Singleton in Delphi

[himitsu]

Warum nicht die Variable auch noch mit in die Klasse?

Delphi-Quellcode:

type
  TSingleton = class sealed
  private
    class var Singleton : TSingleton;
    constructor Create;
  public
    procedure Tuwas;
    destructor Destroy; override;
    class function GetInstance : TSingleton;
  end;

Und den Destructor würde ich noch absichern, damit keiner die Instanz von extern freigeben kann.

Delphi-Quellcode:

TSingleton = class sealed
private
  class var Singleton : TSingleton;
  class var AllowFree : Boolean;
  constructor Create;
public
  procedure Tuwas;
  destructor Destroy; override;
  procedure FreeInstance; override;
  class function GetInstance : TSingleton;
end;

class function TSingleton.GetInstance: TSingleton;
var 
  S: TSingleton;
begin
  if not Assigned(Singleton) then
  begin
    S := TSingleton.Create;
    if Assigned(InterlockedCompareExchangePointer(Pointer(Singleton), Pointer(S), nil)) then
      S.Free;
  end;
  Result := Singleton;
end;

procedure TSingleton.FreeInstance;
begin
  if AllowFree then
    inherited FreeInstance;
end;

finalization
  TSingleton.AllowFree := True;
  TSingleton.Singleton.Free;