[fpc-pascal] Turbo Pascal and Object Pascal ways of OOP

[Daniël Mantione]

Op Tue, 5 Feb 2008, schreef Marc Weustink:

> Daniël Mantione wrote:
>> 
>> 
>> Op Tue, 5 Feb 2008, schreef Luiz Americo Pereira Camara:
>> 
>>> can i safely use the below object  instead of the record and pass directly 
>>> to the c function?
>>> 
>>> TMyObj = object
>>> x: Integer;
>>> y: Integer;
>>> Method1;
>>> Method2;
>>> end;
>>> PMyObj = ^TMyObj;
>> 
>> Yes, objects (by specification) are defined to have the same binary layout 
>> as records.
>> 
>>> Moreover, whats the difference between objects and records with methods 
>>> (the new Delphi feature)?
>> 
>> Objects can have virtual methods, constructors, destructors. The new Delphi 
>> feature cannot.
>
> however with virtuals or constructors/destructors, the memory layout is not 
> 100% the same as a record.

A constructor doesn't add a vmtlink. A virtual method does, and a 
destructor too because they are virtual. It is never a problem in 
practise, because you can declare the virtual method in a descendent, 
i.e.:

type  Tbinary_compatible_obj=object
         a,b,c,d,e,f:byte;
         constructor init;
       end;

       Tbinary_compatible_obj_with_vmt=object(Tbinary_compatible_record)
         constructur init;
         procedure virtualmethod;virtual;
         desctructor done;virtual;
       end;

In the above example, you can perfectly pass a 
Tbinary_compatible_obj_with_vmt instance to any C library expecting a 
Tbinary_compatible_obj, because the vmtlink is located after the fields 
and therefore does not interfere with the binary structure.

Daniël