Cast from B to base class A

Question

I have the following classes:

template<typename T>
class Vector { ... };

template<typename T>
class Vector2 : public Vector<T> { ... };

Now, i would like to be able to cast a Vector to a Vector2 -even if the object is not really a Vector2- (I mean, dynamic_cast is not what I'm looking for)

What should I implement?

1. the cast operator on Vector operator Vector2 ()
2. the constructor on Vector2 Vector2(const Vector<T> &)
3. both

If i should implement both, when will the cast operator, and when the constructor be calld?

Answer 1

I would implement an explicit constructor Vector2 that takes a Vector<U> -- since you might want to use unrelated types that can be converted from one to another.

template <typename T>
class Vector2 : public Vector< T >
{
public:
    template <typename U>
    explicit Vector2(const Vector< U >& copyFrom)
    {
        // ...
    }
};

Implicit convertion might lead to unpredictable behaviors. Kids, don't try this at home.