Inheritance from multiple inheritance

Question

Have a problem with multiple inheritance. I have solved diamond problem:

class A
{
    int m;
    int n;
public:
    A(int x, int y)
    {
        m = x; n = y
    }
    fA() {}
};

class B : virtual public A // B has fA(),fB()
{
public:
    B(int k) : A(1, k) {}
    fB() {}
};

class C : virtual public A // C has fA(),fC()
{
public:
    C(int k) : C(2, k) {}
    fC() {}
};

class D : public B, public C // D has fA(),fB(),fC()
{
public:
    D(int k) : B(k),C(k),A(3,k)
};

This is working well. Problem with this:

class S : public B // S has fA(),fB()
{
public:
    S() : B(6) {}
};

Compilator shows me: "error: no matching function for call to `A::A()'"

This code is working, but it doesn't satisfied me:

class S : public B // S has fA(),fB()
{
public:
    S() : B(6),A(1,6) {}
};

The most derived class is responsible for constructing a virtual base. That's just how the language works. — T.C.
– T.C., Commented Dec 4, 2014 at 22:22
Can you please check your code listings to be correct C++? There are a number of reasons this code won't compile and it's better if we don't have to guess which ones are related to your question and which ones are just typos. — 5gon12eder
– 5gon12eder, Commented Dec 4, 2014 at 22:24

jrok · Accepted Answer · 2014-12-04 22:23:24Z

In virtual inheritance, the constructor of virtual base is called from the constructor of most derived class:

class S : public B // S has fA(),fB()
{
public:
    S() :   B(6) {}
};  //    ^  A base class is initialized at this point, before B

This also means that other explicit calls to A's constructor in initialization-lists further down the inheritance chain are ignored:

class B : virtual public A // B has fA(),fB()
{
public:
    B(int k) : A(1, k) {}
    fB() {} // ^^^^^^^    this call is ignored when B is a part of S object
};

If there's no explicit call to virtual base's constructor in initialization list of most derived class, the compiler will (of course) try to call the default constructor. But A doesn't have one and that's your problem.

One solution you already discovered yourself. The other is to write default constructor for A.

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