From: Paul Harness <gapa83 AT udcf DOT gla DOT ac DOT uk>
Subject: Re: Internal function pointers
To: wrh AT placer1 DOT wimsey DOT com
Date: Fri, 12 Aug 1994 10:30:03 +0100 (BST)
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu (djgpp)

> class x
> {
>   protected:
>   void (*paction)(void *p);
>   ...
>   public: 
>   void setaction(void (*)(void *));  // stores to paction
>   ...
> }
> 
> class y:x
> {...}
> 
> class z:x
> {
>   protected:
>   class y one,two,three;
>   ...
>   void myfunc(void *);
>   void myotherfunc(void);
>   ...
>   public:
>   ...
> }
> 
> void z::myotherfunc(void)
> {
>   one.setaction(myfunc);  
> }
>
> Now, djgpp doesn't compile this, it complains about my prototyping.  
> What gives?
>
The problem is the difference between a `pointer-to-a-function' and a
`pointer-to-a-member-function'. A pointer to member function also
contains information about the class of which it is a member. i.e.,

	returntype (x::*)(args) ptr-member-fn
	returntype (*)(args)	ptr-fn

If you really need pointer to function syntax, the only way around this
is to either (a) use static member fns or (b) call the member fn via a
ordinary `wrapper fn' that accesses the object via, for example, a global.

Paul


-- 
Paul Harness,                        |  Internet: gapa83 AT udcf DOT gla DOT ac DOT uk
Department of Physics and Astronomy, |
University of Glasgow,               |  Tel +44-41-339-8855 ext 8359
Glasgow, G12 8QQ, SCOTLAND.          |  Fax +44-41-334-9029