Delphi2009 Generics and Type Constraints

The .NET type system is rooted at System.Object. The Delphi/Win32 type system isn’t rooted (built-in simple types, records, and classes don’t have a common ancestor), and primitive types can’t/don’t implement interfaces, so someone in the newsgroups asked how to deal with generics which must perform operations on primitive types, without the use of IComparable, et. al. Here’s one way. I’m not sure if it’s the best way, but it works.

unit Unit5;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TAdder<T> = class
  public
    function AddEm(A, B: T): T; virtual; abstract;
  end;

  TIntAdder = class(TAdder<integer>)
  public
    function AddEm(A, B: integer): integer; override;
  end;

  TStringAdder = class(TAdder<string>)
  public
    function AddEm(A, B: string): string; override;
  end;

  TMath<T; A: TAdder<T>, constructor> = class
  public
     function AddTwoDigits(Left, Right: T): T;
  end;

  TForm5 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  end;

var
  Form5: TForm5;

implementation

{$R *.dfm}

procedure TForm5.Button1Click(Sender: TObject);
var
  AddInt: TMath<integer, TIntAdder>;
  AddString: TMath<string, TStringAdder>;
begin
  AddInt := TMath<integer, TIntAdder>.Create;
  try
    ShowMessage(IntToStr(AddInt.AddTwoDigits(2, 2)));
  finally
    AddInt.Free;
  end;
  AddString := TMath<string, TStringAdder>.Create;
  try
    ShowMessage(AddString.AddTwoDigits('2', '2'));
  finally
    AddString.Free;
  end;
end;

{ TIntAdder }

function TIntAdder.AddEm(A, B: integer): integer;
begin
  Result := A + B;
end;

{ TStringAdder }

function TStringAdder.AddEm(A, B: string): string;
begin
  Result := IntToStr(StrToInt(A) + StrToInt(B));
end;

{ TMath<T, A> }

function TMath<T, A>.AddTwoDigits(Left, Right: T): T;
var
  Add: A;
begin
  Add := A.Create;
  try
    Result := Add.AddEm(Left, Right);
  finally
    Add.Free;
  end;
end;

end.