Variant to generic T, how?

[Jacek Laskowski 57]

How to set the generic class field to the value provided by Variant?

 

The following code is not compiled.

  ToField<T> = class
  private
    fData : T;
  protected
    procedure SetFromVariant<T>(const aValue : Variant);

 [...]


procedure ToField<T>.SetFromVariant<T>(const aValue : Variant);
var
  val: TValue;
begin
  val := TValue.FromVariant(aValue);
  fData := val.AsType<T>; <=== [dcc32 Error] E2008 Incompatible types

[dcc32 Error] E2008 Incompatible types

 

[Vandrovnik 214]

This compiles, but I have not tested, whether it works as expected 🙂

 

     ToField<T> = class(tObject)
      private
       fData : T;
      protected
       procedure SetFromVariant(const aValue : Variant);
     end;


procedure ToField<T>.SetFromVariant(const aValue: Variant);
 var val: TValue;
 begin
 val := TValue.FromVariant(aValue);
 fData := val.AsType<T>;
end;

 

Edited April 20, 2020 by Vandrovnik

[Jacek Laskowski 57]

Your code is identical to mine, don't I notice anything?
I still get compilation error on Delphi 10.3

[Tommi Prami 130]

procedure SetFromVariant<T>(const aValue : Variant);
procedure SetFromVariant(const aValue : Variant);

[Jacek Laskowski 57]

11 minutes ago, Tommi Prami said:

procedure SetFromVariant<T>(const aValue : Variant);
procedure SetFromVariant(const aValue : Variant);

I'm blind, sorry and thx! 🙂

 

[Remy Lebeau 1393]

Can't you just assign the Variant to the field directly and let the Variant handle the conversion?  Why do you need to go through TValue at all?

procedure ToField<T>.SetFromVariant(const aValue : Variant);
begin
  fData := aValue;
end;

Internally, when TValue is holding a Variant, TValue.AsType<T>() will look at the Variant's VType, copy the appropriate TVarData field to a temp TValue, and then assign that temp to the Result variable via TValue.Get<T>().  So just take out the middle man (TValue), Variant already knows how to do that same conversion directly.

Edited April 20, 2020 by Remy Lebeau

[Jacek Laskowski 57]

@Remy Lebeau

 

That's impossible, fData is generic:

 

[dcc32 Error] E2010 Incompatible types: 'T' and 'Variant'

[Remy Lebeau 1393]

@Jacek Laskowski I can't test this myself (no working IDE), but what about if the 'record' constraint is added to the Generic so the compiler knows that T is a value type?

ToField<T: record> = class
private
  fData : T;
protected
  procedure SetFromVariant(const aValue : Variant);
end;

procedure ToField<T>.SetFromVariant(const aValue : Variant);
begin
  fData := aValue;
end;

 

[Jacek Laskowski 57]

@Remy Lebeau I cannot use this constraint because it blocks the use of string types (because it is not a valuable type).

[Remy Lebeau 1393]

And this is why I HATE Generics!  They cause more headaches than they solve.

Edited April 22, 2020 by Remy Lebeau

[Georgge Bakh 29]

@Remy Lebeau

Generics are OK. And very useful. But some implementations are not. 😉

Btw in FPC your code compiles and working properly.