More complex diamond problem instance. How to make it work with constant fields?

Question

Let us consider ordinary diamond problem but slightly improved. Now class A have constant fields.

struct A {
   const int a;

   A (int _a): a(_a) {}
};

struct B: virtual A {
   const int b;

   B(int _b, int _a): A(_a), b(_b) {}
};

struct C: virtual A {
   const int c;

   C(int _c, int _a): A(_a), c(_c) {}
};

struct D: B, C {
    D(int _a, int _b, int _c): ??? {}
};

So, what would be sensible to write instead of question marks? Or maybe it can be solved different way?

Answer 1

The constructor may be written as follows:

D(int _a, int _b, int _c): A(_a), B(_b, _a), C(_c, _a) {}

Because A is a virtual base class, the D constructor must initialize it despite the fact that it is not a direct base class. When an object of most derived type D is being constructed, D::D will first initialize A, and then run the constructors for B and C. When the B and C constructors run, they will ignore the virtual base class A, since D already initialized it.