Re: Calling methods based on derived type

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 26 Apr 2007 11:35:15 -0400

Message-ID:

<f0qgrk$ick$1@news.datemas.de>

flopbucket wrote:

Say I have a baseclass B that has many derived classes, let's say
C..Z, for example:

class B
{
};

class C : public B {};
class D : public B {};
class E : public B {};
...

Also assume that these classes are provided as is, that is, I can not
make any changes to them (the real classes contain many data members,
etc).

Now, there is a factory method that creates the correct class based on
input from some external source. Something like:

B *b = SomeFactoryMethod();

Of course SomeFactoryMethod will actually return any of the dervied
classes depending on the external information.

Now here is my question. There are a set of methods such as:

processObject(C *);
processObject(D *);
processObject(E *);
... for all derived types.

Given the base clas pointer returned from the factory, how can I call
the correct method? Obviously I can have a bunch of dynamic_cast<>'s
but that does not seem the best way.

I think that's the only way available. Wrap it in a function and you
are going to have a very clear and concise way. I would even create
some kind of table for either using in a 'switch' statement or for
dispatching the call (through a function pointer).

I was thinking I could do
something with templates and typeinfo?

I don't see how it's better. 'typeid' will give you the most derived
class. What if somebody derives something from 'C' (making 'CC' class)
so you won't be able to compare type_info for 'C' with it. 'dynamic_cast',
OTOH will definitely tell you whether the class is part of the hierarchy.

Any ideas? Please remember that I can not change the existing
classes.

I believe you'd be better off with dynamic_cast (or something based on
it).

V
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