NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/16011

Text File | 1996-08-05 | 1.9 KB | 101 lines

Path: ix.netcom.com!news
From: [e-mail](Giuliano Carlini)
Newsgroups: comp.lang.c++
Subject: Re: How do I put DIFFERENT classes in the same linked list?
Date: 8 Apr 1996 08:55:55 GMT
Organization: Netcom
Message-ID: <4kakar$[e-mail]>
References: <4ka938$[e-mail]>
NNTP-Posting-Host: lbx-ca6-22.ix.netcom.com
X-NETCOM-Date: Mon Apr 08  3:55:55 AM CDT 1996

In <4ka938$[e-mail]> grosin@titan (Gil Rosin)
writes:
>
>Here is the source code I have worked up so far:
>
>#include <iostream.h>
>
>class A
> {
>  public:
>    A *next;
>    A *prev;
>    void Print();
> };
>
>class Demo
> {
>  public:
>    void Insert(A *Temp);
>    void Insert2(A *Temp);
>    void Test(void);
>    A *First;
> };
>
>class B : public A
> {
>  void Print();
> };
>
>class C : public A
> {
>  void Print();
> };
>
>main()
> {
>  Demo *T = new Demo;
>  A *Test = new B;
>  T->Insert(Test);
>  Test = new C;
>  T->Insert2(Test);
>  T->Test();
>  return 0;
> }
>
>void B::Print()
> {
>  cout << "I am in class B" << '\n';
> }
>
>void C::Print()
> {
>  cout << "I am in class C" << '\n';
> }
>
>void Demo::Insert(A *Temp)
> {
>  First = Temp;
> }
>
>void Demo::Insert2(A *Temp)
> {
>  First->next = Temp;
> }
>
>void Demo::Test(void)
> {
>  First->Print();
>  First->next->Print();
> }
>
>void A::Print()
> {
>  cout << "I am in class A" << '\n';
> }
>

Make Print() virtual:
    virtual void Print();

If you don't say "virtual" in the declaration, then C++ uses the static
type - that is the type you declare for the variable - to select the
function to call. Since Demo::First and A::Next are declared to be A*,
the compiler selects A::Print no matter what the actual type is.
Declaring Print to be virtual changes all that. Virtual tells the
compiler to use the actual type when selecting the Print function to
use.

giuliano