Re: About instantiation of an abstract class

From:

"Giovanni Dicanio" <giovanniDOTdicanio@REMOVEMEgmail.com>

Newsgroups:

microsoft.public.vc.language

Date:

Sat, 18 Apr 2009 11:57:02 +0200

Message-ID:

<usG4LwAwJHA.5516@TK2MSFTNGP02.phx.gbl>

"Jack" <jl@knight.com> ha scritto nel messaggio
news:eymK7gAwJHA.5684@TK2MSFTNGP03.phx.gbl...

If I have something like the following,

Mtl* pkMtl = pNode->GetMtl();

where GetMtl() is pure abstract
like
class INode {

virtual Mtl* GetMrl() = 0;
};

When I called GetMtl() in the statement above,
pkMtl became NULL.

pNode should point to an instance of some class derived from INode, that
implements the GetMtl() method (which is declared as pure virtual in INode).

Have you tried following the code step-by-step using a debugger?

I have been thinking about writing
Mtl* pkMtl = new Mtl();
pkMtl = pNode->GetMtl();

This will cause a leak (memory leak, and in general leak of resources
allocated in Mtl constructor), beacuse in the first statement you allocate a
new instance of Mtl() on the heap, but then in the second statment you loose
the pointer pkMtl returned by new (because you overwrite it with the return
value of pNode->GetMtl()).

Giovanni