C++ copy constructor in inheritance

Question

#include<iostream>
using namespace std;
class A
{
public:
     A(){ cout <<"1";}
     A(const A &obj){cout <<"2";}
};

class B: virtual A
{
public:
    B(){cout <<"3";}
    B(const B & obj):A(obj){cout<<"4";}
};

class C: virtual A
{
public:
   C(){cout<<"5";}
   C(const C & obj):A(obj){cout <<"6";}
};

class D:B,C
{
public:
    D(){cout<<"7";}
    D(const D & obj):C(obj),B(obj){cout <<"8";}
};

int main()
{
   D d1;
   D d(d1);
}

I am getting 13571468 as output. But I think that output should be 13572468. Why normal constructor is running instead on of copy constructor of class A?

Answer 1

Your code makes a copy of an instance of D, invoking its copy constructor.

Your class D's copy constructor only invokes the copy constructors of its C and B's superclasses. Because it does not invoke A's copy constructor, it gets default-constructed.

Virtually-inherited classes can be thought of as direct superclasses of the most-derived class. That's what virtual inheritance means. As such, in your instance of D, its virtually-inherited A is a direct superclass of D, and not of B or C; as such, B and C's invocations of A copy-constructor is not invoked.

When you have a virtually-inherited class, all your constructors really have two versions created "behind the scenes": one that's responsible for constructing any virtually-inherited classes, and one that's not. The one that's not does not call the virtually-inherited classes's constructors.