Classes from pointers?

[zRaze]

Scenario:
I have a base class called "BaseCls", and two derived classes called
"DerCls1" and "DerCls2"

I declare a variable -
BaseCls *MyClassObj;

I have some code -

If( Condition )
{
MyClassObj = new DerCls1;
} else {
MyClassObj = new DerCls2;
}

How can I tell if MyClassObj is pointing to a DerCls1 or DerCls2
object?

Thanks in advance...

[Jakob Bieling]

Re: Classes from pointers?

"zRaze" <abuse@msn.co m> wrote in message
news:o2agov8svc pv69oah2mus8ebk a1v1fq0sl@4ax.c om...[color=blue]
> Scenario:
> I have a base class called "BaseCls", and two derived classes called
> "DerCls1" and "DerCls2"
>
> I declare a variable -
> BaseCls *MyClassObj;
>
> I have some code -
>
> If( Condition )
> {
> MyClassObj = new DerCls1;
> } else {
> MyClassObj = new DerCls2;
> }
>
> How can I tell if MyClassObj is pointing to a DerCls1 or DerCls2
> object?[/color]


One way would be to have a virtual function in your base class, that
returns some info that it is the base class. The derived classes are
required to overload it and return info depending on what class they are.

Another way is to use dynamic_cast. If you cast the base class pointer
to the wrong derived class pointer type, you will get a null pointer.

Personally, I would prefer the former method, even tho you have the
disadvantage of requiring all derived classes to overload that function. If
the base class is (or can be) abstract anyway, you could make that make the
'info' function pure virtual even.

hth
--
jb

(replace y with x if you want to reply by e-mail)

[zRaze]

Re: Classes from pointers?

Thanks!

On Sat, 11 Oct 2003 18:05:13 +0200, "Jakob Bieling" <netsurf@gmy.ne t>
wrote:
[color=blue]
>"zRaze" <abuse@msn.co m> wrote in message
>news:o2agov8sv cpv69oah2mus8eb ka1v1fq0sl@4ax. com...[color=green]
>> Scenario:
>> I have a base class called "BaseCls", and two derived classes called
>> "DerCls1" and "DerCls2"
>>
>> I declare a variable -
>> BaseCls *MyClassObj;
>>
>> I have some code -
>>
>> If( Condition )
>> {
>> MyClassObj = new DerCls1;
>> } else {
>> MyClassObj = new DerCls2;
>> }
>>
>> How can I tell if MyClassObj is pointing to a DerCls1 or DerCls2
>> object?[/color]
>
>
> One way would be to have a virtual function in your base class, that
>returns some info that it is the base class. The derived classes are
>required to overload it and return info depending on what class they are.
>
> Another way is to use dynamic_cast. If you cast the base class pointer
>to the wrong derived class pointer type, you will get a null pointer.
>
> Personally, I would prefer the former method, even tho you have the
>disadvantage of requiring all derived classes to overload that function. If
>the base class is (or can be) abstract anyway, you could make that make the
>'info' function pure virtual even.
>
>hth[/color]

[Cy Edmunds]

Re: Classes from pointers?

"zRaze" <abuse@msn.co m> wrote in message
news:o2agov8svc pv69oah2mus8ebk a1v1fq0sl@4ax.c om...[color=blue]
> Scenario:
> I have a base class called "BaseCls", and two derived classes called
> "DerCls1" and "DerCls2"
>
> I declare a variable -
> BaseCls *MyClassObj;
>
> I have some code -
>
> If( Condition )
> {
> MyClassObj = new DerCls1;
> } else {
> MyClassObj = new DerCls2;
> }
>
> How can I tell if MyClassObj is pointing to a DerCls1 or DerCls2
> object?
>
> Thanks in advance...[/color]

If you have to make that distinction you probably have a poor design. When
you have a pointer to a base class which really points to a derived class,
you are using polymorphism. As Jakob implied, the best way to deal with
polymorphism is to call a virtual function which does the right thing no
matter what type it is. This scheme is efficient and maintainable. (For
instance it would be easy to add a DerCls3 later.) If it is impossible
because you can't implement any such virtual function then maybe
polymorphism was the wrong approach in the first place.

