Generics constructor with parameter constraint?

Question

TMyBaseClass=class
  constructor(test:integer);
end;

TMyClass=class(TMyBaseClass);

TClass1=class()
  public
    FItem: T;
    pr         


        

Answer 1

What seems to work in Delphi XE, is to call T.Create first, and then call the class-specific Create as a method afterwards. This is similar to Rudy Velthuis' (deleted) answer, although I don't introduce an overloaded constructor. This method also seems to work correctly if T is of TControl or classes like that, so you could construct visual controls in this fashion.

I can't test on Delphi 2010.

type
  TMyBaseClass = class
    FTest: Integer;
    constructor Create(test: integer);
  end;

  TMyClass = class(TMyBaseClass);

  TClass1 = class
  public
    FItem: T;
    procedure Test;
  end;

constructor TMyBaseClass.Create(test: integer);
begin
  FTest := Test;
end;

procedure TClass1.Test;
begin
  FItem := T.Create; // Allocation + 'dummy' constructor in TObject
  try
    TMyBaseClass(FItem).Create(42); // Call actual constructor as a method
  except
    // Normally this is done automatically when constructor fails
    FItem.Free;
    raise;
  end;
end;


// Calling:
var
  o: TClass1;
begin
  o := TClass1.Create();
  o.Test;
  ShowMessageFmt('%d', [o.FItem.FTest]);
end;