General Info: Class Visibility Specifiers

Languages Focus: Member Visibility

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.

• Reference: /ASPSuite/KB/Document_View.asp?QID=100889

TCyborg = class(System.Object)
private
  //Don't use accept when you really want private friendly members.
strict private
  //Use as your default private members.

  FName: String;
protected
  //Don't use accept when you really want protected friendly members.

strict protected
  //Use as your default protected members.

public

  

published

  //RTTI Info

end;

Now let's dive deeper...

Class Scope Specifiers

Private, Protected, Public, and Published Members

In OOP languages, members of a class have a specific scope that indicates visibility (Class Visibility Specifiers). In OOP languages, private, protected, and public members are defined as follows:

• Private = Used by the defining class only. Invisible outside of the class except by friendly classes.
• Protected = Used by the defining class and descendant classes only (plus friendly classes).
• Public = Visible wherever its class can be referenced.

Each OOP language implements these standard scope specifiers in slightly different ways. The following is a short description of Delphi's scope specifiers:

• Private = Used by the defining class and classes defined in the same unit. Consider classes in the same unit as friendly.
• Strict Private = Use the new strict visibility specifier which enforces the traditional definition of private members. With strict private, only the class that defines a private member can use the private member.
• Protected = Same as private plus any descendant classes whether or not they are in the same unit.
• Strict Protected = Use the new strict visibility specifier to enforce the traditional definition of protected members. With strict protected, classes defined in the same unit do not have access to the protected members that are not on its ancestor path.
• Public = Visible wherever its class can be referenced.
• Published = Same as public members but runtime type information (RTTI) is generated. RTTI allows an application to query the fields and properties of an object dynamically and to locate its methods. It's used to access properties when saving and loading form files, to display properties in the Object Inspector, and to associate event handlers with events.

Here is a traditional Delphi syntax example available since Delphi 1.0:

type

  TPerson = class

  private

  protected

  public

  published

  end;

Strict private and strict protected were introduced in Delphi 7 and available for Win32 with D2005:

type

  TPerson = class

  private

    //Don't use accept when you really want private friendly members.

  strict private

    //Use as your default private members.

  protected

    //Don't use accept when you really want protected friendly members.
  strict protected

    //Use as your default protected members.
  public

  published

  end;