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Polymorphism

Polymorphism. Polymorphism Polymorphism is a major strength of an object centered paradigm Same general type of action….. Accomplished in different ways By different types of objects The underlying software system Decides how to achieve the action. Polymorphism. Polymorphism

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Polymorphism

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  1. Polymorphism • Polymorphism • Polymorphism is a major strength of an object centered paradigm • Same general type of action….. • Accomplished in different ways • By different types of objects • The underlying software system • Decides how to achieve the action 1 February2006

  2. Polymorphism • Polymorphism • Key issue….. • When to implement the action • Compile time • Early Binding • Allows greater execution speed • Achieved through optimized code • Run time • Late Binding • Allows for greater flexibility • Opportunity for abstraction 1 February2006

  3. Polymorphism • Polymorphism and C++ • Early • Binding occurs at compile time • Early binding polymorphism • Process of overloading members • Late • Binding occurs at runtime • Late binding polymorphism • The code to implement the method is chosen at runtime • Appropriate code chosen sending a message to • the object …. Not to the pointer to the object • Implemented through virtual functions 1 February2006

  4. Polymorphism • Virtual Functions • A virtual function must be declared in a parent class • syntax • virtual function • virtual returnType functionName ( argsi ) { function body ;} • purevirtual function • virtual returnType functionName ( argsi ) = 0; 1 February2006

  5. Polymorphism • Virtual Functions • Declaration • A function name is preceded by the keyword virtual • Function name can only be used once in the parent class • Cannot overload virtual functions • Only class member functions can be declared virtual • A function is virtual….. • If it is declared virtual • There is a base class function with the same signature • declared virtual • Any or all class member functions (except constructors) • can be declared virtual 1 February2006

  6. Polymorphism • Virtual Functions • Implementation • The body of the virtual function must be supplied in the parent • class unless declared to be a pure virtual function • A derived class can override the definition by providing its own • implementation • If the re-declaration does not match exactly…... • The function not considered virtual for that class • A virtual function still permitted in a subsequently derived class 1 February2006

  7. Public Interface Base Derived1 Derived2 Derived3 …….. Derivedn Gives Uniform Function Call Interface Polymorphism • Virtual Functions • Such a capability permits multiple functions to be called through a • common interface. • Can be overridden by explicit qualification with the scopeoperator. 1 February2006

  8. Polymorphism • Virtual Functions • When function in a class is declared virtual • Keyword virtual tells compiler • Don’t perform early binding • Install mechanisms to perform late binding • Compiler responds by creating • Table of function pointers • Installing a data member to the class to point to the table 1 February2006

  9. Polymorphism • Virtual Functions • The compiler created table is called the vtable (vtbl) • Contains pointers to all functions declared virtual within the class • and derived classes. • Each class gets its own vtable • A data member called the vpointer (vPtr) • Usually placed as the first element in object in memory. • Initialized to the starting address of the vtable. • The function call through a base class pointer • Indexes into the vtable calls the function located at the address. 1 February2006

  10. Polymorphism vtable[0] &f ( ) class A { public: int i; virtual void f ( ); virtual void g( ); }; &g ( ) vtable[1] vptr class A vtable i class A 1 February2006

  11. vtable[0] &f ( ) Polymorphism class A { public: int i; virtual void f ( ); }; class B : public A { public: virtual void f ( ); // override f( ) virtual void g ( ); // define g () }; vtable[0] &f ( ) &g ( ) class A vtable vtable[1] class B vtable 1 February2006

  12. Polymorphism • Virtual Functions - vtable • Contains pointers to all virtual functions. • Each class gets its own vtable. • Abstract classes have no vtable. • Vtable size is proportional to the number of virtual functions. 1 February2006

  13. Polymorphism • Virtual Functions - Invocation • A virtual function is invoked through a public base class pointer or reference. • Runtime Binding • Typically polymorphic binding is done dynamically at runtime • Virtual functions are not inlined • Compile Time Binding • Occasionally have compile time polymorphic binding • Invoked through an object of the class type • Invoked using the scope operator • Invoked through a constructor or destructor 1 February2006

  14. Polymorphism • Virtual Functions - Access Protection • The access level of a virtual function is determined by • Access level in class of the pointer through which it’s invoked • Not by the class in which it’s defined. 1 February2006

  15. Polymorphism • A • B : public A • C : public B • D : public C • E : public D Declare a and public virtual function f() public virtual function f() in B protected virtual function f() in C private virtual function f() in D Write: 1. A* a = new B; a-> f(); // f() accessible through *a (as a *B - *a in public area) 2. a = new C; a-> f(); // f() accessible through *a (as a *C - a* in public area) 3. C* c = new D; c-> f(); // f() not accessible through *c (as a *D - c* in protected area) 1 February2006

