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.
You can change A
to:
class A {
private:
A() {};
friend class B;
friend class C;
protected:
A(const char *x) { }
};
And then B(): A() {}
and C(): A() {}
would work, yet D(const char*): A() {}
won't. But this is really awkward. I'd stick with the NULL
you're using at the moment.
This is indeed an interesting case, though. I could think of no technical reason why you'd have to specify a constructor for A
in B
and C
given they'll never be created, and whoever inherits them is going to initialize A
anyway.
You can specify default argument in A(const char*)
class A {
protected:
A(const char *x = 0) { ... }
^^^^
};
However, this will also allow D
to avoid A()
.
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) { ... }
};
A variation of your approach would be
class A {
protected:
A(const char *x = NULL) {
assert(x && "A mustn't be default constructed!");
}
};
Thereby adding a more meaningful diagnostic.
However, you may want to explicitly allow x
to be NULL
(as a legal construction via C
), then you could use a Maybe
type.
template <typename T> class Maybe {
T const t; // must be default constructible!
bool const invalid;
public:
Maybe() : t(), invalid(true) {}
Maybe(T t) : t(t), invalid(false) {}
bool nothing() const {
return invalid;
}
T just() const {
assert(!invalid);
return t;
}
};
Then you can change your constructor to
A::A(Maybe<const char*> mx) {
// either
assert(!mx.nothing());
// or
mx.just();
}
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