Questions regarding PB OOP

[Andreas Kage]

Hi,
I would like to take advantage of the PB OOP structure with an adequate documentation of the program. One of the commonly used documentation of the objects is the UML type with the three basic elements NAME, ATTRIBUTES and ACTIONS. Below I tried to do a projection of this documentation type to the syntax and structure of PB objects/classes. This might illuminate the relation between the objects of the real world with the classes in PB OOP.

####################### OBJECT and Class Definition #############################################
++ Object (UML like structure) ++| +++++++++ CLASS (PB – structure and syntax) ++++++++++++++++++
|
===========NAME=============== | ==CLASS name [$GUID] [AS COM | AS EVENT]========================================================================
| Class Method code blocks...
| [CLASS|OVERRIDE] METHOD name_class_level [<DispID>] [ALIAS "altname"] (var AS type...) [AS type]
| statements...
| METHOD = expression
| END METHOD'
|
========ATTRIBUTES============ | ======INSTANCE attribute AS type ===============================================================================
- - action on generation - -| CLASS METHOD CREATE()
| statements...
| METHOD = expression
| END METHOD
|
- - action on destruction - | CLASS METHOD Destroy()
| statements...
| METHOD = expression
| END METHOD
| |
-----------interface-----------| INTERFACE interfacename [$GUID] [AS EVENT] [AS HIDDEN]
- - - -inheritance- - - -| INHERIT IUNKNOWN
| INHERIT IAUTOMATION
OR | INHERIT IDISPATCH ' only one INHERITANCE per CLASS
|
- - - - new - - - - |
===========ACTIONS============ | ========methods=================================================================================================
|
- - - common action - - - | [CLASS|OVERRIDE] METHOD name_interface_level [<DispID>] [ALIAS "altname"] (var AS type...) [AS type]
| statements...
| METHOD = expression
| END METHOD
| [OVERRIDE] PROPERTY GET|SET name_interface_level [<DispID>] [ALIAS "altname"] (var AS type...) [AS type]
| statements...
| PROPERTY = expression
| END PROPERTY
|
|
----------#######----------- | END INTERFACE
| END CLASS




The projection of OBJECT-NAME and CLASS-NAME, ATTRIBUTES and INSTANCES, and ACTIONS and METHODS are obvious. However, I found in the handbook indication that PB differentiates between METHODS on the class_LEVEL (syntactic position above the INSTANCE definition) and METHODS on the interface_LEVEL. The later are used in combination with the INTERFACE and can be accessed from the outside of the CLASS. The variables (ATTRIBUTES) used in the interface_LEVEL METHODS are defined as INSTANCES.

QUESTION: How are the variables defined/declared for the class_LEVEL METHODS located above the INSTANCE block ?

QUESTION: How are the class_LEVEL METHODS accessed/called inside of the CLASS (any examples ?) ?

QUESTION: Are there any relevant limitation of number, size or type of variables transferred through a METHOD command ?


INHERITANCE implies a hierarchical structure from great grand father/mother to great grand son/daughter. The handbook of PB does not mention such limitation.

QUESTION: If no hierarchical structure is required for the PB INHERITANCE, can we combine CLASSES with a circular referencing INHERITANCE ?

QUESTION: What types of errors can occur if an unlimited circular referencing of CLASSES occurs ?

Thank you for suggestions and replies

Andreas
[/LEFT]

[José Roca]

QUESTION: How are the variables defined/declared for the class_LEVEL METHODS located above the INSTANCE block ?

INSTANCE variables must be located ABOVE anything else, just below CLASS. There are not instance variables for class methods and instance variables for the rest.

QUESTION: Are there any relevant limitation of number, size or type of variables transferred through a METHOD command ?

Maximum number of parameters: 32. The same that for standard procedures.

Type of variables: For IUnknwon interfaces, the same that for standard procedures, excepting you can't pass an array descriptor. For IAutomation and IDispatch interfaces, only automation compatible types: BYTE, WORD, DWORD, INTEGER, LONG, QUAD, SINGLE, DOUBLE, CURRENCY, OBJECT, STRING, and VARIANT.

QUESTION: How are the class_LEVEL METHODS accessed/called inside of the CLASS (any examples ?) ?

From the help file:

What is a Class Method?
A CLASS METHOD is one which is private to the class in which it is located. That is, it may only be called from a METHOD or PROPERTY in the same class. It is invisible elsewhere. The CLASS METHOD must be located within a CLASS block, but outside of any INTERFACE blocks. This shows it is a direct member of the class, rather than a member of an interface.

CLASS MyClass
INSTANCE MyVar AS LONG

CLASS METHOD MyClassMethod(BYVAL param AS LONG) AS STRING
METHOD = "My" + STR$(param + MyVar)
END METHOD

INTERFACE MyInterface
INHERIT IUNKNOWN
METHOD MyMethod()
Result$ = ME.MyClassMethod(66)
END METHOD
END INTERFACE
END CLASS

In the above example, MyClassMethod() is a CLASS METHOD, and is always accessed using the pseudo-object ME (in this case ME.MyClassMethod). Class methods are never accessible from outside a class, nor are they ever described or published in a type library. By definition, there is no reason to have a private PROPERTY, so PowerBASIC does not offer a CLASS PROPERTY structure.

Regarding circular reference, it must be avoided by all means. If class A holds a reference to class B, and class B holds a reference to class A, neither A nor B will never be destroyed.