23 implementationThe constructor TItem.Create, called in line 48, sets aside the memory to hold the object, initialises the object’s data fields, and returns the address of the object just created. So we typically create the object and assign the return value to the name we have already declared (the reference) in a single program step. This creates both the object and the link between the object name and the object itself:
24 {$R *.dfm}
25 uses
26 ItemU;
27 var
28 ItemCount: TItem;
29 procedure TfrmCount.btnAddItemClick(Sender: TObject) ;
30 begin
31 ItemCount.AddItem;
32 end;
33 procedure TfrmCount.btnDisplayMouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer) ;
35 begin
36 lblTotal.Caption := IntToStr(ItemCount.GetCount) ;
37 end;
38 procedure TfrmCount.btnDisplayMouseUp(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer) ;
40 begin
41 lblTotal.Caption := '';
42 end;
43 procedure TfrmCount.bmbResetClick(Sender: TObject) ;
44 begin
45 ItemCount.ZeroCount;
46 end;
47 initialization
48 ItemCount := TItem.Create;
49 finalization
50 ItemCount.Free;
end.
ItemCount := TItem.Create;
Notice that we create (instantiate) an object from a class. So when creating an object we call the class’s Create method (TItem.Create, line 48) and assign that to the object name declared in the variable declaration (line 28). When using an object, once it has been created, the method calls are to that object (ItemCount.AddItem in line 31, ItemCount.GetCount in line 36 and ItemCount.ZeroCount in line 45, and not TItem.AddItem etc).
You always have to make sure that you free the objects you create. The ItemCount object is freed in the finalization section of the unit.
Save all the code from example 3.2 as we’ll elaborate further on it in exs 3.3 and 3.4. Run and test this program.
If you want to trace what is happening in this program, return to the editor and place breakpoints in the bodies of the OnClick event handlers (lines 31, 36 and 45 in OODriverU.pas). (To place a breakpoint, place the cursor on the specific line in the editor and then press <F5>.) Run the program again. Click on the different buttons, step through the program line by line by using <F7> and notice how execution moves between units. Resume normal operation through <F9>. (<F5> is a toggle command and will also remove a breakpoint.)
That's it for today, next time we'll take a look at subclassing and inheritance (Chapter 7).
