Teach Yourself Borland C++ in 14 Days - portal

108 Day 4
“But,” you ask, “what does this mean?” Remember that each class instance gets its own copy
of the class’s data members. But all class instances share the same set of functions for the class
(there’s no point in duplicating that code for each instance of the class). How does the
compiler figure out which instance goes with which function call? Each class member
function has a hidden this parameter that goes with it. To illustrate, let’s say you have a
function for the Rect class called GetWidth(). It would look like this (no pun intended):
int Rect::GetWidth()
{
return right - left;
}
That’s how the function looks to you and me. To the compiler, though, it looks something
like this:
int Rect::GetWidth(Rect* this)
{
return this->right - this->left;
}
That’s not exactly accurate from a technical perspective, but it’s close enough for this
discussion. From this code you can see that this is working behind the scenes to keep
everything straight for you. You don’t have to worry about how that happens, but only that
it does happen.
WARNING
Never modify the this pointer. You can use it to pass a pointer to your
class to other functions, or as a parameter in constructing other classes,
but don’t change its value. Learn to treat this as a read-only variable.
Although this works behind the scenes, it is still a variable that you can access from within
the class. As an illustration, let’s take a quick peek into VCL. Most of the time you will create
components in VCL by dropping them on the form at design time. When you do that,
C++Builder creates a pointer to the component and does all sorts of housekeeping chores on
your behalf, saving you from concerning yourself with the technical end of things.
Sometimes, however, you will create a component at runtime. VCL has this funny insistence
(as all good frameworks do) on wanting to keep track of which child objects belong to which
parent. For instance, let’s say you wanted to create a button on a form when another button
is clicked. You need to tell VCL who the parent of the new button is. The code would look
like this:
void __fastcall TMyForm::Button1Click(TObject *Sender)
{
TButton* button = new TButton(this);
button->Parent = this;