Delphi: How delegate interface implementation to child object?

Question

i have an object which delegates implementation of a particularly complex interface to a child object. This is exactly i think is the job of TAggregat

Answer 1

You have to add a field instance for the created child object:

type
  TRobot = class(TInterfacedObject, IStream)
  private
     FStream: TRobotStream;
     function GetStream: IStream;
  public
     property Stream: IStream read GetStream implements IStream;
  end;

destructor TRobot.Destroy;
begin
  FStream.Free; 
  inherited; 
end;

function TRobot.GetStream: IStream;
begin
  if FStream = nil then 
    FStream := TRobotStream.Create(Self);
  result := FStream;
end;

Update TRobotStream should be derived from TAggregatedObject as you already guessed. The declaration should be:

type
  TRobotStream = class(TAggregatedObject, IStream)
   ...
  end;

It is not necessary to mention IUnknown.

In TRobot.GetStream the line result := FStream does an implicite FStream as IStream so writing this out isn't necessary either.

FStream has to be declared as TRobotStream and not as IStream so it can be destroyed when the TRobot instance is destroyed. Note: TAggregatedObject has no reference counting so the container has to take care of its lifetime.

Update (Delphi 5 code):

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    Button1: TButton;
    Edit1: TEdit;
    procedure Button1Click(Sender: TObject);
  private
    procedure LoadRobotFromDatabase(rs: IStream);
  public
  end;

type
  TRobotStream = class(TAggregatedObject, IStream)
  public
    { IStream }
    function Seek(dlibMove: Largeint; dwOrigin: Longint;
       out libNewPosition: Largeint): HResult; stdcall;
    function SetSize(libNewSize: Largeint): HResult; stdcall;
    function CopyTo(stm: IStream; cb: Largeint; out cbRead: Largeint; out cbWritten: Largeint): HResult; stdcall;
    function Commit(grfCommitFlags: Longint): HResult; stdcall;
    function Revert: HResult; stdcall;
    function LockRegion(libOffset: Largeint; cb: Largeint; dwLockType: Longint): HResult; stdcall;
    function UnlockRegion(libOffset: Largeint; cb: Largeint; dwLockType: Longint): HResult; stdcall;
    function Stat(out statstg: TStatStg; grfStatFlag: Longint): HResult; stdcall;
    function Clone(out stm: IStream): HResult; stdcall;
    function Read(pv: Pointer; cb: Longint; pcbRead: PLongint): HResult; stdcall;
    function Write(pv: Pointer; cb: Longint; pcbWritten: PLongint): HResult; stdcall;
  end;

type
  TRobot = class(TInterfacedObject, IStream)
  private
    FStream: TRobotStream;
    function GetStream: IStream;
  public
    destructor Destroy; override;
    property Stream: IStream read GetStream implements IStream;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  rs: IStream;
begin
  rs := TRobot.Create;
  LoadRobotFromDatabase(rs); //dummy method, just to demonstrate we use the stream
  rs := nil;
end;

procedure TForm1.LoadRobotFromDatabase(rs: IStream);
begin
  rs.Revert; //dummy method call, just to prove we can call it
end;

function TRobotStream.Clone(out stm: IStream): HResult;
begin
end;

function TRobotStream.Commit(grfCommitFlags: Integer): HResult;
begin
end;

function TRobotStream.CopyTo(stm: IStream; cb: Largeint; out cbRead, cbWritten: Largeint): HResult;
begin
end;

function TRobotStream.LockRegion(libOffset, cb: Largeint; dwLockType: Integer): HResult;
begin
end;

function TRobotStream.Read(pv: Pointer; cb: Integer; pcbRead: PLongint): HResult;
begin
end;

function TRobotStream.Revert: HResult;
begin
end;

function TRobotStream.Seek(dlibMove: Largeint; dwOrigin: Integer;
  out libNewPosition: Largeint): HResult;
begin
end;

function TRobotStream.SetSize(libNewSize: Largeint): HResult;
begin
end;

function TRobotStream.Stat(out statstg: TStatStg; grfStatFlag: Integer): HResult;
begin
end;

function TRobotStream.UnlockRegion(libOffset, cb: Largeint; dwLockType: Integer): HResult;
begin
end;

function TRobotStream.Write(pv: Pointer; cb: Integer; pcbWritten: PLongint): HResult;
begin
end;

destructor TRobot.Destroy;
begin
  FStream.Free;
  inherited;
end;

function TRobot.GetStream: IStream;
begin
  if FStream = nil then
     FStream := TRobotStream.Create(Self);
  result := FStream;
end;

end.

Answer 2

There is no need for your class that does the delegation to inherit from any particular class. You could inherit from TObject provided the appropriate methods have been implemented. I'll keep things simple and illustrate using TInterfacedObject which provides the 3 core methods which you have already identified.

Also, you should not need TRobotStream = class(TAggregatedObject, IUnknown, IStream). You could instead simply declare that IStream inherits from IUnknown. By the way, I always give my interfaces a GUID (Press the conbination Ctrl+Shift+G).

There are a number of different approaches and techniques that can be applied depending on your particular needs.

• Delegating to interface type
• Delegating to class Type
• Method aliasing

The simplest delegation is by interface.

TRobotStream = class(TinterfacedObject, IStream)

TRobot = class(TInterfacedObject, IStream)
private
  //The delegator delegates the implementations of IStream to the child object.
  //Ensure the child object is created at an appropriate time before it is used.
  FRobotStream: IStream;
  property RobotStream: IStream read FRobotStream implements IStream;
end;

There are perhaps a few thing to watch out for:

• Ensure the objects you're delegating to have an appropriate lifetime.
• Be sure to hold a reference to the delegatee. Remember that interfaces are reference counted, and will be destroyed as soon as the count drops to zero. This may actually have been the cause of your headaches.