6.1 Class de�nitions

6.1 Class denitions

The prototype declaration of a class is as follows:

_________________________________________________________________________________________________________ Class types
-- ------------ ---------------------------- ----------------
-packed -|class -heritage| --component list-| end
6-------------|

-- -- - - ----------------------- ------------
heritage ( class type identi er - -| )
implemented interfaces

-- --implemented interfaces-6, -interface identi er------------------------
--------------------

-- -- ------------------|-----------------
component list -visibility speci er-||-- eld de nition ---|
6------------|
- -----------------------------------------------------------------
----method de nition-----|
6|-property de nition -||
--------------------

-- -- eld de nition identi er list- :- type- ;------------------------------

-- -- ------------- ----- --
method de nition | -class-| -pfurnoccetdiounr hee haedaederr-|| ;
------constructor header-----|
-------desctuctor header------|
- -----------------------------------------------------------------
---- virtual--|--------------- ;--|-call modi ers -;--|
| -;- abstract -- |
|---------override ---------|
-message -|integer constant--
--string constant---

-- --class visibility speci er|-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 dierence between objects and classes is in the way they are created (see further in this chapter). The visibility of the dierent sections is as follows:

Private: All elds and methods that are in a private block, can only be accessed in the module (i.e. unit) that contains the class denition. 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 dened in a published section must be of class type. Array properties cannot be in a published section.

In the syntax diagram, it can be seen that a class lists implemented interfaces. This will be discussed in the next chapter.

It is also possible to dene 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 specied at run time. Essentially, a variable of a class reference type contains a pointer to the denition of the specied class. This can be used to construct an instance of the class corresponding to the denition, or to check inheritance. 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;

This function can be passed a class reference of any class that descends from TComponent. The following is a valid call:

Var
C : TComponent;

begin
C:=CreateComponent(TEdit,Form1);
end;

On return of the CreateComponent function, C will contain an instance of the class TEdit. Note that the following call will fail to compile:

Var
C : TComponent;

begin
C:=CreateComponent(TStream,Form1);
end;

because TStream does not descend from TComponent, and AClass refers to a TComponent class. The compiler can (and will) check this at compile time, and will produce an error.

References to classes can also be used to check inheritance:

  TMinClass = Class of TMyClass;
  TMaxClass = Class of TMyClassChild;

Function CheckObjectBetween(Instance : TObject) : boolean;

begin
  If not (Instance is TMinClass)
     or ((Instance is TMaxClass)
          and (Instance.ClassType<>TMaxClass)) then
    Raise Exception.Create(SomeError)
end;

The above example will More about instantiating a class can be found in the next section.

[next] [front] [up]