Re: function without a definition

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