static variables in derived classes

[Nils]

Hi.

I ahve three classes A,B,C, where B and C are derived from A.
A:>B,C.
Furthermore each class has an static int variable with name id,
static int id;
Besides that the class A implements the function
int GetId()
{
return id;
}

If I instanciate an object from the class B (where id is set to 1). The
function GetId() returns 0 (thats the value of the id in A).

Is there a way that GetId() delivers the right number, here 1, without
implementing GetId() in the classes B and C?

Thanks a lot
Nils

[Thomas Matthews]

Nils wrote:

Hi.

I ahve three classes A,B,C, where B and C are derived from A.
A:>B,C.
Furthermore each class has an static int variable with name id,
static int id;
Besides that the class A implements the function
int GetId()
{
return id;
}

If I instanciate an object from the class B (where id is set to 1). The
function GetId() returns 0 (thats the value of the id in A).

Is there a way that GetId() delivers the right number, here 1, without
implementing GetId() in the classes B and C?

Thanks a lot
Nils

A variable declared as static in a class means that
there is only one instance of that variable no matter
how many instances of the class there are.

In your case, there will only be one "id" variable
for every instance of class A, B, and C.

I would say to either remove the "static" from the
declaration, or create static variables in each
of the child classes. I can't give better advice
unless you give a description of what you are
trying to accomplish.
--
Thomas Matthews

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[Howard]

"Nils" <sl***@web.de> wrote in message
news:cv***********@ariadne.rz.tu-clausthal.de...

Hi.

I ahve three classes A,B,C, where B and C are derived from A.
A:>B,C.
Furthermore each class has an static int variable with name id,
static int id;
Besides that the class A implements the function
int GetId()
{
return id;
}

If I instanciate an object from the class B (where id is set to 1). The
function GetId() returns 0 (thats the value of the id in A).

Is there a way that GetId() delivers the right number, here 1, without
implementing GetId() in the classes B and C?

Thanks a lot
Nils

I don't think it's a good idea to have a variable of the same name in
multiple classes in a class hierarchy. I'd probably design this whole thing
differently. How about just having a virtual function GetId(), which would
return a different value in each class? Why store the Id at all, especially
if it's static? Simply returning a numeric literal from a virtual function
would accomplish the same thing. (It's not *that* much extra work, after
all!)

It is also potentially an indication of poor design that you need to have a
unique identifier for each class type in the first place. How are you using
that information? If you're using it in a switch statement in order to
decide on some action, then the better alternative is usually to just use
virtual functions for the action(s), and let each class do what it needs to
do in its override of the virtual function.

-Howard

[Nils]

I don't think it's a good idea to have a variable of the same name in
multiple classes in a class hierarchy. I'd probably design this whole
thing differently. How about just having a virtual function GetId(),
which would return a different value in each class? Why store the Id at
all, especially if it's static? Simply returning a numeric literal from a
virtual function would accomplish the same thing. (It's not *that* much
extra work, after all!)

It is also potentially an indication of poor design that you need to have
a unique identifier for each class type in the first place. How are you
using that information? If you're using it in a switch statement in order
to decide on some action, then the better alternative is usually to just
use virtual functions for the action(s), and let each class do what it
needs to do in its override of the virtual function.

-Howard

I have done it with virutual functions now, almost like you suggested it.
Thanks!

Don't need it explicit for switch statements. the classes stands for
mathematical fuzzy-functions. The Id discribes some charakteristik of that
functions. I need it because not every operation is allowed to apply on
every function.

Thanks to all
Nils

[Ron Natalie]

Thomas Matthews wrote:

A variable declared as static in a class means that
there is only one instance of that variable no matter
how many instances of the class there are.

In your case, there will only be one "id" variable
for every instance of class A, B, and C.

Yes, I'm sure he understands that. The issue is that there
are three id variables, A::id, B::id, and C::id and he is
trying to differentiate between them. There's no such
thing as a virtual static nor a virtual data type so he'll
have to implement a virtual member function to what he's
describing.

Removing static from the declartion doesn't affect the
problem. The A::id, B::id, and C::id would still be
distinct in the C object.

[Nils]

"Ron Natalie" <ro*@sensor.com> schrieb im Newsbeitrag
news:42**********************@news.newshosting.com ...

Thomas Matthews wrote:

A variable declared as static in a class means that
there is only one instance of that variable no matter
how many instances of the class there are.

In your case, there will only be one "id" variable
for every instance of class A, B, and C.

Yes, I'm sure he understands that. The issue is that there
are three id variables, A::id, B::id, and C::id and he is
trying to differentiate between them. There's no such
thing as a virtual static nor a virtual data type so he'll
have to implement a virtual member function to what he's
describing.

Removing static from the declartion doesn't affect the
problem. The A::id, B::id, and C::id would still be
distinct in the C object.

Yes I wanted to say that exactly. Thanks :)