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Class Interface Diagram

Private data:

hrs

mins

secs

Set

Increment

Write

Time

Time class

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Friend function

A friend function of a class is defined

Outside that class' scope
Has the right to access all private and protected members of the class.

Friends are not member functions.

Their prototypes appear in the class definition.

A friend can be a

function

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Properties of friend functions

Friend of the class can be member of more than one class.
Friend of one class can be GLOBAL FRIEND.
Can access the private or protected members of the friend.
Do not get “this” pointer.
They can be used for message passing between the classes.
Can be declared anywhere in

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When to use friend function

Three different circumstances where friend functions are useful
Operator overloading - for certain types of operators
Creation of I/O operations

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Friend Class

Friendship may allow a class to be better encapsulated

Granting per-class access to parts of its API
That would otherwise have to be public.

This increased encapsulation comes at the cost of tighter coupling between classes

Friendships are not symmetric
Friendships are not transitive
Friendships are not inherited
Access due to friendship is

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Construction and Destruction

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Constructors

void foo(){

int n = 5;

double z[10] = { 0.0 };

struct gizmo { int i, j; } w = { 3, 4 };

·····

}

All of the variables are created at block entry when foo() is invoked.

Uses a runtime system stack.

The class needs a mechanism to specify object creation and destruction so that a client can use objects like native types.

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Constructors

It is a member function whose name is the same as the class name
It creates objects of the class type
It involves initializing data members
Allocating storage from the heap by using new.
The simplest use of a constructor is for initialization.
Demonstration

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Implicit Constructor

An implicitly-declared default constructor is an inline public member of its class

It performs the initialization operations

That are needed to create an object of this type.

These operations do not involve initialization of user-declared data members or allocation of memory from the free store

Demonstration

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Parameterized Constructors

It may be necessary to initialize different member variables with different values

To achieve this we can pass arguments to a constructor

The constructor that can take arguments are called parameterized constructors

Implicit and Explicit calling of parameterized constructors

Demonstration

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Multiple Constructors in a Class

There can be multiple constructors in a class

Each function has its own significance

In some cases it is necessary to have both parameterized and default constructors

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Efficient constructor design

class string {

private:

char * pc;

size_t capacity;

size_t length;

enum { DEFAULT_SIZE = 32};

public:

string(const char * s);

string(size_t initial_capacity );

string();

//...

};

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Efficient … (contd.)

class string {

private:

char * pc;

size_t capacity;

size_t length;

enum { DEFAULT_SIZE = 32};

// following function is called by every constructor

void init( size_t cap = DEFAULT_SIZE);

public:

string(const char * s);

string(size_t initial_capacity );

string();

//...other member functions

};

void string::init( size_t cap)

{

pc = new char[cap];

capacity = cap;

}

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Efficient … (contd.)

string::string(const char * s){

size_t size = strlen (s);

init(size + 1);

length = size;

strcpy(pc, s);

}

string::string(size_t initial_capacity ){

init(initial_capacity);

length=0;

}

string::string(){

init();

length = 0;

}

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Default Argument

class Fraction{

private:

int m_nNumerator;

int m_nDenominator;

public:

// Default constructor

Fraction(int nNumerator=0, int nDenominator=1) {

m_nNumerator = nNumerator;

m_nDenominator = nDenominator;

}

int GetNumerator() { return m_nNumerator; }

int GetDenominator() { return m_nDenominator; }

};

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A special case

Applicable to constructor with exactly one parameter

Implicit type conversion

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Is A Default Constructor Always Necessary?

The existence of a user-defined constructor blocks the synthesis of an implicitly-declared default constructor.

A class with no default constructor limits its users to a narrower set of allowed uses.

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Constant Object

Class objects can be made constant using the const keyword

Constant variables must be initialized at time of creation

In the case of classes, this initialization is done via constructors

Once initialized via constructor, any attempt to modify the member variables of the object is disallowed

It would violate the const(ant)-ness of the object

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Constant object

Changing member variables directly (if they are public) is not allowed

Calling member functions that sets the value of member variables is not allowed

const class objects can only call const member functions

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Trivial Constructors

Compilers synthesize a default constructor for every class unless a constructor was already defined by the user

Such a synthesized constructor is redundant

A constructor is considered trivial when all the following hold true:

Its class has no virtual member functions and no virtual base classes.
All the direct base classes of the constructor's class have trivial constructors.
All the member objects in the constructor's class have trivial constructors.

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Destructors

It is a member function whose name is the class name preceded by the tilde ~ character

A destructor’s usual purpose is finalizing or destroying objects of the class type.

Finalizing objects involves retrieving resources allocated to the object.

Requires using delete to deallocate store assigned to the object.

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When constructor and destructors are called

An object's constructor is called when the object comes into existence

An object's destructor is called when the object is destroyed

For a local object: constructor 🡪 object's declaration,

destructor 🡪 reverse order of constructor

For a global object: constructor 🡪 before main(), , in order of their declaration, within the same file

destructor 🡪 reverse order of constructor, after main() has terminated

Object Oriented Anslysis and Design (CS 212)