Download - Lecture04 polymorphism

TCP1201 OOPDS

Lecture 4

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http://fit.mmu.edu.my/



Learning ObjectivesTo understand static polymorphism and dynamic

polymorphismTo understand the problem of static

polymorphismTo enable dynamic polymorphism via virtual

functionTo understand the benefit of dynamic

polymorphismTo understand abstract classTo understand why virtual destructor is needed

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What is Polymorphism?- Polymorphism is allowing objects of different types

to respond differently to the same method call.

- Polymorphism occurs when there is function overriding.- In lecture 3, superclass Human has a speak

method and is overridden by subclass Student. Invoking the speak method from Human has different result than Invoking the speak method from Student.

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Different objects invoke different version of methods with the same name (speak())

class Animal { public: void speak() { cout << "I'm an animal\n"; }};class Bird : public Animal { public: void speak() { // overriding cout << "I'm a bird\n"; }};

int main() { Animal* a = new Animal; a->speak(); // call Animal::speak() Bird* b = new Bird; b->speak(); // call Bird::speak() delete a; delete b;}

Output:I'm an animalI'm a bird

How does C++ knows which method to call? Through function call binding

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class Animal { public: void speak() { ... }};class Bird : public Animal { public: void speak() { ... }};int main() { Animal* a = new Animal; a->speak(); Bird* b = new Bird; b->speak(); delete a; delete b;}

Bird::speak()Definition

Animal::speak()Definition

Memory

0x7723

0x77b4

Function Call Binding• Connecting a function call to a function body is called binding• Function call binding is the process of determining what block

of function code is executed when a function call is made.

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There are 2 types of function call binding (polymorphism):

Early/Static/Compile-time binding/polymorphism: Binding is performed during compile-time, and is decided by the compiler and linker based on the variable used to invoke the method.

Late/Dynamic/Runtime binding/polymorphism: Binding occurs at runtime, based on the type of the object calling the method.

Function Call Binding

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Static Polymorphism• It is the polymorphism that is implemented using

static binding.

• The compiler determines beforehand what method definition will be executed for a particular method call, by looking at the type of the variable invoking the method.

• We have been using static polymorphism thus far without realizing it.

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Animal

+ move():void

Bird

+ move():void

Consider the following Animal hierarchy, subclass Bird overrides superclass Animal's move() method.

Static Polymorphism

class Animal { public: void move() { cout << "Moving"; }};

class Bird : public Animal { public: void move() { cout << "Flying"; }};

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int main() { Animal a1; a1.move(); // cout "Moving". Animal *a2 = new Animal; // Pointer. a2->move(); // cout "Moving". Animal& a3 = a1; // Reference. a3.move(); // cout "Moving".

Bird b1; b1.move(); // cout "Flying". Bird *b2 = new Bird; // Pointer. b2->move(); // cout "Flying". Bird& b3 = b1; b3.move(); // cout "Flying".}

Animal::move() is called. No problem

Call the move() method from an Animal object, an Animal pointer/reference to an Animal object, a Bird object, or an Bird pointer/reference to a Bird object, got overriding but no upcasting => no problem

Static Polymorphism

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Bird::move() is called. No problem

Call the move() method from a Bird object using an Animal pointer/reference, got overriding and got upcasting => big problem.

int main() { Bird* b1 = new Bird; Animal* a1 = b1; // Upcasting via pointer. a1->move(); // cout "Moving". Bird b2; a1 = &b2; // Upcasting via pointer. a1->move(); // cout "Moving". Animal& a2 = b2; // Upcasting via reference. a2.move(); // cout "Moving". delete b1;} Animal::move() is still called but

Bird::move() is more appropriate/precise/accurate in the context.

Static Polymorphism: Problem

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Consider the following inheritance hierarchy:

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class Animal { public: void move() { cout << "Moving"; }};class Bird : public Animal { public: void move() { cout << "Flying"; }};class Fish : public Animal { public: void move() { cout << "Swimming"; }};class Mammal : public Animal { public: void move() { cout << "Walking"; }};

Static Polymorphism: Problem

To make a function callMove() to support every class in the animal inheritance hierarchy, we have to write/overload the callMove() function for every class.

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// 1 callMove() only.void callMove (Animal &

a) { ... a.move();}

int main() { Animal a; Bird b; Fish f; Mammal m; callMove (a); callMove (b); callMove (f); callMove (m);}

Static Polymorphism: Problem 1

Output:

MovingMovingMovingMoving// 4 callMove() for

// 4 different classes.void callMove (Animal &

a) { ... a.move(); }void callMove (Bird & b){ ... b.move(); }void callMove (Fish & f){ ... f.move(); }void callMove (Mammal &

m){ ... m.move(); }

Output:

MovingFlyingSwimmingWalking

We can actually solve the problem in previous slides by using template. However the downside is the template function won't be limited to the Animal hierarchy anymore.

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// template callMove().// Work but not limited to Animal hierarchy. template <typename T>void callMove (T & a) { ... a.move();}


We cannot use one for loop to call the correct version of move() method in an array/vector of objects from Animal hierarchy.

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int main() { // Array of different animals. Animal* a[4] = {new Animal, new Bird, new Fish, new Mammal}; for (int i = 0 ; i < 4; i++) a[i]->move(); for (int i = 0 ; i < 4; i++) delete a[i];}

Current output:

MovingMovingMovingMoving


Preferred output:


• The solution to the upcasting problem we just discussed is to enable dynamic polymorphism or dynamic binding via the use of virtual function.

