C++ Tutorial Index

C++ Tutorial C++ History C++ Installation C++ First Program C++ cin and cout C++ Data type C++ Variable C++ operator C++ Keywords

C++ Control Statements

C++ If C++ Nested if C++ If-else C++ If-else-if C++ Switch C++ Break C++ Continue C++ Goto C++ For loop C++ While loop C++ Do while loop

C++ Functions

C++ Call by Value C++ Call by Reference C++ Recursion Function C++ Inline function C++ Friend function

C++ Arrays

Single dimension array Two dimension array

C++ Strings

C++ Strings

C++ Inheritance

C++ Inheritance Single level Inheritance Multilevel Inheritance Multiple Inheritance Hierarchical Inheritance Hybrid Inheritance

C++ Polymorphism

C++ Polymorphism C++ Overloading C++ Overriding C++ Virtual Function

C++ Pointers

C++ Pointers C++ this pointer

C++ Exception Handling

C++ Exception Handling

C++ Constructors

C++ Constructors Default Constructor Parameterize Constructor Copy constructor Constructor Overloading Destructor

C++ File Handling

C++ File Handling C++ Writing to file C++ Reading file C++ Close file

Miscellaneous

C Vs C++ C++ Comments C++ Data Abstraction C++ Identifier C++ Memory Management C++ Storage Classes C++ Void Pointer C++ Array To Function C++ Expressions C++ Features C++ Interfaces C++ Encapsulation std::min in C++ External merge sort in C++ Remove duplicates from sorted array in C++ Precision of floating point numbers Using these functions floor(), ceil(), trunc(), round() and setprecision() in C++ C++ References C++ Friend Functions C++ Mutable keyword Unary Operators in C++ Initialize Array of objects with parameterized constructors in C++ Differences between #define & const in C/C++ C++ Program to Implement Shell Sort C++ Program to Implement Merge Sort Storage Classes in C Vector resize() in C++ Passing by Reference Vs. Passing by the pointer in C++ Free vs delete() in C++ goto statement in C and C++ C++ program to read string using cin.getline() C++ String Concatenation Heap Sort in C++ Swap numbers in C++ Input Iterators in C++ Fibonacci Series in C++ C ++ Program: Alphabet Triangle and Number Triangle C++ Program: Matrix Multiplication C++ Program to Print Fibonacci Triangle Stack in C++ Maps in C++ Queue in C++ C++ Bitset C++ Algorithms Priority Queue in C++ C++ Multimap C++ Deque Function Pointer in C++ Sizeof() Operators in C++ C++ array of Pointers free() Vs delete in C Timsort Implementation Using C++ CPP Templates C++ Aggregation C++ Enumeration C++ Math Functions C++ Object Class C++ Queue Initialize Vector in C++ Vector in C++ C++ STL Components Function overloading in C++ C++ Maximum Index Problem C++ find missing in the second array C++ Program to find the product array puzzle C++ Program To Find Largest Subarray With 0 Sum C++ Program To Move All Zeros To The End Of The Array C++ Program to find the element that occurs once C++ Program to find the largest number formed from an array Constructor Vs Destructor C++ Namespaces C++ OOPs Concept C++ Static C++ Structs C++ Try-Catch C++ User Defined Exceptions C++ Virtual Destructor C++ vs C# Malloc() and new in C++ Palindrome Number Program in C++ Snake Code in C++ Splitting a string in C++ Structure Vs Class in C++ Virtual Function Vs Pure Virtual Function C++ Bidirectional Iterators C++ Forward Iterators C++ Iterators C++ Output Iterators C++ Range-based For Loop Converting string into integer in C++ LCM Program in C++ Type conversion in C++ Add two numbers using the function in C++ Advantage and disadvantage friend function C++ Armstrong Number Program in C++ ATM machine program in C++ using functions Binary to Decimal in C++ Bit Manipulation in C++ C++ Constructor C++ Dijkstra Algorithm Using the Priority Queue C++ int into String C++ Signal Handling Decimal to Binary in C++ Decimal to Hexadecimal in C++ Decimal to Octal in C++ Factorial Program in C++ Function in C++ Hexadecimal to Decimal in C++ Octal to Decimal in C++ Reverse a Number in C++ Structure Vs Class in C++ C++ Forward Iterators C++ Output Iterators C++ Prime number program Char Array to String in C++ Constructor Overloading in C++ Default arguments in C++ Different Ways to Compare Strings in C++ Dynamic Binding in C++ Program to convert infix to postfix expression in C++ SET Data Structure in C++ Upcasting and Downcasting in C++ Reverse an Array in C++ Fast Input and Output in C++ Delete Operator in C++ Copy elision in C++ C++ Date and Time C++ Bitwise XOR Operator Array of sets in C++ Binary Operator Overloading in C++ Binary Search in C++ Implementing the sets without C++ STL containers Scope Resolution Operator in C++ Smart pointers in C++ Types of polymorphism in C++ Exception Handling in C++ vs Java Const Keyword in C++ Type Casting in C++ Static keyword in C++ vs Java Inheritance in C++ vs Java How to concatenate two strings in C++ Programs to Print Pyramid Patterns in C++ swap() function in C++ Structure of C++ Program Stringstream in C++ and its applications rand() and srand() in C / C++ C++ Ternary Operator C++ Scope of Variables While Loop Examples in C++ Star pattern in C++ using For Loops For Loop Examples in C++ Do-While Loop Examples in C++ Top 5 IDEs for C++ That You Should Try Once Assertions in C/C++ C++ Convert Int to String Continue in C++ While loop Diamond Pattern in C++ using For Loop How to Reverse a String in C++ using Do-While Loop How to Reverse a String in C++ using For Loop How to Reverse a String in C++ using While Loop Infinite loop in C++ Loops in C++ Returning Multiple Values from a Function using Tuple and Pair in C++ wcscpy(), wcslen(), wcscmp() Functions in C++ Auto keyword in C++ C++ 11 vs C++ 14 vs C++ 17 C++ STL (Standard Template Library) Differences Between C Structures and C++ Structures Divide by Zero Exception in C++ Dynamic Constructor in C++ Dynamic Memory Allocation in C++ Find the Size of Array in C/C++ without using sizeof() function Floating Point Operations and Associativity in C, C++ and Java Hello World Program in C++ How to create a table in C++ How to Setup Environment for C++ Programming on Mac Implementation of a Falling Matrix in C++ Message Passing in C++ Pointer to Object in C++ Templates in C++ vs Generics in Java Ways to Copy a Vector in C++ What does Buffer Flush mean in C++ sort() function in C++ Structure Sorting (By Multiple Rules) in C++ Similarities between C++ and Java std::distance in C++

