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Message from discussion Frasncis Glassboro wrote.
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Garrett Hartshaw  
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 More options Jan 7 2011, 5:47 pm
Newsgroups: alt.comp.lang.learn.c-c++, comp.lang.c++
From: Garrett Hartshaw <gharts...@gmail.com>
Date: Fri, 07 Jan 2011 17:47:12 -0500
Local: Fri, Jan 7 2011 5:47 pm
Subject: Re: Frasncis Glassboro wrote.
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-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 01/07/2011 02:06 PM, Paul wrote:

> "James Kanze" <james.ka...@gmail.com> wrote in message
> news:7148a492-1586-4421-bc6c-4afa38770f53@w29g2000vba.googlegroups.com...
>> On Jan 7, 2:07 pm, "Paul" <pchris...@yahoo.co.uk> wrote:
>>> "James Kanze" <james.ka...@gmail.com> wrote in message

>>    [...]
>>> >> I can understand the concept you express but
>>> >> a) how do you get the address of a member function?

>>> > &ClassName::functionName

>>> > Concretely:

>>> >    struct C { void f(); };
>>> >    void (C::*pf)() = &C::f;

>>> >> b) what happens if this member function is virtual?

>>> > It works correctly.  That's why pointer to member functions are
>>> > often larger than any other pointer types (but there are other
>>> > ways of solving the problem).

>>> What would your pointer point to ?

>> That's the compiler writers problem, not mine:-).

> It certainly is a problem for the compiler, and perhaps the program too.
> Especially if you didn't initialised the empty pointer.
> Let me put it another way, where would you get the address for the
> virtual function?

>>> You cannot do this with virtual functions and  you are wrong
>>> to suggest it works correctly.

>> It does work, and I've done it.  More than once.

> It simply can't be done as the concept of virtual functions only lives
> in the world of objects.
> Please show some basic code. I guarantee you cannot.

#include <iostream>

class C1 {
public:
    virtual void f() {
        std::cout << "C1::f" << std::endl;
    }

};

class C2 : public C1 {
public:
    virtual void f() {
        std::cout << "C2::f" << std::endl;
    }

};

int main () {
  void (C1::*p)() = &C1::f; //create a pointer to a member function

  C1 a;                         //create a object of type C1
  C1 * b;                       //create a object of type pointer to C1
  b = new C2();                 //allocate a object of type C2

  (a.*p)();                     //call the member function pointed to
                                //by p, with &a as this
  (b->*p)();                    //call the member function pointed to
                                //by p (virtually), with b as this

  return 0;

}

This code prints the following.

C1::f
C2::f
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FbMuQtdtJm6baWiB455Z
=u89w
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