Re: Templates and inheritance

From:

Maxim Yegorushkin <maxim.yegorushkin@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 16 Oct 2008 08:03:30 -0700 (PDT)

Message-ID:

<a5c91111-d540-4f79-9cd9-d9f9aa182efd@s50g2000hsb.googlegroups.com>

On Oct 16, 3:35 pm, Isaac Gelado <igel...@gmail.com> wrote:

I am having problems with inheritance in templates classes. Say I
have the following classes:

class A {};
class B: public A {};
template<typename T> class C {};

Now in my code I have something like:

C<A *> *myAPtr;
myAPtr = new C();

I'm getting an error saying that conversion from C to C<A *> is
not possible.

The actual error is that C<B*>* can not be converted to C<A*>*.

This is expected, as each instantiation of a class template produces a
distinct class, unless template arguments are the same.

Does it mean I can no use subclassing with templates
without using ugly casts?

To make derived-to-base conversion from C* from C<A>* work C has
to derive from C<A>.

If, on the other hand, you just want to make a conversion from C to
C<A> possible, you need to have a template conversion constructor,
just like the standard smart-pointers do (std and boost):

template<class T> struct C
{
template<class U>
C(C const&);
};

--
Max