When does the Delphi compiler create an implicit interface variable?

Question

I tested the code block for comment here in StackOverflow, and I came across a situation where an implicit interface variable lifted its head. What I do not see is what causes it in this case. I have a mini factory that returns an interface for a newly created object. If I call the method in the procedure block, then I get only reference count 1. If I call it from the main program block, I get reference count 2.

I tested this with Delphi 10 Seattle. Is there a resource that contains rules for creating an implicit interface and is a template in which the interface returned by the factory is not a reliable template?

program TestRefCount;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils,
  Vcl.Dialogs;

type
  IMyInterface = interface(IInterface)
    ['{62EB2C46-9B8A-47CE-A881-DB96E6F6437D}']
    procedure DoSomething;
    function GetRefCount: Integer;
  end;

  TMyObject = class(TInterfacedObject, IMyInterface)
  strict private
    FMyValue: Integer;
  public
    procedure Init;
    procedure DoSomething;
    function GetRefCount: Integer;
  end;

  TMyFactory = class(TObject)
  private
    function CreateMyInt: IMyInterface;
  end;

procedure TMyObject.DoSomething;
begin
  MessageDlg(IntToStr(FMyValue), mtInformation, [mbok], 0);
end;

function TMyObject.GetRefCount: Integer;
begin
  Result := FRefCount;
end;

procedure TMyObject.Init;
begin
  FMyValue := 100;
end;

function TMyFactory.CreateMyInt: IMyInterface;
var
  myObject: TMyObject;
begin
  myObject := TMyObject.Create;
  Assert(myObject.GetRefCount = 0);
  myObject.Init;
  Assert(myObject.GetRefCount = 0);
  Result := myObject;
  Assert(myObject.GetRefCount = 1);
  Assert(Result.GetRefCount = 1);
end;

procedure WorkWithIntf;
var
  myFactory: TMyFactory;
  myInt: IMyInterface;
begin
  myFactory := TMyFactory.Create;
  try
    myInt := myFactory.CreateMyInt;
    Assert(myInt.GetRefCount = 1);
    myInt.DoSomething;
    Assert(myInt.GetRefCount = 1);
  finally
    myFactory.Free;
  end;
end;

var
  myFactory: TMyFactory;
  myInt: IMyInterface;
begin
  try
    // This case doesn't have an implicit interface variable
    WorkWithIntf;
    // This case does have an implicit interface variable
    myFactory := TMyFactory.Create;
    try
      myInt := myFactory.CreateMyInt;
      Assert(myInt.GetRefCount = 1); // This fails because the refcount is 2
      myInt.DoSomething;
      Assert(myInt.GetRefCount = 1);
    finally
      myFactory.Free;
    end;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

Here are the first blocks that do not have an implict interface variable:

TestRefCount.dpr.67: myInt := myFactory.CreateMyInt;
005C6A5A 8D55F8           lea edx,[ebp-$08]
005C6A5D 8B45FC           mov eax,[ebp-$04]
005C6A60 E83BFEFFFF       call TMyFactory.CreateMyInt
TestRefCount.dpr.68: Assert(myInt.GetRefCount = 1);
005C6A65 8B45F8           mov eax,[ebp-$08]

, :

TestRefCount.dpr.86: myInt := myFactory.CreateMyInt;
005CF513 8D55EC           lea edx,[ebp-$14]
005CF516 A19CB75D00       mov eax,[$005db79c]
005CF51B E88073FFFF       call TMyFactory.CreateMyInt
005CF520 8B55EC           mov edx,[ebp-$14]
005CF523 B8A0B75D00       mov eax,$005db7a0
005CF528 E8C7E3E3FF       call @IntfCopy
TestRefCount.dpr.87: Assert(myInt.GetRefCount = 1); // This fails because the refcount is 2
005CF52D A1A0B75D00       mov eax,[$005db7a0]

Answer 1

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