[Booch 91, p. 45] defines: "Encapsulation is the process of hiding all of the details of an object that do not contribute to its essential characteristics."
[Coad 91, 1.1.2] defines: "Encapsulation (Information Hiding). A principle, used when developing an overall program structure, that each component of a program should encapsulate or hide a single design decision... The interface to each module is defined in such a way as to reveal as little as possible about its inner workings. [Oxford, 1986]"
Some languages permit arbitrary access to objects and allow methods to be defined outside of a class as in conventional programming. Simula and Object Pascal provide no protection for objects, meaning instance variables may be accessed wherever visible. CLOS and Ada allow methods to be defined outside of a class, providing functions and procedures. While both CLOS and Ada have packages for encapsulation, CLOS's are optional while Ada's methodology clearly specifies class-like encapsulation (Adts).
However most object-oriented languages provide a well defined interface to their objects thru classes. C++ has a very general encapsulation/protection mechanism with public, private and protected members. Public members (member data and member functions) may be accessed from anywhere. A Stack's Push and Pop methods will be public. Private members are only accessible from within a class. A Stack's representation, such as a list or array, will usually be private. Protected members are accessible from within a class and also from within subclasses (also called derived classes). A Stack's representation could be declared protected allowing subclass access. C++ also allows a class to specify friends (other (sub)classes and functions), that can access all members (its representation). Eiffel 3.0 allows exporting access to specific classes.
For another example, Smalltalk's class instance variables are not accessible from outside of their class (they are not only private, but invisible). Smalltalk's methods are all public (can be invoked from anywhere), but a private specifier indicates methods should not be used from outside of the class. All Smalltalk instance variables can be accessed by subclasses, helping with abstract classes and overriding.
Another issue is per-object or per-class protection. Per-class protection is most common (e.g. Ada, C++, Eiffel), where class methods can access any object of that class and not just the receiver. Methods can only access the receiver in per-object protection. This supports a subtyping model, as any object other than the receiver is only satisfying an abstract type interface, whereby no method or object structure can be inferred in the general case.
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