The Delphi Object Model (PART II)

A derived class inherits the interfaces implemented by the ancestors'''' classes. Thus, TRectangle inherits from TBaseShape, and TBaseShape implements IShape so TRectangle implements IShape. Inheritance of interfaces works a little differently. Interface inheritance is merely a typing convenience, so you don''''t have to retype a lot of method declarations. When a class implements an interface, that does not automatically mean the class implements the ancestor interfaces. A class implements only those interfaces that are listed in its class declaration (and in the declaration for ancestor classes). Thus, even though IRectangle inherits from IPolygon, the TRectangle class must list IRectangle and IPolygon explicitly.

To implement a type hierarchy, you might not want to use reference counting. Instead, you will rely on explicit memory management, the way you do for normal Delphi objects. In this case, it''''s best to implement the _AddRef and _Release methods as stubs, such as those in the TNoRefCount class in Example 2-13. Just be careful not to have any variables that hold stale references. A variable that refers to an object that has been freed can cause problems if you use the variable. An interface variable that refers to an object that has been freed will certainly cause problems, because Delphi will automatically call its _Release method. In other words, you never want to have variables that contain invalid pointers, and working with interfaces that do not use reference counting forces you to behave.

COM and Corba

Delphi interfaces are also useful for implementing and using COM and Corba objects. You can define a COM server that implements many interfaces, and Delphi automatically manages the COM aggregation for you. The runtime library contains many classes that make it easier to define COM servers, class factories, and so on. Because these classes are not part of the Delphi Pascal language, they are not covered in this book. Consult the product documentation to learn more.

Reference Counting

The previous section discusses how Delphi uses reference counting to manage the lifetime of interfaces. Strings and dynamic arrays also use reference counting to manage their lifetimes. The compiler generates appropriate code to keep track of when interface references, strings, and dynamic arrays are created and when the variables go out of scope and the objects, strings, and arrays must be destroyed.

Usually, the compiler can handle the reference counting automatically, and everything works the way the you expect it to. Sometimes, though, you need to give a hint to the compiler. For example, if you declare a record that contains a reference counted field, and you use GetMem to allocate a new instance of the record, you must call Initialize, passing the record as an argument. Before calling FreeMem, you must call Finalize.

Sometimes, you want to keep a reference to a string or interface after the variable goes out of scope, that is, at the end of the block where the variable is declared. For example, maybe you want to associate an interface with each item in a TListView. You can do this by explicitly managing the reference count. When storing the interface, be sure to cast it to IUnknown, call _AddRef, and cast the IUnknown reference to a raw pointer. When extracting the data, type cast the pointer to IUnknown. You can then use the as operator to cast the interface to any desired type, or just let Delphi release the interface. For convenience, declare a couple of subroutines to do the dirty work for you, and you can reuse these subroutines any time you need to retain an interface reference. Example 2-15 shows an example of how you can store an interface reference as the data associated with a list view item.

Example 2-15: Storing Interfaces in a List View

// Cast an interface to a Pointer such that the reference
// count is incremented and the interface will not be freed
// until you call ReleaseIUnknown.
function RefIUnknown(const Intf: IUnknown): Pointer;
begin
  Intf._AddRef;               // Increment the reference count.
  Result := Pointer(Intf);    // Save the interface pointer.
end;
 
// Release the interface whose value is stored in the pointer P.
procedure ReleaseIUnknown(P: Pointer);
var
  Intf: IUnknown;
begin
  Pointer(Intf) := P;
  // Delphi releases the interface when Intf goes out of scope.
end;
 
// When the user clicks the button, add an interface to the list.
procedure TForm1.Button1Click(Sender: TObject);
var
  Item: TListItem;
begin
  Item := ListView1.Items.Add;
  Item.Caption := ''''Stuff'''';
  Item.Data := RefIUnknown(GetIntf as IUnknown);
end;
 
// When the list view is destroyed or the list item is destroyed
// for any other reason, release the interface, too.
procedure TForm1.ListView1Deletion(Sender: TObject; Item: TListItem);
begin
   ReleaseIUnknown(Item.Data);
end;
 
// When the user selects the list view item, do something with the
// associated interface.
procedure TForm1.ListView1Click(Sender: TObject);
var
  Intf: IMyInterface;
begin
  Intf := IUnknown(ListView1.Selected.Data) as IMyInterface;
  Intf.DoSomethingUseful;
end;

You can also store strings as data. Instead of using _AddRef, cast the string to a Pointer to store the reference to the string, then force the variable to forget about the string. When the variable goes out of scope, Delphi will not free the string, because the variable has forgotten all about it. After retrieving the pointer, assign it to a string variable that is cast to a pointer. When the subroutine returns, Delphi automatically frees the string''''s memory. Be sure your program does not retain any pointers to memory that is about to be freed. Again, convenience subroutines simplify the task. Example 2-16 shows one way to store strings.

Example 2-16: Storing Strings in a List View

// Save a reference to a string and return a raw pointer
// to the string.
function RefString(const S: string): Pointer;
var
  Local: string;
begin
  Local := S;                // Increment the reference count.
  Result := Pointer(Local);  // Save the string pointer.
  Pointer(Local) := nil;     // Prevent decrementing the ref count.
end;
 
// Release a string that was referenced with RefString.
procedure ReleaseString(P: Pointer);
var
  Local: string;
begin
  Pointer(Local) := P;
  // Delphi frees the string when Local goes out of scope.
end;
 
// When the user clicks the button, add an item to the list view
// and save an additional, hidden string.
procedure TForm1.Button1Click(Sender: TObject);
var
  Item: TListItem;
begin
  Item := ListView1.Items.Add;
  Item.Caption := Edit1.Text;
  Item.Data := RefString(Edit2.Text);
end;
 
// Release the string when the list view item is destroyed
// for any reason.
procedure TForm1.ListView1Deletion(Sender: TObject; Item: TListItem);
begin
  ReleaseString(Item.Data);
end;
 
// Retrieve the string when the user selects the list view item.
procedure TForm1.ListView1Click(Sender: TObject);
var
  Str: string;
begin
  if ListView1.Selected <> nil then
  begin
    Str := string(ListView1.Selected.Data);
    ShowMessage(Str);
  end;
end;

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