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   ► KBTo/From GuidesDelphi Prism  Print This     

Member Visibility (Delphi Prism and C++ Cross Reference Guide)

By Mike Prestwood

Delphi Prism versus C++: A side by side comparison between Delphi Prism and C++.

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).

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.

Delphi Prism:  "Class Member Visibility Levels"

In Prism, you specify each class and each class member's visibility with a Class Member Visibility Level preceding the return type. Like Delphi, you group member declarations as part of defining the interface for a class in the Interface section of a unit.

Unlike Delphi, Prism supports a traditional OO approach to member visibility with additional .Net type assembly visibility. For example, private members are truly private to the class they are declared in. In Delphi for Win32, you use strict private for true traditional private visibility.

Prism also supports assembly and protected and assembly or protected which modify the visibility of protected members to include only descendants in the same assembly (and) or publicly accessible from assembly and descendant only outside (or). OO purist might object to assembly and protected and assembly or protected and I suggest you choose the traditional private, protected, and public as your first chose at least until you both fully understand them and have a specific need for them.

Syntax Example:
Cyborg = public class(System.Object)
  //private properties, methods, etc. here.
  FName: String;
  //protected properties, methods, etc. here.
assembly and protected
  //properties, methods, etc. here.
assembly or protected
  //properties, methods, etc. here.
  //properties, methods, etc. here.

C++ implements class and member visibility specifiers traditionally. Note the colon at the end of each visibility specifier and the semi-colon at the end of the class (the end of the statement).

Syntax Example:
class Cyborg: Public AParentClass {

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