Question

Consider this code:

struct A {};

struct B
{
    B(A* a) : a(a) {}
private:
    A* a;
};

struct base
{
    base(B b) : b(b) {}
protected:
    A a;
private:
    B b;
};

struct derived : public base
{
    derived() : base(B(&a)) // <-- IS THIS OK?
    {}
};

Here, the base class needs a B object passed to its constructor by the derived class, and the B object refers to an A object, but the A object lives inside the base class.

The constructor of the B object does not do anything to the A pointer except store it, so I’m thinking this should be OK, but it still feels wrong because technically the A object doesn’t yet exist at the time I’m passing it to the base constructor.

Can I get into trouble doing this or should this be OK?

Answer 1

It is safe as long as you don’t use a in B‘s constructor, as the object a isn’t constructed yet.

I mean, you shouldn’t dereference the pointer a in B's constructor; however after base‘s constructor is executed, you can safely dereference B::a in other methods of B.