C++ Friend Functions

C++ Friend Functions

A class friend function is defined from outside class, but it does have access to all members of the private and protected class. With the definitions for friend functions occurring in the explanation of class, friends are not features of members.

The friend may be an attribute, a function template, or a member function, or a class or class framework, in which case friends are the entire class itself and its members.

To declare a function as a class friend, precede the function prototype with keyword friend in class definition as follows –

class Box {
   double width;
   public:
      double length;
      friend void printWidth( Box box );
      void setWidth( double wid );
};

To declare all ClassTwo member functions as ClassOne mates, place the following declaration in the ClassOne description

friend class ClassTwo;

Consider the following program –

#include <iostream>
using namespace std;
class bag {
   double width;
   public:
      friend void printWidth( bag bag );
      void setWidth( double wid );
};
// Member function definition
void bag::setWidth( double wid ) {
   width = wid;
}
// Note: printWidth() is not a member function of any class.
void printWidth( bag bag ) {
   /* Because printWidth() is a friend of bag, it can
   directly access any member of this class */
   cout << "Width of bag : " << bag.width <<endl;
}
// Main function for the program
int main() {
   bag bag;
   // set bag width without member function
   bag.setWidth(25.0);
   // Use friend function to print the wdith.
   printWidth( bag );
   return 0;
}

Output:

C++ Friend Functions

Friend Function Special grant for access to private and protected members can be given to a friend function like friend class. Could be a friend's function:

  • A method of another class
  •  A global function
class Node {
private:
    int key;
    Node* next;
    /* Other members of Node Class */
    friend int LinkedList::search();
    // Only search() of linkedList
    // can access internal members
};

Following are some important points regarding the functions and classes of friends:

  • Friends are only to be used for a specific time. Too many functions or external classes are clarified as friends of a class with protected or private data, decreasing the value of encapsulation in object-oriented programming of separate classes.
  • Friendship is not mutual. When Class K is a P friend, then P won't become a P friend immediately.
  • Friendship is not hereditary (see more details here)
  • In Java, there isn't the concept of friends.

A simple and complete C++ program to demonstrate friend Class

#include <iostream>
class B {
private:
    int b;
public:
    B() { b = 10; }
    friend class A; // Friend Class
};
class A {
private:
    int a;
public:
    void showA(B& x)
    {
        // Since B is friend of A, it can access
        // private members of A
        std::cout << "B::b=" << x.b;
    }
};
int main()
{
    B b;
    A a;
    a.showA(b);
    return 0;
}

Output:

C++ Friend Functions

A easy and complete C++ program to explain other class friendly function

#include <iostream>
class T;
class K {
public:
    void showT(T&);
};
class T {
private:
    int t;
public:
    T() { t = 05; }
    friend void K::showT(T& y); // Friend function
};
void K::showT(T& y)
{
    // Since showT() is friend of T, it can
    // access private members of T
    std::cout << "T::t = " << y.t;
}
int main()
{
    K k;
    T y;
    k.showT(y);
    return 0;
}

Output:

C++ Friend Functions

A simple and comprehensive C++ program to prove a global friend

#include <iostream>
class K {
    int k;
public:
    K() { k = 0; }
    // global friend function
    friend void showK(K&);
};
void showK(K& r)
{
    // Since showK() is a friend, it can access
    // private members of K
    std::cout << "K::k=" << r.k;
}
int main()
{
    K k;
    showK(k);
    return 0;
}

Output:

C++ Friend Functions



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