[fpc-pascal] Classes and class methods

[Mark Morgan Lloyd]

Mark Morgan Lloyd wrote:
> Marco van de Voort wrote:
>> In our previous episode, Mark Morgan Lloyd said:
>>> Is it possible to put a class (not an instance) in a variable or 
>>> array element, and to use this to call a class method?
>>>
>>> I'm defining a hierarchy of classes and it would be convenient if it 
>>> were possible to register the fact that a class had a particular 
>>> subclass, such that the information was available before instantiation.
>>>
>>> I'm assuming that the code that inspects the array has imported all 
>>> units so is aware of the relevant class declarations, but I am still 
>>> having problems with calling class methods and/or casting to a class 
>>> (as distinct from an instance).
>>
>> Afaik this route you can only call static methods of the basetype of the
>> class-type.
>>
>> So if it is             A= class
>>           end;
>>      b=class(a)
>>           end;
>>      c=class(b);
>>
>>      TClasstype  = class of  B;
>>
>>
>> var x : TClasstype;
>>
>> begin
>>   x:=C;
>>
>>   x.staticmethod
>>
>> then staticmethod must be visible in B. It doesn't matter which 
>> descendant
>> of B you assign to X.
>>
>> This is logical, since static methods are entirely compiletime, and the
>> compiler can't know what type will be runtime assigned.
> 
> Thanks Marco, I was beginning to suspect that I was hitting limitations 
> and that what I was trying to do might be ill-advised.
> 
> I'm finding virtual class methods useful when I'm able to refer directly 
> to a class, rather than going via a reference to a class.

I'm doing something wrong- it works.

I've got a sequence of classes, each with a Parent class procedure and 
Children() class function which reference a global array associated with 
each class (hardcoded name). Each class also has an Oid() function which 
appends a hardcoded number to whatever it's inherited.

If a test program does this

WriteLn(OidToStr(TIso.Oid));
WriteLn(OidToStr(TIso.Children[0].Oid));
WriteLn(OidToStr(TIso.Children[0].Children[0].Oid));
WriteLn(OidToStr(TIso.Children[0].Children[0].Children[0].Oid));

I get this

.1
.1.3
.1.3.6
.1.3.6.1

i.e. each reference to Children() is identifying the correct method, 
despite the fact that this is spread over more than one file.

-- 
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]