Encapsulation grouping of subprograms and the data Encapsulation - - PowerPoint PPT Presentation

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Encapsulation

• Encapsulation

– grouping of subprograms and the data they manipulate

• Information hiding

– abstract data types

• type definition is hidden from the user
• variables of the type can be declared
• variables of the type can be used via the
• perations defined for the type
• Mechanisms

– modules, packages, classes – nested subprograms, ...

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Classes

• Modules encapsulate data types and
• perations that operate on them, data and
• perations are still separate

– modules can be used by importing them

• Classes support abstract data types better

than modules

– classes are types, objects variables, operations methods of objects/classes

• Class hierarchy (inheritance)

– behavior of the superclass is copied to be part

• f the behavior the subclass

– multiple inheritance vs. single inheritance

• Meta class

– a class itself is described as an object of a meta-class

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Design issues for

• bject-
• riented

languages

• Objects-only "pure" language vs mixed

– consistency (only objects) vs. efficiency (primitive types)

• Are subclasses subtypes of the

superclass?

– consistency between subclasses and superclasses

• Dynamic/static binding of methods
• Are pure interfaces (no implementation)

provides as a separate concept

• Single vs multiple inheritance
• Dynamic/static typing
• Allocation and deallocation of objects

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Object creation

• Stack memory allocation for objects: size

must be known during compilation

• Inheritance: objects that look like base

class objects can really be of derived class! So size depends on situation

• In many languages memory for objects is

always allocated from the heap (often implies reference semantics)

• Same goes for data members in a class
• C++ allows stack allocation & embedded

data members, this causes problems (slicing) and restricts use of inheritance

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Method binding

• Dynamic binding = run time binding

– enables polymorphism (typical for object

• rientation)

– implemented via virtual operations

• Virtual operation

– operation to be called is selected on the basis of the object’s real class, not the class of the reference used to access the object

• Polymorphism: a single interface to entities
• f different types

– in object-oriented languages, inheritance: all subclass objects can be used via superclass interface – genericity, templates (parametric polymorphism) – overloading (ad hoc polymorphism)

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Static and dynamic binding in programming languages

• Dynamic binding for all operations

– Python, Smalltalk, Modula-3

• Dynamic binding as default

– Java, static with final keyword – Eiffel, static with frozen keyword

• Static binding as default

– C++, dynamic with virtual and override (C++11) specifiers – C++11, overriding prohibited with final specifier

Static binding: Operations can't/shouldn't be overridden in subclasses

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Implementing classes and

• bjects (static

typing)

• Features in memory

– class resembles a record (especially in C++)

• CIR (class instance record)

– static structure

• each feature can be found by evaluating the offset of

the feature

– subclass adds its own features after the CIR of the superclass

• Dynamically bound operations

– references to these operations can be found from CIR – list of operations that can be bound dynamically

• virtual method table (VMT, vtable)

information about an object information about a class

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Implementing classes and

• bjects

(dynamic typing)

• Dynamically bound operations in

dynamically typed object-oriented languages

– Each class has a method parser function – Method parser receives method name and parameters and decides what code to call – If parser finds no suitable method, it gives a run- time error – If subclass parser finds no suitable method, it calls base class parser(s) before giving an error – Slower than static methods, but more flexible – In some languages programmer may write custom method parsers for a class

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Object and method table

class Foo { int x; double y; char z; public: virtual void k ( ... ); virtual int l ( ... ); virtual void m ( ... ); virtual double n ( ... ); ... }; Foo f; n k l m x y z CIR of f VMT of Foo code of Foo:m code of Foo:n code of Foo:l code of Foo:k

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class Bar: public Foo { int w; public: virtual void m ( ... ); // = override virtual double s ( ... ); virtual char *t ( ... ); ... }; Bar b; n k l m x y z CIR of b VMT of Bar code of Bar::m w s t code of Bar::s

Implementing single inheritance

n k l m x y z CIR of f VMT of Foo code of Foo:m code of Foo:n code of Foo:l code of Foo:k code of Bar::t

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Choices for multiple inheritance

• Non-repeated multiple inheritance

– inherited superclasses are separate in the class hierarchy

• Repeated multiple inheritance

– replicated multiple inheritance – shared multiple inheritance

• diamond inheritance

B C D B C D A A B C D A

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Implementat- ion issues for multiple inheritance

• Name conflict

– inherited classes have features with the same name – how to refer to the features of the different superclasses

• C++: operator ::
• Eiffel: renaming
• Shared inheritance (diamond

inheritance)

– how does D access features of A? – any feature inherited from A can be redefined in B or C (or both!)

B C D A

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Non-repeated multiple inheritance

B fields C fields D (only) fields

B C D

B operations D (only) operations C operations CIR of D obj. VMT of D (D/B part) VMT of D (C part)

D view, B view C view

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Replicated multiple inheritance

B (only) fields C (only) fields D (only) fields B (only) operations D (only) operations C (only) operations CIR of D obj. VMT of D (D/B part) VMT of D (C part)

D view, B view, B::A view C view, C::A view B C D A A

B::A fields C::A fields B::A operations C::A operations

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Shared multiple inheritance

B C D A

B (only) fields C (only) fields D (only) fields B operations D operations CIR of D obj. VMT of D (D/B part) VMT of D (C part)

D view, B view C view A view

A fields C operations A operations VMT of D (A part)