• Requirements for C++ dynamic polymorphism:• There must be an inheritance hierarchy.• The superclass must have a virtual method.• Subclass must override superclass virtual

method.• A superclass pointer/reference to a subclass

object (upcasting) is used to invoke the virtual method.

Dynamic Polymorphism

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Virtual Function

class Animal { public: virtual void move() { // Enable dynamic binding. cout << "Moving"; }};

class Bird : public Animal { public: void move() { // Automatically virtual. cout << "Flying"; }};

• In order to allow a method to be bound at run-time, it must be declared as virtual in the superclass.

• By declaring a method as virtual in superclass, it is automatically virtual in all subclasses.

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Virtual Function

We can use the keyword virtual at subclass' virtual function to remind ourselves that it is using dynamic binding.

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class Animal { public: virtual void move() { cout << "Moving"; }};

class Bird : public Animal { public: virtual void move() { // Optional, as reminder. cout << "Flying"; }};

After making the move() method virtual:

int main() { Bird* b1 = new Bird; Animal* a1 = b1; // Upcasting via pointer. a1->move(); // cout "Flying". Bird b2; a1 = &b2; // Upcasting via pointer. a1->move(); // cout "Flying". Animal& a2 = b2; // Upcasting via reference. a2.move(); // cout "Flying". delete b1;} Bird::move() is called now, which

is more appropriate/precise/accurate in the context.

Dynamic Polymorphism

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The primary benefit of dynamic polymorphism is it enables to write codes that work for all objects from the same inheritance hierarchy via upcasted pointer / reference.

Consider the following inheritance hierarchy:

Dynamic Polymorphism: Benefit

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class Animal { public: virtual void move() { cout << "Moving"; }};class Bird : public Animal { public: virtual void move() { cout << "Flying"; }};class Fish : public Animal { public: virtual void move() { cout << "Swimming"; }};class Mammal : public Animal { public: virtual void move() { cout << "Walking"; }};

We can write a single callMove() function that call the correct version of move() method for all objects in the Animal hierarchy.


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void callMove (Animal & a) { ... a.move();}int main() { Animal a; Bird b; Fish f; Mammal m; callMove (a); callMove (b); callMove (f); callMove (m);}

Output:


We can also write a loop to call the correct version of move() method for all objects in the Animal hierarchy.


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int main() { // Array of different animals. Animal* a[4] = {new Animal, new Bird, new Fish, new Mammal}; for (int i = 0 ; i < 4; i++) a[i]->move(); for (int i = 0 ; i < 4; i++) delete a[i];}

Output:


Abstract Class

An abstract class is a class that cannot be instantiated.

A class that can be instantiated is called a concrete class.– All classes that we have learned so far are concrete

classes. The reason to make a class abstract is, due to

some requirements/constraints, we want to make it impossible to create instance(s) of the class.

To be an abstract class: a class must have at least a pure virtual function.

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Pure Virtual Function

A pure virtual function is a method that is initialized to zero (0) in its declaration and no implementation is given in the class.

Pure virtual function makes the class abstract and no instance of the class can be created.

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class Animal { // Abstract class. public: virtual void move() = 0; // Pure virtual function.};int main() { Animal a1; // ERROR: Animal is an abstract class. Animal* a2 = new Animal; // ERROR: same reason. ...}


Subclass of an abstract class must override all of the superclass pure virtual functions in order to become a concrete class (instantiate-able).

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class Animal { // Abstract superclass. public: virtual void move() = 0; // Pure virtual function.};class Bird : public Animal { // Concrete subclass. public: virtual void move() { // Overriding pure virtual function. cout << "Flying"; }};int main() { Animal *a = new Bird; // Fine. create a Bird object. a->move(); // cout "Flying“.}


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class Animal { // Abstract class. public: virtual greet() {} virtual void move() = 0; // Pure virtual function.};class Bird : public Animal { // Abstract class. public: virtual void greet() { ... }};class Eagle : public Bird { // Concrete class.public: virtual void greet() { ... } virtual void move() { ... } // Overriding.};

Subclass that does not override superclass' pure virtual function is an abstract class too.

Virtual Destructor Superclass destructor should always be virtual in

order for the delete operator to call the destructors correctly for an subclass object that is created via upcasting.

Otherwise, the delete operator will call only superclass destructor, not the subclass destructor.– This will cause only the base part of the object to be

destroyed.

// Problemclass Super { public: ~Super() { // Non virtual. cout << "Super destroyed\n"; }};class Sub : public Super { public: ~Sub() { cout << "Sub destroyed\n"; }};

int main() { Super* p = new Sub; // Upcasting. delete p; // Invoke destructor.}

Output: // Where is Sub's destructor?Super destroyed

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Virtual Destructor

// Solutionclass Super { public: virtual ~Super() { cout << "Super destroyed\n"; }};class Sub : public Super { public: virtual ~Sub() { cout << "Sub destroyed\n"; }};int main() { Super* p = new Sub; // Upcasting. delete p; // Invoke destructor.}

To ensure that subclass objects are destroyed properly, make superclass destructor virtual.

Output:Sub destroyedSuper destroyed

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Good Programming Practices

Use inheritance to promote code reusability. Use virtual function to describe common behavior

within a family of classes. Use pure virtual function to force subclasses to

define the virtual function. Use a pointer/reference to an abstract base class to

invoke virtual function, thus implementing dynamic polymorphism.

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Download - Lecture04 polymorphism

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