[DUG] Interfaces

[Alister Christie]

If I have
  TPerson = class(TInterfacedObject, ISpeakable)....

Procedure MakeANoise(n : INoiseable);
begin
  n.MakeNoise;
end;

and
var
  Person : TPerson;
begin
  Person := TPerson.Create;
  MakeANoise(Person); //Incompatible types: 'INoiseable' and 'TPerson'
end;

I can't pass a TPerson to an INoiseable, unless I add INoisable to its 
class definition.  Supporting ISpeakable does not make it support 
INoisable (annoyingly - and I don't yet understand why not).

The background to what I'm trying to do is process an xml file (lots of 
them really).  I have generated a bunch of code (about 7k lines, 
involving over 80 interfaces and as many classes) using the XML 
databinding wizard (from a DTD), but there is a significant amount of 
overlap between many of the objects (and interfaces).  So I have been 
trying to clean up the interfaces so that when I process the files I can 
reduce the amount of duplicate code.  It's taking a lot longer that it 
should as my understanding of interfaces is limited (although I'm slowly 
getting there).  I'd probably be finished by now if I just did a lot of 
copy & paste - but that would be wrong ;-)

Anyway - I'll keep persisting.

Alister Christie
Computers for People
Ph: 04 471 1849 Fax: 04 471 1266
http://www.salespartner.co.nz
PO Box 13085
Johnsonville
Wellington 



Todd Martin wrote:
> That is exactly right, but why would you want to, when ISpeakable 
> supports makeNoise() anyway?
> You can still pass your TPerson object into a method(AObject : 
> INoiseable) without problems.
>
> Todd.
>   
>> However if I change my TPerson to the following
>>   TPerson = class(TInterfacedObject, ISpeakable, INoiseable)
>> then all is happy.  I don't quite understand fully why as yet - I 
>> guess you can't get an interface pointer to an object unless it 
>> explicitly states that it supports that interface.
>>     
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