Question

I’m checking out the Delphi 2009 Trial, but run into problems with the generics stuff right away.

The following code does not compile, and I haven’t the slightest idea why it’s giving me E2015 for the Equals() method:

type   TPrimaryKey<T> = class(TObject)   strict private     fValue: T;   public     constructor Create(AValue: T);     function Equals(Obj: TObject): boolean; override;     function GetValue: T;   end;  constructor TPrimaryKey<T>.Create(AValue: T); begin   inherited Create;   fValue := AValue; end;  function TPrimaryKey<T>.Equals(Obj: TObject): boolean; begin   Result := (Obj <> nil) and (Obj is TPrimaryKey<T>)     and (TPrimaryKey<T>(Obj).GetValue = fValue); end;  function TPrimaryKey<T>.GetValue: T; begin   Result := fValue; end; 

Why does the compiler think that fValue and the result of GetValue() can not be compared?

Answer 1

What if T is a string? What if it’s a TSize record?

Without constraining T (e.g. with <T :class>), you can’t be sure that the comparison will be meaningful.

If, instead, you wanted to compare two values of type T, you can use the Generics.Defaults unit and use:

TEqualityComparer<T>.Default.Equals(x, y) 

to compare values x and y of type T.