Question

I have a classical virtual inheritance diamond:

class A {
protected:
    A(const char *x) { ... }
}

class B: public virtual A {
protected:
    B(): A(NULL) { ... }
public:
    virtual void foo() = 0;
}

class C: public virtual A {
protected:
    C(): A(NULL) { ... }
public:
    virtual void bar() = 0;
}

class D: public B, public C {
public:
    D(const char *x): A(x) { ... }
    void foo();
    void bar();
}

I use NULL in B and C because as they’re abstract classes, the A ctor will never be called in their constructurs. Is there a nicer way to do it, otherwise than specify NULL in the constructor or declare parameterless constructor in A? I want the constructor to be called with a parameter, therefore the A() {} ctor should be allowed only in abstract classes.

Answer 1

I am closing this as the rigth answer probably does not exist. IMO the best workaround is the

struct AbstractPlaceholder {
    AbstractPlaceholder() {
        assert(false);
    }
};

class A {
protected:
    A(const AbstractPlaceholder &ap) {}
    A(const char *x) { ... }
};