Tài liệu C++ Language - Session 9: Polymorphism - FPT University

Nội dung text: Tài liệu C++ Language - Session 9: Polymorphism - FPT University

1. Session 9 –Polymorphism Ad-Hoc Polymorphism Inclusion Polymorphism Session 9 - Polymorphism 1/26
2. Objectives • Ad-Hoc Polymorphism – Types – Ad-Hoc Polymorphism – Universal Polymorphism • Inclusion Polymorphism – Function Bindings – Polymorphic Objects Session 9 - Polymorphism 2/26
3. Polymorphism • Polymorphism means many forms. • In programming languages, polymorphism refers to the flexibility of associating different data types with objects and functions. – Polymorphic objects may have different data types. – Polymorphic functions may accept arguments of different data type. • In OOP polymorphism finds its most powerful application in inheritance hierarchies Session 9 - Polymorphism 3/26
4. Types (1) • A type describes how to interpret the bits stored in memory. Session 9 - Polymorphism 4/26
5. Types(2) • The type describes the object's (or variable's) properties and the operations that may be performed upon the object (or variable). • The type does not change throughout the lifetime of the variable or object. Session 9 - Polymorphism 5/26
6. Static Type Checking • We use compilers to check for type consistency and to identify undefined operations. This is called static type checking. • Static type checking applies to monomorphic objects (an object declared as an instance of a particular class). Session 9 - Polymorphism 6/26
7. Dynamic Type Checking • Rather than declare the object as an instance of a particular class, we only declare a pointer to the object. • We identify its type only when we allocate memory for the object Human* h; Since the execution path if( ){ is unknown at compile h = new Student(); time, type checking } else { occurs at run time. This is h= new Professor (); called dynamic type } checking. Session 9 - Polymorphism 7/26
8. Christopher Strachey • The notion of polymorphism as applied to functions dates back four decades. In 1967, Christopher Strachey distinguished – functions that work differently on different argument types from (ad- hoc). – functions that work uniformly on a range of argument types (parametric). Session 9 - Polymorphism 8/26
9. Cardelli and Wegner • In 1985, Cardelli and Wegner refined Strachey's distinction. Session 9 - Polymorphism 9/26
10. Ad-Hoc Polymorphism • Ad-Hoc polymorphism refers to those cases where we use the same syntax with objects of different type (single function that can be called using different argument types). Session 9 - Polymorphism 10/26
11. Ad-Hoc Polymorphism -Overloading- • Overloading redefines a function using the same name but a different signature. We write a distinct definition for each signature. • Compilers implement overloading at compile time. Session 9 - Polymorphism 11/26
12. Ad-Hoc Polymorphism -Coercion(1)- • Coercion: Automatic type conversions invoked by the compiler to change one type to another as a result of the compiler's attempts to match a function definition to the argument types in a function call. • Coercion is implemented at compile time. Session 9 - Polymorphism 12/26
13. Ad-Hoc Polymorphism -Coercion(2)- • Coercion may be further subdivided into – narrowing and – widening (promotion) conversions as well as – implicit and – explicit (casting) conversions. Session 9 - Polymorphism 13/26
14. Universal Polymorphism • Universal polymorphism (true polymorphism) refers to a single function definition that applies to a variety of types Session 9 - Polymorphism 14/26
15. Universal Polymorphism -Inclusion- • Implemented at run time. • It refers to those cases where an object is identified with several types that are related by an inclusion relation (inheritance). Session 9 - Polymorphism 15/26
16. Universal Polymorphism -Parametric- • Parametric (generic) polymorphism is implemented at compile time. It refers to the use of type parameters with functions. Session 9 - Polymorphism 16/26
17. Inclusion Polymorphism • The classes that belong to an inheritance hierarchy exhibit inclusion relations amongst themselves. • These relations enable us to associate an object with the inheritance hierarchy, but to postpone identification with a specific class until run time. Session 9 - Polymorphism 17/26
18. Function Bindings • The compiler generates code that associates each function call in a program with a function definition. This code is called a function binding. • 02 forms: – early binding or – late binding. Session 9 - Polymorphism 18/26
19. Early Binding • An early binding is a binding between a function call and a function definition that is defined at compile time. • Demo Session 9 - Polymorphism 19/26
20. Late Binding • A late binding is a binding between a function call and a function definition that is defined at run time. • To implement late binding, the compiler inserts a virtual table that holds the information needed to select the appropriate function at run time. • Demo Session 9 - Polymorphism 20/26
21. Virtual Destructors • If a derived class allocates resources, it needs its own destructor to release those resources. • The presence of a virtual base class destructor ensures that the most derived destructor will be called if and when a class is derived from the base class. • Demo Session 9 - Polymorphism 21/26
22. Polymorphic Objects(1) • An object fully declared at compile time is monomorphic. Its type does not change during its lifetime. • An object partly declared at compile time and only fully declared at run time may be polymorphic. Space for an object fully declared at run time is allocated on freestore. Session 9 - Polymorphism 22/26
23. Polymorphic Objects(2) • C++ objects are monomorphic by default. To declare a polymorphic object, we define: – a pointer to the object, – a reference to the object, – a receive-by-reference parameter for the object, or – a receive-by-address parameter for the object. Session 9 - Polymorphism 23/26
24. Inclusion polymorphism Advantages • Inclusion polymorphism is extremely useful in minimizing the amount of code written. • Sample code: Human h[ ]={new Student(), new Professor(), new Employee()}; Session 9 - Polymorphism 24/26
25. In-Class Practice Inclusion Polymorphism Handout Session 9 - Polymorphism 25/26
26. Summary • Ad-Hoc Polymorphism – Types – Ad-Hoc Polymorphism – Universal Polymorphism • Inclusion Polymorphism – Function Bindings – Polymorphic Objects Q&A Session 9 - Polymorphism 26/26