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Object-Oriented Programming Encapsulation Control/ Visibility (VRH 7.3.3)

Object-Oriented Programming Encapsulation Control/ Visibility (VRH 7.3.3). Carlos Varela RPI Adapted with permission from: Seif Haridi KTH Peter Van Roy UCL April 16, 2015. Controlling visibility.

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Object-Oriented Programming Encapsulation Control/ Visibility (VRH 7.3.3)

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  1. Object-OrientedProgrammingEncapsulation Control/Visibility (VRH 7.3.3) Carlos Varela RPI Adaptedwith permission from: SeifHaridi KTH Peter Van Roy UCL April 16, 2015 C. Varela; Adapted from S. Haridi and P. Van Roy

  2. Controlling visibility • Visibility is the control given to the user to limit access to members of a class (attributes, methods and properties) • Each member is defined with a scope (part of program text that the member can be accessed by name) • Programming languages use words like public, private and protected to define visibility • Unfortunately different languages use these keywords to define different scopes C. Varela; Adapted from S. Haridi and P. Van Roy

  3. Public and private scopes in ADTs • A private member is one which is only visible in the object instance (it is used for implementing the ADT) • The object instance can see all the private members in its class and its super classes • A public member is visible anywhere in the program • It is part of the interface of the ADT • In Oz (and Smalltalk) attributes are private and methods are public (the default rule) • In Java and C++ private has another meaning C. Varela; Adapted from S. Haridi and P. Van Roy

  4. The meaning of Private C Class Hierarchy SubC SubSubC I1 I2 I3 I4 Instances C. Varela; Adapted from S. Haridi and P. Van Roy

  5. The meaning of Private C Class Hierarchy According to Smalltalk and Oz SubC All private members in this region are visible to I3 SubSubC I1 I2 I3 I4 Instances C. Varela; Adapted from S. Haridi and P. Van Roy

  6. The meaning of Private C Class Hierarchy According to C++ and Java All private members in this region are visible to I3 SubC SubSubC I1 I2 I3 I4 Instances C. Varela; Adapted from S. Haridi and P. Van Roy

  7. Public and private scopes in ADTs • In Oz (and Smalltalk) attributes are private and methods are public • It is possible in Oz to make a method private within a class • Using a variable identifier as a method head will make the method local to the class • The variable is automatically bound to a unique name class C meth A(...) ... end .... end C. Varela; Adapted from S. Haridi and P. Van Roy

  8. Public and private scopes in ADTs • In Oz (and Smalltalk) attributes are private and methods are public • It is possible in Oz to make a method private within a class • Using a variable identifier as a method head will make the method local to the class • The variable is automatically bound to a unique name • ! is an escape character, !A means escape the class scope class C meth A(...) ... end .... end local A = {NewName} in class C meth!A(...) ... end .... end end C. Varela; Adapted from S. Haridi and P. Van Roy

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