default method for objects

[speter]

G'Day Folks,

Is it possible to have a default method or similar for objects (not classes). For example:

Code: Pascal  [Select][+][-]

1. type
2.   tfoo = object
3.     v : byte;
4.     procedure init;
5.     function getit : byte;
6.   end;
7. procedure tfoo.init;
8. begin
9.   v := 255;
10. end;
11. function tfoo.getit : byte;
12. begin
13.   result := v;
14. end;
15.  
16. var
17.   zz : tfoo;
18.   k : byte;
19. begin
20.   zz.init;
21.   k := zz.v;      // this works
22.   k := zz.getit;  // this works
23.   k := zz;        // i'd like to do this - have it return 'v' or call getit()
24. end;

In other words, can I code it so that function getit is called as a default action.

cheers
S.

[jamie]

You need to use an operator overload but  you can not do that with OBJECTS, you need to use the ADVANCEDRECORDS mode which gives you everything you are using objects for atm.

 You can define an operator := for the record.

 fpc also supports this outside too but its a little tricky.

[speter]

Thanks for the quick answer, jamie.

I tried:

Code: Pascal  [Select][+][-]

1. {$mode objfpc}{$H+}
2. {$modeswitch advancedrecords}
3. ...
4. type
5.   tfoo = record
6.     v : byte;
7.  
8.     procedure init;
9.     function getit : byte;
10.     class operator Implicit(a : byte) : tfoo;
11.   end;

But received the error msg : "Error, it is not possible to overload this operator" (pointing to line 10 above).

So I'm still missing something. 

Also, if I understand the implicit operator, it is like:

Code: Pascal  [Select][+][-]

1. zz := 22;

rather than

Code: Pascal  [Select][+][-]

1. k := zz;

cheers
S.

[Thaddy]

You use the wrong syntax for the assignment operator. You use delphi syntax but you specify mode objfpc.
Here a sketch how it would work with the correct objfpc syntax:

Code: Pascal  [Select][+][-]

1. {$mode objfpc}{$H+}
2. {$modeswitch advancedrecords}
3. type
4.   Tfoo = record
5.     v : byte;
6.     class operator initialize(var foo:Tfoo);
7.     class operator :=(a : byte) : tfoo;
8.     class operator :=(a : TFoo) : Byte;
9.   end;
10.  
11.   class operator Tfoo.:=(a:byte):Tfoo;
12.   begin
13.     Result.v := a;
14.   end;
15.  
16.   class operator Tfoo.:=(a:TFoo):Byte;
17.   begin
18.     Result := a.v;
19.   end;
20.  
21.   class operator TFoo.Initialize(var Foo:Tfoo);
22.   begin
23.     Foo := Default(TFoo);
24.     Foo.v :=255;
25.   end;
26.  
27. var
28.   f,f2:Tfoo;
29.   b:byte;
30. begin
31.   writeln(f.v);
32.   f2 := 10;
33.   writeln(f2.v);
34.   b := f;
35.   writeln(b);
36. end.

This should get you on the right track.

[speter]

Thanks very much for your help Thaddy (and jamie)!

My original question was: can I do something like:

Code: Pascal  [Select][+][-]

1. var
2.   zz : tfoo;
3.   k : byte;
4. begin
5.   zz.init;
6.   k := zz;        // i'd like to do this - have it return 'v' or call getit()
7. end;

is this possible?

cheers
S.

[Thaddy]

Well I just demo'd that, isn't it? You should look at how I implemented the class operator Initialize.
That is fully automatic. So yes, it is possible. Also look at the two class operators.
  b := f;
  writeln(b);

This is exactly what you asked. b = byte and f = Tfoo.

Try to understand my - complete - example. It works as intended and with the correct syntax.

[speter]

Yes! Thanks Thaddy.

cheers
S.

[Thaddy]

I would also add the two comparison operators =
I leave that as an exercise for you.