Examples of friend class by using C++

FRIEND CLASSES

Just like functions are made friends of classes, we can also make one class to be a friend of another class. Then, the friend class will have access to all the private members of the other class.

#include <iostream. h>

class Square;

class Rectangle {
int width, height;

public:
Rectangle(int w = 1, int h = 1):width(w),height(h){}
void display() {
cout << "Rectangle: " << width * height << endl;
};
void morph(Square &);
};

class Square {
int side;

public:
Square(int s = 1):side(s){}
void display() {
cout << "Square: " << side * side << endl;
};
friend class Rectangle;
};

void Rectangle::morph(Square &s) {
width = s.side;
height = s.side;
}

int main () {
Rectangle rec(5,10);
Square sq(5);
cout << "Before:" << endl;
rec.display();
sq.display();

rec.morph(sq);
cout << "\nAfter:" << endl;
rec.display();
sq.display();
return 0;
}

Friend classes

It is also possible to make an entire class a friend of another class. This gives all of the members of the friend class access to the private members of the other class. Here is an example:

class Storage
{
private:
    int m_nValue;
    double m_dValue;
public:
    Storage(int nValue, double dValue)
    {
        m_nValue = nValue;
        m_dValue = dValue;
    }

    // Make the Display class a friend of Storage
    friend class Display;
};

class Display
{
private:
    bool m_bDisplayIntFirst;

public:
    Display(bool bDisplayIntFirst) { m_bDisplayIntFirst = bDisplayIntFirst; }

    void DisplayItem(Storage &cStorage)
    {
        if (m_bDisplayIntFirst)
            std::cout << cStorage.m_nValue << " " << cStorage.m_dValue << std::endl;
        else // display double first
            std::cout << cStorage.m_dValue << " " << cStorage.m_nValue << std::endl;
    }
};

int main()
{
    Storage cStorage(5, 6.7);
    Display cDisplay(false);

    cDisplay.DisplayItem(cStorage);

    return 0;
}

The below example will completely illustrate the use of a friend classes.

For Example:-

#include<iostream.h>
#include<conio.h>
class A
{
private:
int a,b;
public:
A()
{
a=10;
b=20;
}
friend class B;
};
class B
{
public:
B()
void showA(A obj)
{
cout<<”The value of a: ”<<obj.a<<endl;
}
void showB(A obj)
{
cout<<”The value of b: ”<<obj.b<<endl;
}
};
main()
{
A x;
B y;
y.showA(x);
y.showB(x);
getch();
}

// friend class
#include <iostream. h>

class Square;

class Rectangle {
    int width, height;
  public:
    int area ()
      {return (width * height);}
    void convert (Square a);
};

class Square {
  friend class Rectangle;
  private:
    int side;
  public:
    Square (int a) : side(a) {}
};

void Rectangle::convert (Square a) {
  width = a.side;
  height = a.side;
}
 
int main () {
  Rectangle rect;
  Square sqr (4);
  rect.convert(sqr);
  cout << rect.area();
  return 0;
}

// friclass.cpp
// friend classes
#include <iostream.h>

////////////////////////////////////////////////////////////////
class alpha
{
private:
int data1;
public:
alpha() : data1(99) { } //constructor
friend class beta; //beta is a friend class
};
////////////////////////////////////////////////////////////////
class beta
{
public: //access private alpha data
void func1(alpha a) { cout << "\ndata1=" << a.data1; }
void func2(alpha a) { cout << "\ndata1=" << a.data1; }
};
////////////////////////////////////////////////////////////////
int main()
{
alpha a;
beta b;
b.func1(a);
b.func2(a);
cout << endl;
return 0;
}

  #include<iostream.h>
class summation{
friend class mean;
private:
int n,dd;float x ,sum;
public:
void input(){
cout<<"Enter number of the student"<<endl;
cin>>dd;
for(int j=1;j<=dd;j++){
cout<<"Enter number of degree for the stude"<<"["<<j<<"]"<<endl;
cin>>n;
sum=0;
for (int i=1;i<=n;i++){
cout<<" Enter degree "<<"["<<i<<"]"<<endl;
cin>>x;
sum+=x;}
cout<<"sum"<<"["<<i<<"]"<<sum<<endl;
}
}};

main(){
summation ob1;
ob1.input();

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