[fpc-pascal] Quick Modern Object Pascal Introduction, for Programmers

[Michalis Kamburelis]

2016-06-21 12:14 GMT+02:00 Marco van de Voort <marcov at stack.nl>:
> In our previous episode, Michalis Kamburelis said:
>>
>> - A section about TPersistent.Assign. This is the "basic approach to
>> cloning" that should probably be shown first. See it here:
>> http://michalis.ii.uni.wroc.pl/~michalis/modern_pascal_introduction/modern_pascal_introduction.html#_cloning_tpersistent_assign
>
> See also
> http://stackoverflow.com/questions/4041760/correct-way-to-duplicate-delphi-object/4041906#4041906
>
> That is for an own root class and pre class helpers, but you could do it also in or TPersistent
> helper:
>
> type tpersisthelp = class helper for TPersistent
>     function Clone(t: TPersistentClass = nil): TPersistent;
> end;
>
> function tpersisthelp.Clone(t: TPersistentClass = nil): TPersistent;
>   begin
>     if Assigned(t) then
>       Result := t.Create
>     else
>       Result := TPersistentClass(Self.ClassType).Create;
>     Result.Assign(Self);
>   end;
>
> Since TComponent declares a virtual destructor that might even be better.

I saw your post on stackoverflow before writing my article:)

But doing this on TPersistent means that you avoid calling the
constructor of the actual class. If you use this helper on a class
TMyClass, you will have an instance of TMyClass that "bypassed" the
constructor of TMyClass. Only "TPersistent.Create" was called. I
suspect that in real-world usage, a lot of classes will fail to work
correctly then, since code inside TMyClass often assumes that TMyClass
constructor was performed. I know that a lot of my own classes will
fail because of this...

That's why I would limit this usage only to the cases when you have a
virtual constructor on the base class. Only then the final class has a
chance to execute it's proper constructor.

Regards,
Michalis
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