[rounin]

Hola, roman,
(6) I meant that I could forget about something

(5)
Mix-in classes is particular case of the multiple ihneritance.
When you don't want to use interface references,
and don't need the automatic life time management,
but want to add to your classes some different
orthogonal functionalities.

Mix-in interfaces allow make the interface of the base class compact.

Código Delphi [-]
type
  //Base class for mix-in support. Disables reference counting
  TMixInBase = class(TPersistent, IUnknown)
  public
    function TInterfacedObject.QueryInterface(const IID: TGUID; out Obj): HResult;
    //const E_NOINTERFACE = HResult($80004002);
    //begin
    //  if GetInterface(IID, Obj)
    //    then Result := 0
    //    else Result := E_NOINTERFACE;
    //end;
    function TInterfacedObject._AddRef: Integer;
    //begin
    //  Result := -1;
    //end;
    function TInterfacedObject._Release: Integer;
    //begin
    //  Result := -1;
    //end;
  end; 
  TSomeBaseObject = class(TMixInBase)
    {...}
  end;

  {----------------------------------------------------------------------------}

  // Some of descendant of the TSomeBaseObject have name, other don't have.
  IName = interface
  ['{D1693279-7EEA-417B-BCFC-BD7DC29E2763}']
    function GetName: string;
    procedure SetName(const AName: string);
  end;

  // Some of descendant of the TSomeBaseObject can create clone
  ICloneable = interface
  ['{8990AA78-364E-4BFA-B444-CF12608D39FF}']
    function Clone: TSomeBaseObject;
  end;

  // Some of descendant of the TSomeBaseObject can write their data to a stream
  IStreamable = interface
  ['{3B83C00D-85E0-4CDC-B757-549592C756FA}']
    procedure SaveData(Dest: TStream);
    procedure LoadData(Src: TStream);
  end;

  // Some of descendant of the TSomeBaseObject have child objects
  TChildProc = procedure(Child: TSomeBaseObject) of object;
  IComposite = interface
  ['{07CB94B4-231F-41FF-B51E-1570D69966C5}']
    procedure GetChildren(Proc: TChildProc);
  end;

  // Some of descendant of the TSomeBaseObject have an active area
  IHitTest = interface
  ['{4279B456-5749-44CB-BCBF-B2FFEF0CCB15}']
    function HitTest(const Point: TPoint): Boolean;
  end;

  TSomeDescendant = class(TSomeBaseObject, IName, ICloneable, IStreamable)
  {...}
  end;

  TOtherDescendant = class(TSomeBaseObject, IComposite, IStreamable)
  {...}
  end;

  {----------------------------------------------------------------------------}

// When you work with mix-in interfaces, you must use only local
// variables and must never save interface references in the
// global variables or object members.
// For example:

function GetName(Obj: TSomeBaseObject): string;
var intf: IName;
begin
  if Obj.GetInterface(IName, intf)
    then Result := intf.GetName
    else Result := '';
end;

function Clone(Obj: TSomeBaseObject): TSomeBaseObject;
var intf: ICloneable;
begin
  if Obj.GetInterface(ICloneable, intf)
    then Result := intf.Clone
    else raise Exception.Create('Object does''nt support ICloneable');
end;