Re: Problem with static downcast of base type to derived type
On Jul 2, 11:29 pm, Dom Jackson <nos...@mci2000.com> wrote:
Thanks folks - I had no idea that you couldn't put a derived
type in a set of base types. I can't immediately see anything
in Josuttis about this.
What should he say? He certainly says that the containers
contain objects of type T. In C++, an object of type T is an
object of type T, and not an object of some other type.
Polymorphism only works through references and pointers.
Can anyone tell me what the problem is here?
There is no problem. In C++, containers contain objects, with
value semantics.
Is this common to all STL containers? I had rather naively
assumed that an object of a derived type actually was, to all
intents and purposes, also a base object, but that's obviously
not the case.
It's never the case. An object of a derived type is an object
of a derived type, and an object of a base type is an object of
a base type. Every object has one, and only one type. This is
a fundamental aspect of the C++ object model (and also the
object model of many other languages).
Written correctly, an object of a derived type can be used as an
object of a base type, but only if the "use" involves an
indirection (reference or pointer). The object still has a
specific type, which never changes (but a pointer or a
reference can designate different objects).
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