Re: questions about dynamic binding

From:
=?ISO-8859-1?Q?Erik_Wikstr=F6m?= <Erik-wikstrom@telia.com>
Newsgroups:
comp.lang.c++
Date:
Sat, 12 May 2007 11:04:23 GMT
Message-ID:
<Xah1i.172$Tk3.258@newsb.telia.net>
On 2007-05-12 12:13, Jess wrote:

Hello,

I have some questions to do with dynamic binding. The example program
is:

#include<iostream>

using namespace std;

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

class B:public A{
public:
  void f(){cout << "B::f()" << endl;}
};

int main(){
  B b;
  A* bp = &b;
 // A* bp2 = new B(*bp);
  A* bp2 = new A(*bp);
  bp2->f();
  bp->f();
  return 0;
}

As I expected "bp->f()" calls B's f(). However, "bp2->f()" calls A's
f(). Is it because of the "new A", which only copies the A's part of
"*bp" object, or is it because of the behaviour of the synthesized
copy constructor of A?


It's because new A() will create a new object of type A, so when doing
bp2->f() it will call the f() method of the object pointed to by bp2
(which happens to be of type A). When you do bp->f() the type of the
object pointed to is of type B so its f() is called.

If it is the latter, then can I create a B object by defining my own
copy constructor for A?


No, any constructor in an object (regardless if it's a normal
constructor or a copy-constructor) of type T will create an object of
type T, so when you do 'new A()' the type of the object will always be A
(since that is what you specified).

In addition, the statement that is commented out produced compiler
error. It says "no matching function for call to 'B::B(A&)'". Can't
compiler find out *bp is in fact a B object?


No, bp is a pointer to an object of type A, you can either cast the
pointer to a pointer of type B or create a constructor in B which takes
an A as argument.

--
Erik Wikstr?m

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