Initialization of a Virtual Base Class.

In a Inheritance hierarchy including virtual base classes, for ex:

           A
          / \  (virtual inheritance of B,C)
         /   \
        B     C
         \   /
          \ /
           D

class A{};
class B : virtual public A{};
class C : virtual public A{};
class D : public B, public C{};

Since the A base class occurs twice in class C, how is it ensured that its initialized or contructed only once, and not twice ( via B's and C's ctor )?

Answer:*

In this case, class D (the most derived class) can make direct calls to the ctor of its super base class A (which is not allowed in cases not involving virtual inheritance ). If this is not done, then the default ctor of class A will be called and then the ctors of B and C are called.

 1 class A
  2 {
  3     public:
  4         A(int a){}
  5         A(){}
  6 };
  7 class B : virtual public A{};
  8 class C : virtual public A{};
  9 class D : public B, public C{
 10     public:
 11         D():A(1),B(),C(){} 
 12 };
 13     
 14 int main()
 15 {
 16     D d;
 17 }

If the inheritance was not virtual, then D's ctor cannot access A's ctor.