Teach Yourself Borland C++ in 14 Days - portal

150 Day 5
NOTE
Remember when you changed the Name property of the memo component
to Memo? When you did that, C++Builder went to work behind the
scenes. C++Builder first derived a class from TMemo. Then it created a
dynamic instance of the new class and gave it the variable name Memo.
In the code in this section, Memo is a pointer to the object. That is why
the Show() and Hide() functions are accessed using the indirect member
operator.
That’s all there is to it! But does it work? Let’s find out. Click the Run button on the speedbar.
After being compiled, the program runs and is displayed. It’s the moment of truth. Click the
button, and the memo component is hidden. Click the button again, and the memo
component is again displayed. It works! Hallelujah! After playing with that for a minute, close
the program (use the Close Program button in the upper-left corner of the title bar) and you
are back to the Code Editor.
Hmmm…all that messing with the static bool variable is a bit cumbersome. Think back to
the discussion about properties. Wouldn’t it be nice if the memo component had a property
that could tell us whether the component was currently visible? Is there such a beast? Of
course there is! It’s called, predictably, Visible. Let’s make use of it. Again, edit the function
until it looks like this:
void __fastcall TPMEForm::ButtonClick(TObject *Sender)
{
if (Memo->Visible) Memo->Hide();
else Memo->Show();
}
Again click the Run button. The program is displayed and, lo and behold, the button does
what it’s supposed to. How about that? We managed to use properties, methods, and events
in the same example.
Are you getting the fever yet? Hold on, because there’s lots more to come. Oh, and wipe that
silly grin off your face…your boss thinks you’re working!
As you can see, the ButtonClick() function takes a pointer to a TObject called Sender. Every
event-handling function will have at least a Sender parameter. Depending on the event being
handled, the function might have one or more additional parameters. For instance, the
OnMouseDown event handler looks like this:
void __fastcall TPMEForm::ButtonMouseDown(TObject *Sender,
TMouseButton Button, TShiftState Shift, Integer X, Integer Y)
{
}