Re: function without a definition

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 25 Oct 2009 03:46:22 -0700 (PDT)

Message-ID:

<cb3a3609-3902-4c7d-bf4f-85ba5a6f329d@e18g2000vbe.googlegroups.com>

On Oct 24, 5:38 pm, thomas <freshtho...@gmail.com> wrote:

//-----------------------------------------------------------
#include<iostream>
#include<cstring>
using namespace std;

class B1{
public:
        virtual void fun() = 0;
};

class B2{
public:
        void fun();
};

class A:public B1, private B2{
public:
        A(){
                B1::fun();
                B2::fun();
        }
};

int main(){}
//-------------------------------------------

The above code links well but B1::fun() and B2::fun() are not defined.

It's undefined behavior. The compiler/linker aren't required to
detect the error. Typically, most will *if* the constructor of
A is not inline, or if it is ever used.

//----------------------------------------------
#include<iostream>
#include<cstring>
using namespace std;

class B2{
public:
        B2(){}
        void fun();
};

int main(){
        B2 b;
        b.fun();
}
//---------------------------------------

In this case, the func() definition miss causes a link error.

Any explanation?

Again, failing to provide a needed definition is undefined
behavior. Depending on what you do, it may or may not be
detected as an error.

--
James Kanze