Polymorphism, why can't I do this?

Question

class BaseA
{
}

class B : public BaseA
{
}

template <class T>
class C : public vector<T>
{
}

void someFunction (void)
{
    C<B> myClass;

    // I can't seem to do this...is it not possible?
    vector<BaseA> converted = ((vector<BaseA>) myClass);
}

See comment in code for what I am trying to do.

Answer 1

A vector of B isn't a vector of A even if B is an A (I assume a mixup between A and BaseA)

Try

vector<A> converted(myClass.begin(), myClass.end());

which is probably what you want to express.

(BTW inheriting from vector is bad idea in general, it isn't designed for that.)

Answer 2

C<B> and vector<A> are completely unrelated types and you would have to explicitly define such a conversion (e.g. template <typename TargetValueType> explicit operator vector<TargetValueType> () { ... }).

If such conversion is an uncommon task, something that is not really natural to the nature of your class, it might be preferable to use a range constructor (see AProgrammer's answer).

Also: If vector is std::vector, deriving from it is not a good idea. It isn't defined as a base class, and typically you should prefer containment over derivation (as it imposes looser coupling on your clients).