cs32 - week 4 - uclaweb.cs.ucla.edu/~umut/cs32/slides/w4/w4.pdf · inheritance process of deriving...

CS32 - Week 4

Umut Oztok

Jul 15, 2016

Umut Oztok CS32 - Week 4

Inheritance

Process of deriving a new class using another class as a base.

Base/Parent/Super Class −→ Derived/Child/Sub Class


Inheritance

class Animal{

public:

Animal();

~Animal();

int getAge() const;

void speak() const;

private:

int m_age;

};

class Dog : public Animal{

public:

Dog();

~Dog();

string getName() const;

void setName(string name);

private:

string m_name;

};

Dog inherits Animal.


Inheritance

When to use?

If there exists an ”is-a relationship”.

Dog is an Animal.Student is a Person.SpeedShip is a Ship.

What is inherited?

All member variables.

All member funtions except assignment operator, constructorsand destructor.


Inheritance

What is accessable?

A derived class cannot access private member variables of itsbase class.

A derived class cannot access private member functions of itsbase class.

A derived class can access public member variables of its baseclass.

A derived class can access public member functions of itsbase class.

What if we want to access private functions of the base class?

Use keyword protected instead of private.

Making private member variables protected violates encapsulation!!


Inheritance

How to implement Constructor?

Dog objects are Animal objects. So, when we create a Dogobject, we also create an Animal object.

In general, when a class object is constructed:1 Base class’s constructor is called.2 Data members of the object are created.3 Body of the constructor is executed.


Inheritance

Is Dog’s constructor valid?

class Animal{

public:

Animal(){}

...

private:

int m_age;

};


public:

Dog(int age, string name){

m_age = age;

m_name = name;

}

...

private:

int m_name;

};

NO, m age cannot be accessed by Dog.


Inheritance

This one works for now.

class Animal{

public:

Animal(){}

void setAge(int age);

...

private:

int m_age;

};


public:


setAge(age);

m_name = name;

}

...

private:

int m_name;

};


Inheritance

What if Animal’s constructor takes one argument?

class Animal{

public:

Animal(int age){

m_age = age;

}

void setAge(int age):

...

private:

int m_age;

};


public:


setAge(age);

m_name = name;

}

...

private:

int m_name;

};

Does not work anymore!


Inheritance

Solution is to use initializer list.

class Animal{

public:

Animal(int age){

m_age = age;

}

...

private:

int m_age;

};


public:

Dog(int age, string name)

: Animal(age){

m_name = name;

}

...

private:

int m_name;

};


Inheritance

Destructor : reverse order of constructor

1 Body of the destructor is executed.

2 Member variables are removed.

3 Base class’s destructor is called.

We will revisit destructors!!


Overriding

Assume speak() is implemented as follows:

void Animal::speak() const{

cout << "..." << endl;

}

Dog inherits this function.But we expect a dog to make a specific sound.What to do?


Overriding

We can override member functions.


public:

...

void speak() const;

private:

string m_name;

};

void Dog::speak() const{

cout << "Woof!" << endl;

}

Example:

Animal a1;

a1.speak();

Output: ...

Dog d1;

d1.speak();

Output: Woof!


Overriding

We can still call the base class’s speak() function on a Dog object.

Dog d1;

d1.Animal::speak();


Polymorphism

Is the following allowed?: Animal* ani = new Dog;

Yes, as ”Dog is an Animal”.

Does the following output Woof!?: ani->speak();

No, because ani is a pointer to an Animal instance.But, I really want ani to say ”Woof!”.

Solution: insert the virtual keyword in front of both theoriginal and the replacement functions!

Polymorphism: using a base pointer/reference to access anyvariable that is of a type derived from the base class.


Polymorphism

class Animal{

public:

Animal();

virtual ~Animal();

int getAge() const;

virtual void speak() const;

private:

int m_age;

};


public:

Dog();

virtual ~Dog();

string getName() const;

void setName(string name);

virtual void speak() const;

private:

string m_name;

};

Compiler figures out which object ani points to and calls thecorresponding function.


Polymorphism

Virtual functions allow late binding or dynamic binding:Appropriate version of a method is decided at execution time(runtime), instead of at compilation time.

Called as polymorphism as ani is allowed to have multipleforms.


Polymorphism

Animal* ani;

int x;

cin >> x;

switch (x){

case 1:

ani = new Dog;

break;

case 2:

ani = new Cat;

break;

default:

ani = new Animal;

break;

}

ani->speak();


Polymorphism

class Animal{

public:

void tickle () {

speak();

}

virtual void speak() {

cout << "...";

}

};

class Dog: public Animal {

public:

virtual void speak() {

cout << "Woof!";

}

};

main() {

Animal* ani= new Dog;

ani->tickle();

delete ani;

}

What is the output?


Virtual Destructor

Always make destructors virtual if you use inheritance.

Otherwise, when you use polymorphism, only base class’sdestructor will be called.


Pure Virtual Functions

virtual void speak() { cout << "..."; }

speak function of Animal class doesn’t make sense.

Make it pure virtual by putting ”=0” in declaration of baseclass.

virtual void speak() = 0;

You don’t implement pure virtual functions in base class.

However, derived classes should implement pure virtualfunctions to be able to be instantiated.


Abstract Base Classes

If a base class has at least one pure virtual function, it iscalled abstract base class (ABC).

An ABC cannot be instantiated. So, Animal ani; is notallowed.


Abstract Base Classes

Why do we use pure virtual functions?

Avoid implementing ”dummy” functions when it doesn’t makesense.

Force the programmer to implement functions in a derivedclass to prevent bugs.


Slides

Slides will be available at http://www.cs.ucla.edu/~umut/cs32


http://www.cs.ucla.edu/~umut/cs32

cs32 - week 4 - uclaweb.cs.ucla.edu/~umut/cs32/slides/w4/w4.pdf · inheritance process of deriving...

Documents