Teach Yourself Borland C++ in 14 Days - portal

C++ Class Frameworks and the Visual Component Model
underscores proceed the fastcall keyword). It’s not important that
you understand what __fastcall does, but just that every C++Builder
function uses this calling convention.
You’ll probably want to make the Code Editor window a little wider so you can see all of the
text that is displayed. Before you go on, save the project. Choose File | Save from the main
menu. The first thing you are prompted for is the name of the unit (source file). Type PMEMain
and click OK. Next you are prompted for a filename for the project. Type PMETest and press
Enter or click OK. Now on to the good stuff….
Notice that the function is already set up for you and all you have to do is type the code. If
you take a good look at the function, you will see that the function is called ButtonClick, that
it is a member function of the TPMEForm class, that it returns void, and that it takes a pointer
to a TObject called Sender as a parameter. (I’ll talk about the Sender parameter in just a bit.)
All that is left to do now is type code that alternately shows or hides the button each time the
button is clicked. We’ll borrow a little code from our earlier discussion of methods. Edit the
ButtonClick function until it looks like this:
void __fastcall TPMEForm::ButtonClick(TObject *Sender)
{
static bool isVisible;
isVisible = !isVisible;
if (isVisible) Memo->Hide();
else Memo->Show();
}
This code sets up a static variable named isVisible.
NEW TERM
A static variable is one that retains its value between function calls.
Static variables are the exception to the rule regarding uninitialized variables—static variables
are initially set to 0. In this case, isVisible is a bool variable, so it is initially set to false.
The second line of code in this function flips the bool variable between true and false by
applying a logical NOT to the present value of the variable. It works like this: Initially the static
variable is set to false. The first time the function executes, the variable is assigned NOT false,
which is, of course, true. The next time the function executes, the variable is assigned NOT
true, and so on. So each time the function executes, isVisible contains the opposite value
it had on the previous function call. After that, the if/else pair calls either Show() or Hide()
depending on the value of isVisible.
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