Re: function without a definition

From:

"Johannes Schaub (litb)" <schaub-johannes@web.de>

Newsgroups:

comp.lang.c++

Date:

Sat, 24 Oct 2009 21:02:46 +0200

Message-ID:

<hbviu1$d6f$03$1@news.t-online.com>

thomas 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.

//----------------------------------------------
#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?

This is entirely dependent on the implementation. It may diagnose it, and it
may not. In your case, it diagnoses the second case, but not the first.

From the language point of view, both are ill-formed, but without requiring
a diagnostic from the implementation. Since the definition of A::A happens
in-class, it's inline and the compiler knows that inline functions must be
defined in any TU in that they are used. So it can define it according to
the TU's needs as long as you don't observe the difference. And therefor, it
can optimize out both calls. Try changing to this:

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

You may have more luck getting a diagnostic with that.