I wrote this code so I can understand more about c , I know that I get this warning because of the virtual inheritance in class B and class C is not virtual public inheritance ,and I know that this warning can go if I changed it .

but what I don't understand is : why my code works and why it wont get this warning if I only made this change ---> class C :virtual public A and not made the change for both classes (C and B) :

 #include <iostream>
using namespace std;
class A
{
public:
    A() { cout << "A"; }
    A(const A&) { cout << "a"; }
};
class B : virtual A
{
public:
    B() { cout << "B"; }
    B(const B&) { cout << "b"; }
};
class C : virtual A
{
public:
    C() { cout << "C"; }
    C(const C&) { cout << "c"; }
};
class D :B, C
{
public:
    D() { cout << "D"; }
    D(const D&) { cout << "d"; }
};
int main()
{
    D d1;
    D d2(d1);
}

: warning C4594: class 'D' can never be instantiated - indirect virtual base class 'A' is inaccessible

: message : 'A' is a private base class of 'B'

CodePudding user response：

Actually protected instead of public would suffice...

Now assume your classes would not inherit virtually.

What now happens is that any instance of B incorporates its own instance of A as well as does any instance of C. Both B and C call A's constructor already, an inheriting class D doesn't need to care for.

This changes if virtual inheritance is introduced: Now the task of constructing the sole instance of A is delegated to the most derived class, which is D in given case.

D now needs access to this common instance of A to be able to call it's constructor. This can occur via B or (!) via C – so it suffices if at least one of these provides at least protected access to that single instance – C in given case but could have been B as well.