[fpc-pascal] override and virtual: was SetFileTime

[Giuliano Colla]

Il 03/05/23 16:44, Steve Litt via fpc-pascal ha scritto:
> José Mejuto via fpc-pascal said on Wed, 3 May 2023 11:35:50 +0200
>
>> Hello,
>>
>> Attached is a dirty implementation of "touch" for junctions
> ==========================================================
> TTouchJunction = class(TCustomApplication)
>    protected
>      procedure DoRun; override;
>      function DateTimeToFileTime(DateTime: TDateTime): TFileTime;
>    public
>      constructor Create(TheOwner: TComponent); override;
>      destructor Destroy; override;
>      procedure WriteHelp; virtual;
>    end;
> ==========================================================
>
> I'm an old Turbo Pascal (mainly 3.x, a little 5.5) guy and don't
> remember or never saw override and virtual. What do they do?
>
> SteveT

  *

    Override: When you're dealing with objects, you have the
    inheritance, that is you may derive a new class (the template of an
    object) from an existing class and the newly created class inherits
    all the methods and properties of the parent class. In this case
    TTouchJunction is declared as a descendant of TCustomApplication,
    therefore it inherits all the methods and properties. If you need
    that your object does something different from the methods of its
    parent, but keep the same name, you must declare that your
    declaration overrides the ancestors one. If in the body of your new
    method if you want to execute also the ancestor method you may call
    it with the keyword "inherited".

  *

    Virtual: means that this method can be overridden by a descendant
    class. The same does the keyword Dynamic.  The difference has to do
    with the way the code is allocated in memory. Virtual is optimized
    for speed, Dynamic for memory.

Hope that it helps.

Giuliano

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