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From: landauer@morocco.Eng.Sun.COM (Doug Landauer)
Newsgroups: comp.lang.c++
Subject: Re: Pointers to functions anyone?
Summary: Ptr-to-member-function is NOT just ptr-to-function
Keywords: pointer-to-functions inside class
Message-ID: <1069@exodus.Eng.Sun.COM>
Date: 30 Sep 90 00:40:37 GMT
References: <6472@wolfen.cc.uow.oz>
Sender: news@exodus.Eng.Sun.COM
Distribution: comp
Organization: Sun Microsystems, Mt. View, Ca.
Lines: 70

> Please email replies - I'll summarise here if someone has a solution!

I tried to, but I got an error message back

<<< 554 <bernt@wolfen.cc.uow.oz>... Unknown domain : cc . uow . oz
554 <bernt@wolfen.cc.uow.oz>... Service unavailable

So, this time I'll just post it.

> I'm trying to set up an array of pointers to functions within a class,
> much like the example here (for simplicity no array here):
> 
> #include <stream.h>
> 
> class test {
>   void f1(int i)			// function to be called
>     { cout<<"ok"<<i<<"\n"; }
> 
>   void (test::*ptf)(int);		// pointer-to-function

		Whoops, your comment shows what's missing here.
		ptf is *not* a "pointer-to-function".  Instead,
		it is a "pointer-to-MEMBER-function", which is
		a different beast altogether.
> 
> public:
>   test() { ptf=test::f1; }		// constructor - assigns &f1 to ptf
>   void execute (int i)
>     { (*ptf)(i); }			// try to call f1()
> };
> 
> main() {
>   test variable;
>   variable.execute(1);
> }
> 
> My compiler (Turbo C++ V1.0) complains about the call (*ptf)(i):
> 
> -- Temporary used for parameter 'this' in function test::execute(int)
> -- Type mismatch in parameter 'this' in function test::execute(int)
> -- Parameter 'i' is never used in function test::execute(int)

cfront 2.1 shows a little more clearly what's happening:
	"bernt.cc", line XX: error:  pointer to member dereferenced

Read the E&S ARM section about pointers to members, and your compiler
should be much happier about compiling your code.  In order to make
your little test acceptable to cfront, all I had to do was change the
call to give the ptr-to-member-fn an explicit "this" for which to call
the function that it pointed at.  Like so:

Change the line

>     { (*ptf)(i); }			// try to call f1()

to a line like

>     { (this->*ptf)(i); }                    // try to call f1()

> What am I doing wrong??

All pointers-to-members must be given an object for which to call
their pointed-at functions.  Even if the call is from within another
member function.   Ptrs-to-members are usually implemented as
OFFSETs within the class, rather than as true pointers.

I hope this helps....
-- 
Doug Landauer - Sun Microsystems, Inc. - Languages - landauer@eng.sun.com
    Matt Groening on C++:  "Our language is one great salad."