[fpc-pascal] Constructors & Destructors 101

[Doug Chamberlin]

Bart wrote:
> On 4/3/09, Jonas Maebe <jonas.maebe at elis.ugent.be> wrote:
>>  I think what is meant, is that if you create a direct subclass of TObject,
>> there is no need to call TObject's create constructor (e.g., via "inherited
>> create;") from your own constructors. It doesn't hurt if you do it of
>> course, and may be good practice to account for future situations where the
>> parent class may change.
> 
> I often wondered abou that.
> 
> So if i understand correctly:
> 
> Say I have
> 
> Type
>   TFoo = class;
>   private
>     fSomeField: Integer;
>   public
>     constructor Create;
>   end;
> 
> then
> 
> constructor  TFoo.Create
> begin
>   Inherited Create;
>   fSomeField := -1;
> end;
> 
> would in essence be equal to
> 
> constructor  TFoo.Create
> begin
>   fSomeField := -1;
> end;
> 
> Since TOblect.Create "does nothing".

Essentially, yes.

However, you may create subtle, lurking bugs if you omit that call and 
later refactor your code. For example, if you later change

   type
     TFoo = class

to

   type
     TFoo = class(TSomeClass)

and TSomeClass has some important work done in it's Create constructor. 
If you properly called "inherited Create;" now that important stuff will 
get done, just the way you want it to!

<emphasis>
So, the big lesson here is to stick to the proper structure even though 
you may have outside knowledge that the form you should follow is not 
strictly necessary in all cases.
</emphasis>