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CSC241: Object Oriented Programming

Lecture No 21

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Previous Lecture

• Polymorphism Example program– person class

• Virtual destructor– base class destructor is virtual, derived class

destructor also become virtual• Friend functions– can access private member of a class

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Today’s Lecture

• Friend function – example program: Distance class– Friend function for functional notation

• Friend classes• static functions

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Distance Exampleclass Distance {

private:int feet; float inches;

public: Distance() : feet(0), inches(0.0) { } Distance (float fltfeet) { feet = fltfeet; inches = 12*(fltfeet-feet); }Distance(int ft, float in) : feet(ft), inches(in) { }void showdist() const { cout << feet << “ : ” << inches ; }

Distance operator + ( Distance ) const; };

Distance Distance::operator + ( Distance d2 ) const { int f = feet + d2.feet; float i = inches + d2.inches; if(i >= 12.0) {

i -= 12.0; f++;

} return Distance(f,i); }

When such constructor exists, following statements are allowedd3 = 15.5;d3 = d1 + 10.0;

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Cont.

• d3 = 10.5;– 10.5 is passed to one argument constructor, – 10.5 is converted into Distance object– d3 = nameless object of Distance;

• d3 = d1 + 9.75;– 9.75 is converted into Distance object by calling

one argument constructor– Then operator + function is invoked by d1 object

• d3 = 6.5 + d1; • d3 = Distance(6.5) + d1;

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Note• When a float value pass as argument to function– Definition: void function (Distance d1) { … }– Call : function (10.75);– One argument constructor is called Distance(float)– Convert float value i.e. 10.75 into Distance object– Call : function (Distance (10.75))– function (Nameless Distance object)

• d3 = 10.75 + d1;– Neither float value 10.75 invoke operator +

function– Nor operator knows to convert float to Distance

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Distance Example – Friend functionclass Distance {

.

.

.friend Distance operator + ( Distance, Distance ) const;

};Distance Distance::operator + (Distance d1, Distance d2) const {

int f = d1.feet + d2.feet; float i = d1.inches + d2.inches; if(i >= 12.0)

i -= 12.0; f return Distance(f,i);

}

d3 = 10.0 + d1;

friend function Overloaded () operator took two argument

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friends for Functional Notationclass Distance {

private:int feet;float inches;

public: Distance() : feet(0), inches(0.0) { }Distance(int ft, float in) : feet(ft), inches(in) { }void showdist() const{ cout << feet << “ : ” << inches ; }float square();

};

float Distance::square() { float fltfeet = feet + inches/12; float feetsqrd = fltfeet * fltfeet; return feetsqrd;

}

main() {Distance dist(3, 6.0); float sqft;sqft = dist.square(); cout << “\nDistance = “; dist.showdist();cout << “\nSquare = “ << sqft;

}

sqft = square(dist);

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Cont.class Distance { private:

int feet;float inches;

public: Distance() : feet(0), inches(0.0){ } Distance(int ft, float in) : feet(ft), inches(in){ }void showdist() const{ cout << feet << “:” << inches; }friend float square(Distance);

};

float square(Distance d) {

float fltfeet; fltfeet = d.feet + d.inches/12; float feetsqrd = fltfeet * fltfeet; return feetsqrd;

}

In general, the friend version of a function requires one more argument than when the function is a member.

Go to program

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Friend classes

• Same as a function can be friend of a class– Friend function can access private data member

• A class can be friend of another class – All functions of friend class become friend

function of that class

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Cont.class alpha {

private: int data1;public:alpha() : data1(99) { } friend class beta;

};

main() {alpha a;beta b;b.func1(a);b.func2(a);cout << endl;

}

class beta { public: void func1(alpha a) { cout << “\ndata1=” << a.data1; }void func2(alpha a) { cout << “\ndata1=” << a.data1; }

};

• In class alpha the entire class beta is proclaimed a friend. • Now all the member functions of beta

can access the private data of alpha

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Static function

• A static data member is not duplicated for each object; rather a single data item is shared by all objects of a class

• Example: a class that keep track of how many objects of itself have created

• Function can also be static• A static function can only access static data

member of class

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Example programclass gamma {

private:static int total;int id;

public:gamma() { total++; id = total;

}~gamma() {total--;cout << “Destroying ID “ << id << endl;

}

static void showtotal() {cout << “Total is “ << total;

}void showid() {

cout << “ID number is “ << id;}

};int gamma::total = 0;

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Static vs. non static functions

• Static function can only access static data member of class– className::static_function();– Object_of_class.static function();

• Non static member function can access static and non static data member of class– Object_of_class.Non_static_function();

Go to program

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Assignment and Copy Initialization

• C++ compiler do many things on programmer behalf. For example:– Assignment operator– Copy constructor

Assignmenta2 = a1;

Copy constructoralpha a2(a1);

cause the compiler to copy the data from a1, member by member, into a2.

compiler creates an object, a2, copies the data from a1, member by member, into a2

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Cont.

• Both of these default activities are provided, free of charge, by the compiler

• If member-by member copying is required, then default activities are enough

• If assignment or initialization has to do something more complex, then they must be override

• First, separate example to override default assignment and copy constructor

• String class in more efficient way by overriding these two activities

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Overloading the Assignment Operatorclass alpha {

private: int data;public:

alpha() { }alpha(int d) { data = d; }void display(){ cout << data; }

alpha operator = (alpha& a){data = a.data; cout << “Assignment operator invoked”;return alpha(data);

}};

main() {alpha a1(37);alpha a2;a2 = a1; cout << “\na2=”; a2.display(); alpha a3 = a2; cout << “\na3=”; a3.display();

}

Program OutputAssignment operator invokeda2=37a3=37

Go to program

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Note

• Initialization Is Not Assignment– alpha a3 = a2; // copy initialization

is not an assignment but an initialization, with the same effect as– alpha a3(a2);

• Passing by Reference– Save memory space– Avoid creation of unwanted objects

• Returning a Value- Cannot do return by reference

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Assignment operator – inheritance

• The assignment operator is unique among operators in that it is not inherited

• If you overload the assignment operator in a base class, you can’t use this same function in any derived classes

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The Copy Constructor

• An object can be define and initialize with values of another object of same class– alpha a3(a2); // copy initialization– alpha a3 = a2; // copy initialization, alternate syntax

• Both invoke a copy constructor: – That creates a new object and copies its argument into

it• Default copy construct perform member by

member copy• Distance d2 = d1; 7

6.5feet

inches

d1feet

inches

d27

6.5

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Cont.

• It is similar to what assignment operator (d2 = d1) does

• The difference is that copy initialization also creates a new object

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Example program – assignment and copy constructor

class alpha {private: int data;public:

alpha() { }alpha(int d){ data = d; }alpha(alpha& a) {

data = a.data;cout << “\nCopy constructor invoked”;

}

void display(){ cout << data; }void operator = (alpha& a) {data = a.data;cout << “\nAssignment operator invoked”;

}};

a2 = a1; alpha a3(a1);

Program OutputAssignment operator invokedCopy constructor invoked

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