pure virtual - C / C++

I don't understand. What's the difference in practice between 2 next lines?
When I *must* use first line and when second?
-------------
virtual int a(int b) = 0;
virtual int a(int b);
-------------

thank you

Jul 19 '05 #1

Subscribe Reply

3633

Siana

On Sat, 27 Sep 2003 20:13:31 +0300, "<- Chameleon ->"
<ch******@hotma il.NOSPAM.com> wrote:

I don't understand. What's the difference in practice between 2 next lines?
When I *must* use first line and when second?
-------------
virtual int a(int b) = 0;
virtual int a(int b);
-------------

thank you

The first line declares a function a in some class that has no
definition - it is there only to be overridden by derived classes for
the purposes of polymorphism. It also makes the class abstract (unable
to be instantiated). Derived classes must declare and implement this
function or else they too are abstract and un-instatiable. It is
useful in defining an interface.

The second line declares a function a in some class that must have an
implementation for that class. That implementation will be inherited
by derived classes but can still be overridden and perform
polymorphically .

S.

Jul 19 '05 #2

Thomas Wintschel

"<- Chameleon ->" <ch******@hotma il.NOSPAM.com> wrote in message
news:bl******** **@nic.grnet.gr ...

I don't understand. What's the difference in practice between 2 next lines? When I *must* use first line and when second?
-------------
virtual int a(int b) = 0;
virtual int a(int b);
-------------

thank you

You may want to look up 'pure virtual'

The first one would be used when defining a pure virtual base class. This
is done in situations where code needs to call the method 'a' for objects of
types which derive from the class but where you need the derived class to
implement the actual method (derived class *must* implement the method). If
we define:

class C
{

Jul 19 '05 #3

Kevin Goodsell

Siana wrote:

On Sat, 27 Sep 2003 20:13:31 +0300, "<- Chameleon ->"
<ch******@hotma il.NOSPAM.com> wrote:

I don't understand. What's the difference in practice between 2 next lines?
When I *must* use first line and when second?
-------------
virtual int a(int b) = 0;
virtual int a(int b);
-------------

thank you
The first line declares a function a in some class that has no
definition

That is not true in general. A pure virtual function certainly may have
a definition.
- it is there only to be overridden by derived classes for
the purposes of polymorphism. It also makes the class abstract (unable
to be instantiated). Derived classes must declare and implement this
function or else they too are abstract and un-instatiable. It is
useful in defining an interface.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

Jul 19 '05 #4

Ron Natalie

"Siana" <si************ **************@ yahoo.ca> wrote in message news:vh******** *************** *********@4ax.c om...

The first line declares a function a in some class that has no
definition
Wrong answer. It is the designation of the function as pure virtual.
It does not preclude the function from having a definition. It makes
the class it is defined in abstract. An abstract class is one that can
not be instantiated. It serves only as a base class for others.
- it is there only to be overridden by derived classes for
the purposes of polymorphism.
Not quite, it means that it must be overridden in dervied classes
(as well as all other pure virtual functions) in order to be concrete
(that is, one that can have instances created of it).
The second line declares a function a in some class that must have an
implementation for that class. That implementation will be inherited
by derived classes but can still be overridden and perform
polymorphically .

No it just declares one that is not pure. It means that it doesn't make
the class abstract.

While it is the case that you can omit the implementation of a pure virtual
function, that is secondary to it's meaning.

Jul 19 '05 #5

jeffc

"<- Chameleon ->" <ch******@hotma il.NOSPAM.com> wrote in message
news:bl******** **@nic.grnet.gr ...

I don't understand. What's the difference in practice between 2 next lines? When I *must* use first line and when second?
-------------
virtual int a(int b) = 0;
virtual int a(int b);
-------------

There is no time when you *must* use the first (pure virtual). It's a
design decision on your part. There are 2 reasons you might use it
a) you want to force any class that inherits from this class to define
function a
b) you want that class to be abstract (no one can make an instance of it -
you can only make instances of subclasses). This is a rather confusing
means of creating an abstract class, but that's the way it is.

Jul 19 '05 #6

jeffc

"Thomas Wintschel" <th******@telus .net> wrote in message
news:KYkdb.1203 7$o21.6208@edtn ps84...

The first one would be used when defining a pure virtual base class.

You mean an abstract class.

Jul 19 '05 #7

Ron Natalie

"jeffc" <no****@nowhere .com> wrote in message news:3f******** @news1.prserv.n et...

"Thomas Wintschel" <th******@telus .net> wrote in message
news:KYkdb.1203 7$o21.6208@edtn ps84...

The first one would be used when defining a pure virtual base class.

You mean an abstract class.

Now you're being pedantic :-)

Jul 19 '05 #8

jeffc

"Ron Natalie" <ro*@sensor.com > wrote in message
news:3f******** *************** @news.newshosti ng.com...

"jeffc" <no****@nowhere .com> wrote in message

news:3f******** @news1.prserv.n et...

"Thomas Wintschel" <th******@telus .net> wrote in message
news:KYkdb.1203 7$o21.6208@edtn ps84...

The first one would be used when defining a pure virtual base class.

You mean an abstract class.

Now you're being pedantic :-)

I knew somone was going to say that :-) The point to my way of thinking is
that "pure virtual base class" is not an OO term, it's not a C++ term, it's
just not any term I've ever heard. I did not of course object to "method",
nor would I even have objected if he used the term "abstract method" for
pure virtual function. I'd be interested in knowing if that's what Thomas
really meant to write and where he heard it. Maybe I should have said "You
mean an abstract class, right?"

Jul 19 '05 #9

Ron Natalie

"jeffc" <no****@nowhere .com> wrote in message news:3f******** @news1.prserv.n et...

"Ron Natalie" <ro*@sensor.com > wrote in message
news:3f******** *************** @news.newshosti ng.com...

"jeffc" <no****@nowhere .com> wrote in message

news:3f******** @news1.prserv.n et...

"Thomas Wintschel" <th******@telus .net> wrote in message
news:KYkdb.1203 7$o21.6208@edtn ps84...
>
> The first one would be used when defining a pure virtual base class.

You mean an abstract class.

Now you're being pedantic :-)

I knew somone was going to say that :-)

I was just ribbing ya.

Jul 19 '05 #10

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