Path: utzoo!attcan!uunet!decwrl!fernwood!dumbcat!marc
From: marc@dumbcat.sf.ca.us (Marco S Hyman)
Newsgroups: comp.lang.c++
Subject: Re: Address of member function
Summary: The worrkaround is not to bad
Message-ID: <220@dumbcat.sf.ca.us>
Date: 29 Oct 90 01:00:29 GMT
References: <1399@carol.fwi.uva.nl>
Organization: MH Software, Hayward, Ca.
Lines: 77

In article <1399@carol.fwi.uva.nl> delft@fwi.uva.nl (Andre van Delft) writes:
    
    A few days ago I had a question on comp.lang.c++ about taking the address
    of a member function:

    [[...]]

    I meant the following: why is it allowed to take the address of:
    
    a GLOBAL VARIABLE
    a GLOBAL FUNCTION 
    a MEMBER VARIABLE, i.e. a variable local to an instance of a class
    
    but why is it NOT allowed to take the address of
    
    a MEMBER FUNCTION, i.e. a function local to an instance of a class

I think the problem here is that member functions are not "local to an
instance of a class," but are part of the class itself.  Given the class

    class c {
      public:
        int j;
        int g(int k) {printf("%d\n",j+k);}
    };

    c aC;

there is a j for every instance, therefore you can take the address of 
aC.j or you can get its relative offset in the class by taking the address
of c::j.  There is only one g for the class used by all instances and you
can only get its offset (&c::g).  See section 8.1c of the ARM (pg 155).

Note: by making minor changes to your example you can get what you want,
albeit by special casing pointers to member functions.

#include <stdio.h>

int i;
int f(int k) {printf("%d\n",i+k);return 0;}
	// return 0 added to get rid of warning

class c {
  public:
    int j;
    int g(int k) {printf("%d\n",j+k); return 0;}
	// return 0 added to get rid of warning
};

main() {
  int*ip,*jp;
  int(*fp)(int);
//int(*gp)(int);	// replaced gp with the next two lines
  int (c::*gp)(int);
  c * gpObject;

  c anObject;

  ip  = &         i;        // OK
  jp  = &anObject.j;        // OK
  fp  = &         f;        // OK
//gp  = &anObject.g;        // NOT OK but see the next two lines
  gpObject = &anObject;
  gp  = &c::g;
  *ip = 1;
  *jp = 2;
  fp(3);                     // prints 4
//gp(4);                     // would print 6, if not NOT OK ...
  (gpObject->*gp)(4);	     // this is OK and prints 6 with CC 2.0
}

It's more work than you want to do but gets the job done.

// marc
-- 
// marc@dumbcat.sf.ca.us
// {ames,decwrl,sun}!pacbell!dumbcat!marc