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   ► KBTo/From GuidesVB ClassicOOP Basics  Print This     

Cross Ref > OOP Basics

By Mike Prestwood

VB Classic versus C++/CLI: A side by side comparison between VB Classic and C++/CLI.

 
OOP Basics
 

Some languages support object-based concepts such as Paradox, Access, and VB Classic. Other languages have OO extensions and fully support object orientation in a hybrid fashion (such as C++ and Dephi for Win32). Finally, some lanages such as C#, VB.Net, Prism, and Java are entirely written in OO. Meaning, every line of code written must occur within a class).

Base Class

[Other Languages] 

Languages Focus

When you create a class, it is either a base class or inherits from another class. Some languages require all classes to inherit from a common base class and some do not.

VB Classic:   Not Supported
C++/CLI:   System::Object

In C++/CLI, the Object keyword is an alias for the base System::Object class and is the single base class all classes ultimately inherit from.

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Class..Object

[Other Languages] 

Languages Focus

In short, a class is a data type, and an object is an instance of a class type. A class has methods (routines), properties (member variables), and a constructor. The current values of the properties is the current state of the object. The UML is one of the diagraming disciplines that allows you to document the various changing states of a series of objects.

VB Classic:   Yes

One class per file. File must use a .cls extension (instead of .bas). You can include your classes in your project (a traditional OOP approach) or compile them to a DLL or ActiveX EXE, register them, and use them that way (a uniquely VB approach).

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Inheritance

[Other Languages] 

The concept of a class makes it possible to define subclasses that share some or all of the main class characteristics. This is called inheritance. Inheritance also allows you to reuse code more efficiently. In a class tree, inheritance is used to design classes vertically. (You can use Interfaces to design classes horizontally within a class tree.) With inheritance, you are defining an "is-a" relationship (i.e. a chow is-a dog). Analysts using UML call this generalization where you generalize specific classes into general parent classes.

VB Classic:   No, but see Implements.

VB classic supports only a single "level" of interface inheritance (abstract class to implimentation class). See Implements in VB's help.

[Not specified yet. Coming...]




Member Method

[Other Languages] 

Also known as a Class Method.

A code routine that belongs to the class or an object instance (an instance of the class). Methods that belong to the class are called class methods or static methods. Methods that belong to an object instance are called instance methods, or simply methods.

When a method returns a value, it is a function method. When no value is returned (or void), it is a procedure method.

Methods frequently use method parameters to transfer data. When one object instance calls another object instance using a method with parameters, you call that messaging.

VB Classic:   sub, function

VB classic uses the keywords sub and function. A sub does not return a value and a function does. Many programmers like to use the optional call keyword when calling a sub to indicate the call is to a procedure.

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Member Visibility

[Other Languages] 

General Info: Class Visibility Specifiers

In OOP languages, members of a class have a specific scope that indicates visibility. Standard visibility includes private, protected, and public. Private members are usable by the defining class only (fully encapsulated). They are invisible outside of the class except by friendly classes. Protected members are usable by the defining class and descendant classes only (plus friendly classes). Public members are usable wherever its class can be referenced.

Languages Focus

Traditional member visibility specifiers for fully OOP languages are private, protected, and public. Many modern OOP languages implement additional member visibilities.

Additional member modifiers are documented under the Member Modifiers topic.

VB Classic: 

In VB Classic, the keywords Private, Friend, Public, and Static are used to set access levels for declared elements.

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