[CLOSED] ClassType method

[julkas]

From http://docs.embarcadero.com/products/rad_studio/delphiAndcpp2009/HelpUpdate2/EN/html/delphivclwin32/System__TObject__ClassType.html

Note: In Delphi code, use the is or as operators instead of System::TObject::ClassType.

Why ?

[jamie]

so what's the question ?

if Sender is TSomeClass then …

That does a compare..

 With Sender As TSomeClass do …

 is that what you are referring to ?

basically does type checking of classes..

[Warfley]

Honestly I don't know, ClassType and is have some differences.
E.g. consider foo being of type TFoo, which inherits from TBar then:

Code: Pascal  [Select][+][-]

1. foo is TFoo // True
2. foo is TBar // True
3. foo is TObject // True

while:

Code: Pascal  [Select][+][-]

1. foo.ClassType = TFoo // True
2. foo.ClassType = TBar // False
3. foo.ClassType = TObject // False

So their use is pretty use-case dependent.

And as has nothing to do with that at all. Sure you could implement a check like this:

Code: Pascal  [Select][+][-]

1. var tmp: TFoo;
2. try
3.   tmp := foo as TFoo;
4. except on E: Exception do
5.   tmp := nil;
6. end;
7.  
8. if Assigned(tmp) then

But let's be honest, this code is at most usable as a punchline but should never see the light of day in any production environment

[PascalDragon]

I don't know why the Delphi documentation recommends the is operator (and also the as one) compared to using ClassType in C++, but is also checks whether the class instance is Nil, ClassType will crash in that case. Also is is essentially a call to InheritsFrom and not ClassType.

[egsuh]

Does following discussion have something to do with current thread?

https://forum.lazarus.freepascal.org/index.php/topic,48966.msg353641.html#msg353641

[julkas]

is would be best choice.
Thanks.