Having a subclass accept a reference to a parent in a constructor

Question

I have a class hierarchy that can be simply put like this:

struct Parent {
    Parent() { }
    Parent(Parent& p, std::string s) { }

private:
    // I want this class to be non-copyable
    Parent(const Parent&);
};

struct Child : public Parent {
    Child() { }
    Child(Parent& p) : Parent(p, "hi") { }
};

When I try to create two instances like this:

Child c1;
Child c2(c1);

I get the following error from Clang:

test.cpp:37:8: error: call to deleted constructor of 'Child'
        Child c2(c1);
              ^  ~~
test.cpp:30:8: note: function has been explicitly marked deleted here
struct Child : public Parent {
       ^
1 error generated.

I want this class to be non-copyable, so is there a way to have the Parent& overload called instead of the copy constructor? I know why it behaves the way it does but I am looking for a workaround. I would like Child(Parent& p) to be called without having to cast it.

I get this error in GCC and Visual Studio as well. I don't get it with Intel's compiler though, but the consistent behaviour of the other three seem to indicate that it's wrong and the others are right.

Answer 1

You're relying on the auto-generated copy constructor of the Child while making the copy constructor of the parent inaccessible. What seems to be happening is that the auto-generated copy constructor for Child is trying to call the copy constructor for the parent, but it can't because it's private.

If you don't want to add in a copy constructor, it looks like you need to either explicitly cast c1 to parent reference:

Child c2(static_cast<Parent&>(c1));

or declare copy constructor for Child.

Apart from that, I don't believe there is a workaround.

Answer 2

You say that you don't want your class to be copyable.

You also say in your question that you want the following snippet to work:

Child c1;
Child c2(c1);

Those requirements are contradictory.