compilation failure: generic class

[Bagout]

Hi ! (sorry in advance for my bad English...)

I don't understand why my code doesn't work :

Code: Pascal  [Select][+][-]

1.   generic TNullable<T> = class
2.     private
3.       FValue : T;
4.     public
5.       isAssigned : boolean;
6.       function IsNull: boolean;
7.       procedure SetValue (newValue: T);
8.       Property Value : T read FValue write SetValue; default;
9.     end;
10.  
11.  procedure TNullable.SetValue (newValue: T);
12.  begin
13.    FValue := newValue;
14.    isAssigned := true;
15.  end;
16.  
17.  function TNullable.IsNull: boolean;
18.  begin
19.    result := not isAssigned;
20.  end;
21.  
22.   type
23.     TBoolean = specialize TNullable<boolean>;
24.   Var
25.     bool: TBoolean;
26.   begin
27.     bool := true;
28.     if bool then writeln('bool is true');
29.  end.

[Blaazen]

I made it work in {$mode objfpc} in FPC 3.3.1:

Code: Pascal  [Select][+][-]

1. Property Value : T read FValue write SetValue;// default;

Property can't be default.

Code: Pascal  [Select][+][-]

1.   Var
2.     bool: TBoolean;
3.   begin
4.     bool := TBoolean(true);
5.     if bool= TBoolean(true) then writeln('bool is true');
6.   end;

This is maybe some compiler issue, since retyped had to be even a nil in past (see https://bugs.freepascal.org/view.php?id=34037).

[Thaddy]

The code indeed does not work in FPC, not even trunk because the default keyword for types other than arrays is not (yet) implemented.
(And also not smartpointers, so you have to create/free your object)
That is a NewPascal feature, not FPC.
Your code should have looked something  like this:

Code: Pascal  [Select][+][-]

1. {$mode objfpc}
2. type
3.  generic TNullable<T> = class
4.     private
5.       FValue : T;
6.     public
7.       isAssigned : boolean;
8.       function IsNull: boolean;
9.       procedure SetValue (newValue: T);
10.       Property Value : T read FValue write SetValue;
11.     end;
12.  
13.  procedure TNullable.SetValue (newValue: T);
14.  begin
15.    FValue := newValue;
16.    isAssigned := true;
17.  end;
18.  
19.  function TNullable.IsNull: boolean;
20.  begin
21.    result := not isAssigned;
22.  end;
23.  
24.   type
25.     TBoolean = specialize TNullable<boolean>;
26.   Var
27.     bool: TBoolean;
28.   begin
29.     bool := TBoolean.Create;
30.     bool.value := true;
31.     if bool.value then writeln('bool is true');
32.     bool.free;
33.  end.

@Blaazen: that that cast works is only because the first field in the class stores the boolean. I don't think that is what you mean...

[Blaazen]

I mainly forgot to create the instance and the rest is consequence 

[Thaddy]

If he just needs a nullable Boolean type look here: http://forum.lazarus.freepascal.org/index.php/topic,41144.msg295325.html#msg295325 Much easier.