A couple of other points: you should probably put a do-nothing virtual
destructor in the base class:

virtual ~BaseCls() {}

to make sure possible destructors in derived classes are called properly.
Also, whenever you use new() you must use delete() to avoid a memory leak
and you must use it only once to avoid undefined behaviour. This can be a
source of problems. Consider using a reference counted smart pointer instead
of a raw pointer for MyClassObj:

typedef boost::shared_p tr<BaseCls> BaseClsPtr;

MyClassObj = BaseClsPtr(new DerCls1);

It will automatically delete the object when the pointer goes out of scope
so you don't have to worry about it.You can get such a pointer at

https://www.boost.org

Good luck.

--
Cy

http://home.rochester.rr.com/cyhome/

[zRaze]

Re: Classes from pointers?

Basically, the reason I need to know what class a pointer points to is
because I have a 2 dimensional array of pointers (it's a map)
and my classes are the landmarks etc. (Road, open space etc.)

So therefore, I don't have the problem with adding new classes, but
when I come do save the map to disk, I need to be able to tell what is
on that tile for when reloading.

I have now got a new method of this. I had a function that returned a
graphic handle (for drawing on the screen), however, I was having
problems (which I thought was related, but wasn't) so I changed it to
return an enum value which gets linked to the graphic, and I'm just
going to use that enum value...

Thanks for your help tho...

On Sun, 12 Oct 2003 02:47:54 GMT, "Cy Edmunds"
<cedmunds@spaml ess.rochester.r r.com> wrote:
[color=blue]
>"zRaze" <abuse@msn.co m> wrote in message
>news:o2agov8sv cpv69oah2mus8eb ka1v1fq0sl@4ax. com...[color=green]
>> Scenario:
>> I have a base class called "BaseCls", and two derived classes called
>> "DerCls1" and "DerCls2"
>>
>> I declare a variable -
>> BaseCls *MyClassObj;
>>
>> I have some code -
>>
>> If( Condition )
>> {
>> MyClassObj = new DerCls1;
>> } else {
>> MyClassObj = new DerCls2;
>> }
>>
>> How can I tell if MyClassObj is pointing to a DerCls1 or DerCls2
>> object?
>>
>> Thanks in advance...[/color]
>
>If you have to make that distinction you probably have a poor design. When
>you have a pointer to a base class which really points to a derived class,
>you are using polymorphism. As Jakob implied, the best way to deal with
>polymorphism is to call a virtual function which does the right thing no
>matter what type it is. This scheme is efficient and maintainable. (For
>instance it would be easy to add a DerCls3 later.) If it is impossible
>because you can't implement any such virtual function then maybe
>polymorphism was the wrong approach in the first place.
>
>A couple of other points: you should probably put a do-nothing virtual
>destructor in the base class:
>
>virtual ~BaseCls() {}
>
>to make sure possible destructors in derived classes are called properly.
>Also, whenever you use new() you must use delete() to avoid a memory leak
>and you must use it only once to avoid undefined behaviour. This can be a
>source of problems. Consider using a reference counted smart pointer instead
>of a raw pointer for MyClassObj:
>
>typedef boost::shared_p tr<BaseCls> BaseClsPtr;
>
>MyClassObj = BaseClsPtr(new DerCls1);
>
>It will automatically delete the object when the pointer goes out of scope
>so you don't have to worry about it.You can get such a pointer at
>
>www.boost.org
>
>Good luck.[/color]

[jeffc]

Re: Classes from pointers?


"zRaze" <abuse@msn.co m> wrote in message
news:o2agov8svc pv69oah2mus8ebk a1v1fq0sl@4ax.c om...[color=blue]
> Scenario:
> I have a base class called "BaseCls", and two derived classes called
> "DerCls1" and "DerCls2"
>
> I declare a variable -
> BaseCls *MyClassObj;
>
> I have some code -
>
> If( Condition )
> {
> MyClassObj = new DerCls1;
> } else {
> MyClassObj = new DerCls2;
> }
>
> How can I tell if MyClassObj is pointing to a DerCls1 or DerCls2
> object?[/color]

This is not a hard and fast rule, but one of the basic principles OO code
such as that is you're not *supposed* to know. And you're supposed to write
your code in such a way that the code doesn't care either.