Interfaces without reference counting - delphi

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;

Answer 2

You cannot use FreeAndNil() with an interface reference, only an objct reference. Had you left the interface's reference count enabled, you would simply assign nil to the interface reference (or just let it go out of scope) to free the object correctly, eg:
type
TPersona = class(TInterfacedObject, IPersona)
strict private
FNome: string;
function GetNome: string;
procedure SetNome(const Value: string);
public
constructor Create(const ANome: string);
destructor Destroy; override;
end;
procedure TForm4.btn1Click(Sender: TObject);
var
p: IPersona;
begin
p := TPersona.Create('Fabio');
try
ShowMessage(p.Nome);
finally
p := nil;
end;
end;
But since you have disabled the reference count on the interface, you need to go back to using normal object reference variables in your code, eg:
procedure TForm4.btn1Click(Sender: TObject);
var
p: TPersona;
intf: IPersona;
begin
p := TPersona.Create('Fabio');
try
if Supports(p, IPersona, intf) then
ShowMessage(intf.Nome);
finally
FreeAndNil(p);
end;
end;