Delphi - 通用类型检查是否已创建

Question

I have the following class definition

  TBase<T> = class
  public
    class var Inst: T;
    class function GetClone: T;
  end;

And I want to check if the class var Inst is assigned.

class function TBase<T>.GetClone: T;
begin
 if TBase<T>.Inst = nil then //- error here. Trying with Assigned(TBase<T>.Inst) is also nor recognized.
    TBase<T>.Inst := TBase<T>.Create;
end;

How can I check if my class variable is assigned?

Answer 1

You need to constraint the generic parameter in order to check for nil . For example:

TBase<T: class> = class //...

That way T must be TObject or any descendant of it, so you can check for nil .

Without the constraint T can be integer or any other value type that doesn't support nil .