  16. Polymorphism Virtual Destructors When a base class pointer is used to refer to a derived class object and the object is deleted….. Only the base class destructor will be invoked leaving behind the derived class parts of the object. 1 February2006

  17. Polymorphism • Virtual Destructors • syntax • virtual ~ class ClassName ( ) { destructor body } Specifying a destructor as virtual ensures all appropriate destructors are invoked. Rule of thumb…..If a class is abstract then declare the destructor as virtual. Don’t declare the destructor if there are no othervirtualfunctions. 1 February2006

  18. Polymorphism • Virtual Destructors • Invocation order…. • The derived type destructor • The destructor for each base class….. • …...Invoked in turn in normal fashion 1 February2006

  19. Polymorphism • Polymorphism and Object Slicing • One must exercise caution when treating objects polymorphically • There is a distinct difference between passing objects by value and by reference. • When a derived class object is passed by value to a function expecting a base class value…. • The derived class portion is sliced off. 1 February2006

  20. Polymorphism • Pure Virtual Functions • ….A virtual function must be defined when it is declared • Abstract base class may be defined that is not intended to be instantiated • If virtual function is declared pure …. an implementation • may still be supplied • Derived class may use implementation 1 February2006

  21. Polymorphism • Pure Virtual Functions • When a function is declared pure….. • There is no address to put into the vtable • The 0 keys the compiler that no instances of this class can be created 1 February2006

  22. Polymorphism • Pure Virtual Functions • Restrictions • A class with one or more pure virtual functions. • 1. Can only be used as a base class • 2. Cannot have instances • 3. Cannot be used as • An argument type • Return type • Type for explicit conversion • 4. Can be used as a • Pointer • Reference type • A class cannot define a pure virtual destructor 1 February2006

  23. Polymorphism • Pure Virtual Definitions • There may be occasions when it’s desirable to share code with • derived classes but not duplicate in each class. • Can prevent base class instantiation yet provide a definition for a pure • virtual function. • syntax • virtual returnType functionName ( argsi ) = 0 { function body } • access • BaseClassName :: functionName ( ); 1 February2006

  24. Polymorphism • Virtual Functions • Rules for Virtual Functions • 1. Virtual functions called from within a constructor use the local • version. • 2. The first class in a derivation hierarchy that declares a virtual function it must provide a definition or it must declare the virtual function to be pure • 3. If a definition is provided, the definition serves as the default instance in subsequent derivations • 4. If pure, a subsequently derived class must provide a definition - to have instances or inherit the pure virtual function - have no instances 1 February2006

  25. Polymorphism vf11( ) vf12( ) must define vf11( ) and vf12( ) vf12( ) can have instances pvf13( ) no instances no instances pvf13( ) = defines pvf13( ) can have instances • Virtual Functions Class 1 Class 2 Class 3 Class 4 Class 5 1 February2006

  26. Polymorphism • Virtual Functions - Access Level • The access level of a virtual function is….. • Specified in the class where it is defined • Not by initial definition 1 February2006

  27. Polymorphism • Virtual Base Classes • Parent classes may have a common base class Fruit Fruit Peach Plum Peach Plum Stem Stem Nectarine 1 February2006

  28. Polymorphism • Virtual Base Classes • Problem: • Fruit has a stem data member • Peach and plum each inherit a stem member from Fruit • Nectarine inherits a stem member from each • Could resolve using the scope operator • Plum::stem • Peach::stem 1 February2006

  29. Polymorphism • Virtual Base Classes • Solution: Declare Fruit as a virtual base class • Result: Only a single copy of the base class in the derivation hierarchy. Only a single copy of all inherited data members. Subsequent derivations point to shared members. 1 February2006

  30. Polymorphism • Virtual Base Classes - Specification • Syntax • class DerivedClass : virtual accessSpec BaseClass • DerivedClass - The class being derived • BaseClass - The parent class • Specification - Specify base class member access • public • protected • private • The keyword virtual identifies BaseClass as a virtual base class of • DerivedClass. 1 February2006

  31. A Data Members B Data Members B Data Members Polymorphism • Virtual Base Classes - Implementation Non-Virtual Derivation A Data Members Virtual Derivation 1 February2006

  32. Polymorphism • Virtual Base Classes - Access Protection • When there are multiple paths from a common root … • ….the most public path dominates. Fruit virtual public virtual private Peach Plum Nectarine 1 February2006

  33. Polymorphism • Virtual Base Classes - Initialization • A virtual base class is initialized by the most derived class. • Initialization Order: • 1. Constructors for any virtual base class(es). • 2. Constructors for any non-virtual base class. • 3.The most derived class must provide initialization values. 1 February2006

  34. Polymorphism • Virtual Base Classes - Initialization • Specify class E… • class E : public D, public C, public virtual F virtual base class A virtual virtual B C virtual F D E 1 February2006

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