The Friend Functions of a Class - C++ Class

C++ examples for Class:friend

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Introduction

An individual function can be specified as a friend of a class, or a whole class can be specified as a friend of another class.

In the latter case, all the function members of the friend class have the same access privileges as a normal member of the class.

To make a function a friend of a class, you must declare it within the class definition using the friend keyword.

A friend function can be a global function or it can be a member of another class.

Demo Code

                                                                                                                                    
#include <iostream>
#include <memory>
class Pool/*  ww  w.  j  a  v  a 2 s.  c om*/
{
private:
  double length;
  double width;
  double height;
                                                                                                                                        
public:
  // Constructors
  Pool(double lv = 1.0, double wv = 1.0, double hv = 1.0);
                                                                                                                                        
  double volume();                                  // Function to calculate the volume of a pool
                                                                                                                                        
  friend double surfaceArea(const Pool& aPool);       // Friend function for the surface area
};
                                                                                                                                        
// Constructor definition
Pool::Pool(double lv, double wv, double hv) : length(lv), width(wv), height(hv)
{
  std::cout << "Pool constructor called." << std::endl;
}
                                                                                                                                        
// Function to calculate the volume of a pool
double Pool::volume()
{
  return length*width*height;
}
                                                                                                                                        
                                                                                                                                        
int main()
{
  Pool pool1{ 2.2, 1.1, 0.5 };                              // An arbitrary pool
  Pool pool2;                                              // A default pool
  auto pool3 = std::make_shared<Pool>(15.0, 20.0, 8.0);   // Pool on the heap
                                                                                                                                        
  std::cout << "Volume of pool1 = " << pool1.volume() << std::endl;
  std::cout << "Surface area of pool1 = " << surfaceArea(pool1) << std::endl;
                                                                                                                                        
  std::cout << "Volume of pool2 = " << pool2.volume() << std::endl;
  std::cout << "Surface area of pool2 = " << surfaceArea(pool2) << std::endl;
                                                                                                                                        
  std::cout << "Volume of pool3 = " << pool3->volume() << std::endl;
  std::cout << "Surface area of pool3 = " << surfaceArea(*pool3) << std::endl;
}
                                                                                                                                        
// friend function to calculate the surface area of a Pool object
double surfaceArea(const Pool& aPool)
{
  return 2.0*(aPool.length*aPool.width + aPool.length*aPool.height + aPool.height*aPool.width);
}

Result


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