c++ multiple inheritance and explicit constructor calls

Question

Consider we have a parent class A that has protected members x and y and a constructor:

A::A(int xpos, int ypos) : x(xpos), y(ypos) {}

Now consider we have two classes B and C that inherit from A and have constructors as defined below.

class B : public virtual A
B::B(int xpos, int ypos) : A(xpos,ypos) {}

class C : public virtual A
C::C(int xpos, int ypos) : A(xpos,ypos) {}    

Finally let's have a class D that inherits both B and C.

class D : public B, public C

If I write Ds constructor as follows I get a compile error saying that Ds constructor must explicitly call As constructor.

D::D(int xpos, int ypos) : B(xpos,ypos), C(xpos,ypos) {}  

Why is this? Surely this is problematic if I attempt to inherit from classes that I don't know anything about the classes constructor? Isn't it enough that B and C both explicitly call As constructor?

Answer 1

The rule for a virtual base is that the most-derived class calls its constructor. If that wasn't the case, which of the intermediate base classes should construct the one copy of the virtual base? Pretend for a moment that the constructor for your class B reverses the order of the arguments when it constructs A:

B::B(int xpos, int ypos) : A(ypos, xpos) {}

Now, what should the state of the A sub-object be? First constructor wins? Last one wins? Either way, it's fragile: changing the order of B and C in your D type would change how the A base class gets initialized.