Re: What the hell is dynamic_cast for?

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 28 May 2008 13:48:22 -0400

Message-ID:

<g1k5t7$jis$1@news.datemas.de>

lovecreatesbea...@gmail.com wrote:

Suppose I have the following three classes,

GrandBase <-- Base <-- Child <-- GrandChild

The following cast expression holds true only if pBase points object
of type of ``Child'' or ``GrandChild'', i.e. types not upper than
Child in the above class hierarchy,

dynamic_cast<Child*>pBase

Do I draw this correctly?

You missed the parentheses and your class has to be polymorphic (have at
least one virtual function). That's first. And second, the expression

dynamic_cast<Child*>(pBase)

has the type 'Child*', and not "holds true", although it will evaluate
to (Child*)0 if the cast fails.

Up-casting is safe while down-casting is not. dynamic_cast seems to do
down-casting, so why is dynamic_cast intenting to do the dangerous
work?

Not sure what you're asking here. 'dynamic_cast' is allowed to *fail*
and you can either catch the 'std::bad_cast' exception or check the
result (for pointers the result is a null pointer, for references the
exception is thrown).

V
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