Path: utzoo!attcan!uunet!mcsun!unido!ira.uka.de!fuchs
From: fuchs@it.uka.de (Harald Fuchs)
Newsgroups: comp.lang.c++
Subject: Re: Invoking Member Functions Via Pointers
Message-ID: <fuchs.658164606@t5000>
Date: 9 Nov 90 15:30:06 GMT
References: <27515@mimsy.umd.edu>
Sender: news@ira.uka.de (USENET News System)
Organization: University of Karlsruhe, FRG
Lines: 88

wilson@brillig.cs.umd.edu (Anne Wilson) writes:
>(I tried to post this yesterday, and it has not yet appeared.  My
>apologies if this appears twice. -aw)
Not here.
>class Base_class {
>}

>class Derived_class : public class {
>	void new_function;
>}

>typedef void (*v_fn_ptr) (void *);

>class Container_class {
>	Base_class* *list;	// array of pointers to Base_class elements
>	Container_class (int obj_count, ... );
>	void apply (v_fn_ptr);	// apply function to each element in list
>}

>Derived_class D1, D2;

>Container_class c (2, &D1, &D2);		// create c

>// Idea is to apply new_function to both D1 and D2
>c.apply (&Derived_class::new_function);	// compiles, but crashes

>Can anyone help with this??  I am running g++ V 1.37.1 on a VAX.
>I would be **very** grateful for any suggestions!
Use real "pointers to members". If you don't know what that is, get a
newer C++ book (they were added to the language after 1985).
Then it goes something like that:

  #include <iostream.h>
  #include <stdarg.h>

  class Base_class {
  public:
    virtual void dfunc () = 0;
  };

  class Derived_class: public Base_class {
    int d;
  public:
    Derived_class (int x): d (x) {}
    void dfunc () { cout << "Derived_class\t" << d << "\n"; }
  };

  class Container_class {
    int num;
    Base_class** list;
  public:
    Container_class (int, ...);
    void apply (void (Base_class::*) ());
  };

  Container_class::Container_class (register int x, ...):
  num (x), list (new Base_class*[num]) {
    cout << num << " elements:";
    va_list ap; va_start (ap, x);
    register Base_class** p = list;
    while (x--) {
      *p++ = va_arg (ap, Derived_class*);
      cout << " " << p[-1];
    }
    va_end (ap);
    cout << "\n";
  }

  void Container_class::apply (void (Base_class::*funcp) ()) {
    register int n = num;
    register Base_class** p = list;
  //  while (n--) ((*p++)->*funcp) ();   // Too complicated for AT&T cfront 2.1
    while (n--) ((*p)->*funcp) (), p++;
  }

  main () {
    Derived_class D1 (1), D2 (2);
    cout << "D1@" << &D1 << ", D2@" << &D2 << "\n";
    Container_class c (2, &D1, &D2);
    c.apply (&Base_class::dfunc);
  }

Note that you can only apply a member function of the base class to a
Base_class* object, but it can be a virtual member function.
--

Harald Fuchs <fuchs@it.uka.de> <fuchs%it.uka.de@relay.cs.net> ...
<fuchs@telematik.informatik.uni-karlsruhe.dbp.de>   *gulp*