Lazarus

Free Pascal => Beginners => Topic started by: skepta on January 23, 2023, 10:52:17 am

Title: [Solved] How to get the actually instantiated class name from Abstract Class?
Post by: skepta on January 23, 2023, 10:52:17 am

Hi, I want to know whether it is possible to know the actual class name from the Abstract Class? For example, I have TListBox where I can add to it through the TStrings returned by Items method through the following code:

Code: Pascal  [Select][+][-]

1. TForm1 = class(TForm)
2.   ListBox1: TListBox;
3. private
4.   procedure SomeProcedure;
5. end
6.  
7. procedure TForm1.SomeProcedure;
8. begin
9.   ListBox1.Items.Add('abc') // this works because TListBox1.Items is a TStrings
10.   // I want to know the actual type of ListBox1.Items, for code understanding purposes
11. end
12.  

In this case, I know TStrings is implemented by TStringList through reading the documentation. But is there a way to know it without reading the documentation or the whole the code base? Note: this is possible in PHP through the get_class function https://www.php.net/manual/en/function.get-class.php. (https://www.php.net/manual/en/function.get-class.php.).

In other words, I want to know what implements TStrings?

Please don't ask me to read the documentation of TStrings. Because I want to do it for other scenario where the documentation is poor/non-existent.

Thank you in advance for your help!

Title: Re: How to get the actually instantiated class name from Abstract Class?
Post by: Zvoni on January 23, 2023, 11:04:45 am

Unit TypInfo?
https://www.freepascal.org/docs-html/rtl/typinfo/index-3.html

Title: Re: How to get the actually instantiated class name from Abstract Class?
Post by: cdbc on January 23, 2023, 11:31:43 am

Hi
Every TObject descendant, have these class methods:

Code: Pascal  [Select][+][-]

1. TObject = class
2.     public
3.     ....
4.           class function ClassType : tclass;{$ifdef SYSTEMINLINE}inline;{$endif}
5.           class function ClassInfo : pointer;
6.           class function ClassName : shortstring;
7.           class function ClassNameIs(const name : string) : boolean;
8.    ....
9. end;
10.  
11. this allows you to do stuff like this:
12. ImplClassName:= ListBox1.Items.Classname;
13. if ListBox1.Items.ClassType = TStringList then ...;
14. var TypeInfo: PTypeInfo; // incl. TypInfo in uses clause
15. TypeInfo:= ListBox1.Items.ClassInfo;
16. if TypeInfo^.Kind = tkClass then ...;
17.  

Hth, Regards Benny

Title: Re: How to get the actually instantiated class name from Abstract Class?
Post by: skepta on January 24, 2023, 01:41:00 pm

I think cdbc's answer is good enough and I will close this thread as solved. Thank you!

Title: Re: [Solved] How to get the actually instantiated class name from Abstract Class?
Post by: Thaddy on January 24, 2023, 02:01:08 pm

It is much simpler:

Code: Pascal  [Select][+][-]

1. writeln(TSomeMyClass.Classname); [/end]
2. You can cast somemyclass to its ancestor. If this happens to be a TStrings, listboxes, memo's, comboboxes etc can be assigned to and from.