[fpc-pascal] Constructors & Destructors 101

Jonas Maebe jonas.maebe at elis.ugent.be
Fri Apr 3 11:15:19 CEST 2009

On 03 Apr 2009, at 03:43, Richard Ward wrote:

> A)	The documentation says that for the create constructor:
> (quote}
> Description: Create creates a new instance of TObject. Currently it  
> does nothing. It is also not virtual, so there is in principle no  
> need to call it directly.
> {unquote}
> What is it meant by: "no need to call [create] directly?"  How do  
> you invoke the constructor without calling it?

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.

> ... and ... Why is create not virtual and the destroy destructor is?

Because when creating a class instance, you usually know the exact  
class type of that instance (e.g., TObject.create -> you know that the  
created instance type will be of the type TObject). This only changes  
if you use class reference types, but if you use those you can still  
declare your own virtual constructors.

Conversely, many routines that free a class instance, have no idea  
about the exact type of that instance (such as FreeAndNil: all it  
knows is that the instance inherits from TObject). So to make sure  
that all resources allocated by those class types are properly freed,  
the destructor almost has to be virtual (since otherwise, only  
TObject's dummy destructor would be called by, e.g., FreeAndNil).

> B)	The documentation says that for the destroy destructor:
> (quote}
> Description: Destroy is the destructor of TObject. It will clean up  
> the memory assigned to the instance. Descendent classes should  
> override destroy if they want to do additional clean-up. No other  
> destructor should be implemented.
> {unquote}
> What is it meant by: "No other destructor should be implemented?"

It means that you should not add "destructor  
my_peculiarly_named_destructor; virtual;" to your own classes. The  
reason is that TObject's free method, which is used by pretty much all  
code out there, is hardcoded to call "destroy". So if your destructor  
is named differently, a lot of existing code will not properly destroy  
instances of your class type.

> ...and...  Does it do "something" while the Create constructor  
> doesn't?

No, TObject's default destructor does not do anything either.

> If you had two different create constructors (for whatever reason),  
> might you not also need two different destroy destructors?

No, the default destructor should always free all resources,  
regardless of how the class instance was created. Otherwise, it would  
also make your code more complex, because throughout the code you  
would have to track how the instance was created, so that in the end  
you could call the correct destructor.

> What problems might you get into if you did?

See above.


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