Teach Yourself Borland C++ in 14 Days - portal

492 Day 14
__classid
The __classid keyword generates a pointer to a class’s virtual function table (vtable).
__classid is for C++Builder’s internal use and it is not recommended that you use this
keyword in your applications.
__closure
A closure is a special type of function pointer. A regular function pointer is a 4-byte value that
points to a function within the program’s code segment. A closure, on the other hand, is an
8-byte value that contains not only the function address (in the first 4 bytes), but also the
address of the specific object in which the function resides (in the last 4 bytes). This type of
function pointer is vital to the way C++Builder programs operate. In Pascal this is termed a
method pointer. A closure is the mechanism that allows you, at design time or runtime, to
assign an event handler in one class (in your main form’s class, for instance) with an event
that occurs in another class (such as in a menu class or a button class). While this is very
common in C++Builder programming, it might come as a surprise to learn that the C++
language does not inherently have this ability. The __closure keyword extends C++ to allow
events and event handlers to work as they do.
A closure, then, is used with events. An event is a property that uses a function pointer as its
data type. So an event is a closure. You can declare a closure just as you do a regular function
pointer, only with the addition of the __closure keyword:
void __fastcall (__closure *TMyEvent)(System::TObject *Sender);
You will generally only use the __closure keyword when writing events for components.
While your day-to-day use of C++Builder will rarely make direct use of the __closure
keyword, you will still use closures heavily whether you realize it or not. Closures are
something that should, in my opinion, be in the C++ language standard.
__declspec
The __declspec keyword is not new to Borland C++ compilers, but it has been extended for
use in C++Builder. The delphiclass and delphireturn arguments are new to C++Builder.
You use __delcspec(delphiclass) any time you forward declare a class derived from TObject
(whether derived directly or indirectly). You use __delcspec(delphireturn) any time you make
a forward declaration of a class derived from Currency, AnsiString, Variant, TDateTime, or Set.