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Free Pascal => Beginners => Topic started by: Weitentaaal on October 13, 2021, 01:13:48 pm

Title: Override a Property
Post by: Weitentaaal on October 13, 2021, 01:13:48 pm

Hello :),

Is it possible to override a Property ?
Have some inherited classes and a Property in Super class. Property reutrns a constant which is defined in Main Class. For every class the Property gets set by an Edit(Main Class). But for 1 Class it can be 2 different Values.

Normal Case

property P : Integer read GetP write SetP;
set := 300
get := 300

Other Case

property P : Integer read GetP write SetP;
can be 300 or 400

So is it possible to override it to -->

property P : Integer read GetP(MODE) write SetP(MODE);
set(True):= 300
set(False):= 400
Get(True):= 300
Get(False):= 400

I hope you understand what I mean :-[
I don't even know if Parameters in Property's are possible  :-[
(this is probably something you should never do like this, but im just thinking different  %))

thanks in advice :)

EDIT: Currently doing it like that:

Code: Pascal  [Select][+][-]

1.  
2. fValue1 :Integer;
3. fValue2 :Integer;
4.  
5. property PValGeneral: Integer read GetVALG write SetVALG;
6. property PVal1: integer read GetVal1 write SetVal1;
7. property PVal2: integer read GetVal2 write SetVal2;
8.  
9. Get Valg
10. IF MainForm.MODE = True then begin
11.    Return := fValue1
12. end else begin
13.    Return := fValue2
14. end;
15.  
16. Same for Setter
17.  
18. and for the other Setter/Getter i just do as usual
19.  
20.  

Title: Re: Override a Property
Post by: Bart on October 13, 2021, 01:57:18 pm

Can't you just declare the getter/setter as virtual and override them where necessary?

Bart

Title: Re: Override a Property
Post by: wp on October 13, 2021, 02:24:37 pm

Of course, properties can be indexed. You must use the "Index" identifier in the property declaration to indicate which value is to be used. How about this (untested):

Code: Pascal  [Select][+][-]

1. type
2.   TMode = (MODE1, MODE2);
3.  
4.   TBaseClass = class
5.   private
6.     FP: Integer;
7.   protected
8.     property P: Integer read FP write FP;
9.   public
10.   end;
11.  
12.   TDerivedClass1 = class(TBaseClass)
13.   public
14.     property P;
15.   end;
16.  
17.   TDerivedClass2 = class(TBaseClass)
18.   private
19.     FOtherP: Integer;
20.     function GetP(AMode: TMode): Integer;
21.     procedure SetP(AMode: TMode; AValue: Integer);
22.   public
23.     property P1: Integer index MODE1 read GetP write SetP;
24.     property P2: Integer index MODE2 read GetP write SetP;
25.   end;
26.  
27. function TDerivedClass2.GetP(AMode: TMode): Integer;
28. begin
29.   case AMode of
30.     MODE1: Result := inherited P;
31.     MODE2: Result := FOtherP;
32.   end;
33. end;
34.  
35. procedure TDerivedClass2.SetP(AMode: TMode; AValue: Integer);
36. begin
37.   case AMode of
38.     MODE1: inherited P := AValue;
39.     MODE2: FOtherP := AValue;
40.   end;
41. end;

Title: Re: Override a Property
Post by: simone on October 13, 2021, 05:07:58 pm

You can find useful information on this topic in the following paragraph of the Language reference guide:

6.7.7 Overriding and redeclaring properties: https://www.freepascal.org/docs-html/ref/refsu39.html

Title: Re: Override a Property
Post by: Weitentaaal on October 14, 2021, 09:56:35 am

Quote from: Bart on October 13, 2021, 01:57:18 pm

Can't you just declare the getter/setter as virtual and override them where necessary?

Bart

I Have to take a look at virtual nad overriding Property's first but i guess it will be a Solution ! Thanks

Quote from: simone on October 13, 2021, 05:07:58 pm

You can find useful information on this topic in the following paragraph of the Language reference guide:

6.7.7 Overriding and redeclaring properties: https://www.freepascal.org/docs-html/ref/refsu39.html

So this is very welcome :)

Quote from: wp on October 13, 2021, 02:24:37 pm

Of course, properties can be indexed. You must use the "Index" identifier in the property declaration to indicate which value is to be used. How about this (untested):

Code: Pascal  [Select][+][-]

1. type
2.   TMode = (MODE1, MODE2);
3.  
4.   TBaseClass = class
5.   private
6.     FP: Integer;
7.   protected
8.     property P: Integer read FP write FP;
9.   public
10.   end;
11.  
12.   TDerivedClass1 = class(TBaseClass)
13.   public
14.     property P;
15.   end;
16.  
17.   TDerivedClass2 = class(TBaseClass)
18.   private
19.     FOtherP: Integer;
20.     function GetP(AMode: TMode): Integer;
21.     procedure SetP(AMode: TMode; AValue: Integer);
22.   public
23.     property P1: Integer index MODE1 read GetP write SetP;
24.     property P2: Integer index MODE2 read GetP write SetP;
25.   end;
26.  
27. function TDerivedClass2.GetP(AMode: TMode): Integer;
28. begin
29.   case AMode of
30.     MODE1: Result := inherited P;
31.     MODE2: Result := FOtherP;
32.   end;
33. end;
34.  
35. procedure TDerivedClass2.SetP(AMode: TMode; AValue: Integer);
36. begin
37.   case AMode of
38.     MODE1: inherited P := AValue;
39.     MODE2: FOtherP := AValue;
40.   end;
41. end;

Looks great ! i think with your Reply's i will be able to find a Solution that fits !

Thanks to you Guys :)