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From: andrew@hrc63.co.uk (Andrew Haylett "Baddow")
Newsgroups: comp.lang.c++
Subject: Virtual functions and missing constructors
Message-ID: <583@hrc63.co.uk>
Date: 9 May 89 11:36:49 GMT
Organization: GEC Hirst Research Centre, Wembley, England.
Lines: 40

A mysterious problem with C++: your help would be appreciated.

The problem seems to be that when an instance of a derived class is
created and the only matching constructor is in the base class, this
constructor is used. Fair enough, but this leaves the virtual function
pointer referring to the base class virtual function. Is this:

(a) a language feature
(b) a compiler feature (we use Oregon C++, version 1.2a)

----- cut here ----------------------------------------

#include <stream.h>

class A {
public:
    A() { };
    A(int z) { };
    virtual void f() { cout << "a\n"; };
};

class B : public A {
public:
    B() { };
    // B(int z) { };	// This constructor is missing
    void f() { cout << "b\n"; };
};

main()
{
    A a (1);	// instance of A, one argument
    B b1   ;	// instance of B, calls B's constructor
    B b2(1);	// instance of B, calls A's constructor

    a.f();	// says "a"
    b1.f();	// says "b"
    b2.f();	// says "a" -- WRONG -- should be "b"
}

Andrew Haylett, GEC Research