Question

After reading many post on StackOverflow about the cons of using automatic reference counting for Interfaces, I started trying to manually reference counting each interface instantiation.

After trying for a full afternoon I give up!

Why do I get Access Violation when I call FreeAndNil(p)?

What follow is a complete listing of my simple unit.

unit fMainForm;

interface

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

type
  TForm4 = class(TForm)
    btn1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure btn1Click(Sender: TObject);
  end;

type
  IPersona = interface(IInterface)
  ['{44483AA7-2A22-41E6-BA98-F3380184ACD7}']
    function GetNome: string;
    procedure SetNome(const Value: string);
    property Nome: string read GetNome write SetNome;
  end;

type
  TPersona = class(TObject, IPersona)
  strict private
    FNome: string;
    function GetNome: string;
    procedure SetNome(const Value: string);
  protected
    function _AddRef: Integer; stdcall;
    function _Release: Integer; stdcall;
    function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
  public
    constructor Create(const ANome: string);
    destructor Destroy; override;
  end;

var
  Form4: TForm4;

implementation

{$R *.dfm}

procedure TForm4.FormCreate(Sender: TObject);
begin
  ReportMemoryLeaksOnShutdown := True;
end;

procedure TForm4.btn1Click(Sender: TObject);
var
  p: IPersona;
begin
  p := TPersona.Create('Fabio');
  try
    ShowMessage(p.Nome);
  finally
    FreeAndNil(p);
  end;
end;

constructor TPersona.Create(const ANome: string);
begin
  inherited Create;
  FNome := ANome;
end;

destructor TPersona.Destroy;
begin
  inherited Destroy;
end;

function TPersona._AddRef: Integer;
begin
  Result := -1
end;

function TPersona._Release: Integer;
begin
  Result := -1
end;

function TPersona.QueryInterface(const IID: TGUID; out Obj): HResult;
begin
  if GetInterface(IID, Obj) then
    Result := S_OK
  else
    Result := E_NOINTERFACE;
end;

function TPersona.GetNome: string;
begin
  Result := FNome;
end;

procedure TPersona.SetNome(const Value: string);
begin
  FNome := Value;
end;

end.

Answer 1

The access violation occurs because FreeAndNil receives an untyped var parameter that is expected to be an object reference. You are passing an interface reference which does not meet the requirement. Unfortunately you only find out at runtime. This is, in my view, the strongest point against the use of FreeAndNil.

Your reference counting disables lifetime management by the interface reference counting mechanism. In order to destroy an object you need to call its destructor. And in order to do that you must have access to the destructor. Your interface doesn’t expose the destructor (and it should not). So, we can deduce that, in order to destroy the object, you need to have an object reference.

Here are some options:

var
  obj: TPersona;
  intf: IPersona;
....
obj := TPersona.Create('Fabio');
try
  intf := obj;
  //do stuff with intf
finally
  obj.Free;
  // or FreeAndNil(obj) if you prefer
end;

Or you can do it like this

var
  intf: IPersona;
....
intf := TPersona.Create('Fabio');
try
  //do stuff with intf
finally
  (intf as TObject).Free;
end;