从C++中具有多重继承的类派生的类：派生类试图调用根基类构造函数

Question

I'm sorry if this feels like a cheap sequel to my last question.

I have a diamond inheritance where D is derived from both B and C , who in turn are both derived (virtually) from A . A , B and C are abstract, and thanks to the answers to my previous questions the compiler is now aware of it and all is fine.

Now, I need to create a class E derived from D . As far as I know, normally the constructor E::E should call D::D , and it would be D::D 's job to call all of A::A , B::B , and C::C .

But my compiler really insists on having E::E call A::A itself.

Here is a simple example I made:

class A {                       //abstract
    protected:
        A(int foo) {}
        virtual void f() =0;
};

class B: public virtual A {     // abstract
    protected:
        B() {}
};

class C: public virtual A {     // abstract
    protected:
        C() {}
};

class D: public B, public C {   // concrete
    public:
        D(int foo, int bar) :A(foo) {}
        void f() {}
};

class E: public D {             // concrete
    public:
        E(int foo, int bar, int buz) :D(foo, bar) {}
};


int main()
{
    return 0;
}

And here is the compilation error:

$ g++ test.cpp
test.cpp: In constructor ‘E::E(int, int, int)’:
test.cpp:25:49: error: no matching function for call to ‘A::A()’
   25 |         E(int foo, int bar, int buz) :D(foo, bar) {}
      |                                                 ^
test.cpp:3:9: note: candidate: ‘A::A(int)’
    3 |         A(int foo) {}
      |         ^
test.cpp:3:9: note:   candidate expects 1 argument, 0 provided
test.cpp:1:7: note: candidate: ‘constexpr A::A(const A&)’
    1 | class A {                       //abstract
      |       ^
test.cpp:1:7: note:   candidate expects 1 argument, 0 provided
test.cpp:1:7: note: candidate: ‘constexpr A::A(A&&)’
test.cpp:1:7: note:   candidate expects 1 argument, 0 provided

I know the virtual inheritance is correct and I know the compiler knows which classes I intend to be abstract and which I intend to be instantiable, because if I remove class E , the code compiles.

What am I missing?

Answer 1

But my compiler really insists on having E::E call A::A itself.

Like I explained in anwer to your previous question: "the constructor of the most derived class calls the constructor of the virtual base" .

All non-abstract classes in a hierarchy that contains virtual bases must correctly initialise the virtual bases because they can potentially be instantiated as the most derived class. For example, if you create an instance of E , then it is the constructor of E that initialises the virtual base A .

In your code, the constructor of E attempts to use the default constructor of A by omitting the initialiser. But A is not default constructible, so the program is ill-formed.

What am I missing?

The initialiser for the virtual base A in the constructor of E .

As far as I know, there is no way to delegate the construction to of the virtual base to another concrete base such as D .