OOP_(re)

Wed, 28 May 1997 17:01:51 +0100 (BST)

On Wed, 28 May 1997 16:40:06 +0200 Frank Heckenbach  
wrote:
[...]
>> At the moment, "Parent" and "Child" do nothing, [...]
>> "SelfID" is an integer which holds a unique numeric ID for each
>> living TObject descendant.
>
>AFAICS, they will hold the parent/child *objects*, not *classes*, and
>the ID of instances. Also useful, I think, but not to be confused with
>the parent/child classes, and "ObjID" (= class IDs) which should resonably
>be in the VMT, since they depend only on the class, not on the instance.

True.

>> Can we make TObject automatically become the ancestor of each
>> Object type (i.e., any object which is just defined as "Object")?;
>>  if so, is this desirable?;
>
>No. There are applications of OOP that are quite different from the class
>libraries as TVL, OWL and so on. Sometimes I use objects as "convenient
>records" without any need for inheritance, virtual methods or streams.

I do too - fair enough ...

>AFAICS, objects that are stored in streams, must be descendants of TObject.

Correct.

>But not all types of objects need to be able to be used in streams at all.

Correct again.

>I think the programmer should be allowed to decide for himself if he wants
>his objects to be "streamable" (by making them descandants of TObject or
>its descendants) or not.

True.

[...]
>BTW: Would it be possible at all? Doesn't TObject refer to TStream? 

No, it doesn't. In bpcompat, TObject has a "Load" constructor that takes
a TObject parameter.

>So if TObject was a part of the "core language", also TStream would have to be.

See above.

>> True for a Program, false for a Unit.  While the GPC compiler is *much*
>> smarter than BP and would remove unused code and variables, the GNU linker
>> is not able to remove dead code, it only does so if a whole Unit is unused.
>
>I suppose there's not much hope of getting a smarter linker in the near
>future, is there?

I very much hope so.

>The African Chief wrote:
>
>> Yes (but Handle is present in Delphi as well). However, the main reason
>> for keeping it is because the OBJECTS unit exports a function which returns
>> a pObject, which can be used to locate an object instance by its Handle
>> (i.e., given any Handle, you would thereby be able to locate its owner). 
> 
>I don't know all the details here, but isn't it possible to have a
>TWindow(TObject) with Handle being a field of TWindow, and this function
>returning a PWindow? This would seem natural to me. 

Yes, that is possible. But I don't think it does any harm to have the Handle 
in TObject instead. 

>> This is not an academic issue, because I have seen the utility of 
>> this facility in other contexts (viz - my portable "CHIEFAPP" class
>> library, for BP 7.x, Delphi1 and Delphi2, from which project this
>> OBJECTS unit was borrowed and cannibalised). Thus, the fields
>> need to be in the ultimate "mother of all objects".
>
>Similar arguments might be applied to many kinds of fields, with the
>possible danger of blowing up the "mother of all objects" very much.

True.

>Then it could - at best - be the mother of all objects in a particular
>class library, but it wouldn't promote the goal of making it a widely
>used base class.

Perhaps. But there are many times when I have wished that certain
things existed in BP's TObject and TWindowsObject. Making a 
descendant object is fair enough, but it would have been a lot easier
for me if the things were in the ancestor.

>> It has a present use - to be able to locate every object instance by its 
>> own unique ID. See above.
>
>I'm probably missing something - but isn't the address of an object a
>"unique ID"? (One that makes it very easy to "locate" the object... ;-)

That assumes that you already know the object and its address. If you 
already know  it, there would be no point trying to locate it again. Also, 
there are a whole load of things that you can do with integers that you 
cannot do with addresses. And, suppose that, for some obscure reason, 
I suddenly wanted to locate all currently active object instances and/or 
their parents (if any), how would I do this using addresses? - but I could 
do this with a loop through the SelfIDs.

Best regards, The Chief 
Dr Abimbola A. Olowofoyeku (The African Chief, and the Great Elephant)
Author of:  Chief's Installer Pro v3.50 for Win16 and Win32.
Homepage:  http://ourworld.compuserve.com/homepages/African_Chief/
E-mail: laa12@cc.keele.ac.uk


The African Chief (laa12@cc.keele.ac.uk)