Implicit cast of class derived from template base class

问题

I've got a problem with implicit casting, templates and inheritance from template classes. The following is what I extracted from my project, I have left out that some classes are even abstract, but it does not have to do with the case.

class A {};
class B : public A {};

template <typename T> class Base {};
class Derived : public Base<B> {};

int main() {
    Derived d;
    Base<A>* base = new Derived();
}

Basically, I have a template base class Base that I derive Derived : public Base<B> from. Then I have to cast it to the most general occuring form of Base, which is Base<A>.

I would have thought that I can cast an Object deriving from Base<B> to Base<A> implicitly, as B derives from A. Am I doing something wrong or how could I cast that implicitly? This is important as I need to accept all types of Derived classes in a method of Base as a parameter.

Thanks in advance.

回答1:

This is not possible. Base<A> has no relation to Base<B>, regardless of the relation between A and B.

回答2:

Base<B> does not necessarily need to have a relation to Base<A>. This doesn't have anything to do with Derived. If you want to force such a relation, you're going to need a templated constructor.

template <typename T>
class Base
{
    template <typename TOther>
    Base(const TOther& that)
    {
        // ...
    }
    // ...
};

Obviously, the implementation would have to depend on the actual implementation of Base. Note that this constructor doesn't substitute for the normal copy constructor.

来源：https://stackoverflow.com/questions/6238544/implicit-cast-of-class-derived-from-template-base-class