6.1 Class deﬁnitions

The prototype declaration of a class is as follows:

_________________________________________________________________________________________________________ Class types
------------ --------------------------------- -----------
- -|class - | ----- -----| end
packed heritage -clascso vmipsiobnilietyntsp liestciﬁer-|

--heritage -(- class type identiﬁer-)---------------------------------

-- - - - - ------------------------------
ﬁeld deﬁnition identiﬁer list : type ;

-- ------------- ----- --
method deﬁnition | - -| - function header-|| ;
---class procedure header--|
------codnessctrtuuccttoorr h heeaaddeerr-----|
-----------------------------------------------------------------
---- virtual------------------ ;--|-call modiﬁers-;--|
| -;- abstract -| |
|---------override ---------|
-message -|integer constant--
--string constant---

--class visibility speciﬁer---private ------------------------------------
|protected -|
|-public --|
-published -|
___________________________________________________________________

As many private, protected, published and public blocks as needed can be repeated. Methods are normal function or procedure declarations. As can be seen, the declaration of a class is almost identical to the declaration of an object. The real diﬀerence between objects and classes is in the way they are created (see further in this chapter). The visibility of the diﬀerent sections is as follows:

Private: All ﬁelds and methods that are in a private block, can only be accessed in the module (i.e. unit) that contains the class deﬁnition. They can be accessed from inside the classes’ methods or from outside them (e.g. from other classes’ methods)
Protected: Is the same as Private, except that the members of a Protected section are also accessible to descendent types, even if they are implemented in other modules.
Public: sections are always accessible.
Published: Is the same as a Public section, but the compiler generates also type information that is needed for automatic streaming of these classes. Fields deﬁned in a published section must be of class type. Array properties cannot be in a published section.

It is also possible to deﬁne class reference types:

_________________________________________________________________________________________________________ Class reference type
--class of-classtype-----------------------------------------------
___________________________________________________________________

Class reference types are used to create instances of a certain class, which is not yet known at compile time, but which is speciﬁed at run time. Essentially, a variable of a class reference type contains a pointer to the VMT of the speﬁcied class. This can be used to construct an instance of the class corresponding to the VMT. The following example shows how it works:

Type
  TComponentClass = Class of TComponent;

Function CreateComponent(AClass : TComponentClass; AOwner : TComponent) : TComponent;

begin
  // ...
  Result:=AClass.Create(AOwner);
  // ...
end;

More about instantiating a class can be found in the next section.

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