Default constructor and virtual inheritance

Question

Is possible to have virtual inheritance for class not providing default constructor?

The present diamond diagram (the simplest one with the only change of no default constructor provided) does not compile (g++ 4.4.3).

class A {
 public: 
  A(int ) {}
};
class B : virtual public A {
 public:
  B(int i) : A(i) {}
};
class C : virtual public A {
 public:
  C(int i) : A(i) {}
};
class D : public B, public C {
 public:
  D(int i) : B(i), C(i) {}
};

Thanks, Francesco

Answer 1

I believe your class D also needs to explicitly call A's constructor in its initializer list.

Answer 2

You need to call A's constructor explicitly here

 D(int i) : A(i), B(i), C(i) {}

virtual base classes are special in that they are initialized by the most derived class and not by any intermediate base classes that inherits from the virtual base. Which of the potential multiple initializers would the correct choice for initializing the one base?

If the most derived class being constructed does not list it in its member initalization list then the virtual base class is initialized with its default constructor which must exist and be accessible.

Shamelessly copied from here :-)

Answer 3

The Dr. Dobbs article Multiple Inheritance Considered Useful explains various ways of dealing with this. The recommendation is basically to provide default constructors and init() methods. It adds more work for B and C but prevents D from having to know about A.