How C++ virtual inheritance is implemented in compilers?

Question

How the compilers implement the virtual inheritance?

In the following code:

class A {   public:     A(int) {} };  class B : public virtual A {   public:     B() : A(1) {} };  class C : public B {   public:     C() : A(3), B() {} }; 

Does a compiler generate two instance of B::ctor function, one without A(1) call, and one with it? So when B::constructor is called from derived class's constructor the first instance is used, otherwise the second.

Answer 1

It's implementation-dependent. GCC (see this question), for example, will emit two constructors, one with a call to A(1), another one without.

B1() B2() // no A 

When B is constructed, the "full" version is called:

B1():     A(1)     B() body 

When C is constructed, the base version is called instead:

C():     A(3)     B2()        B() body     C() body 

In fact, two constructors will be emitted even if there is no virtual inheritance, and they will be identical.