Niven's Law # 16