Explain in detail the concept of friend function.

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5 days ago

Explain in detail the concept of friend function.

SOLUTION....

Friend Function in C++

A friend function is a non-member function that is granted access to the private and protected members of a class by declaring it with the friend keyword inside that class.



class A {
    int x;
public:
    A(int v = 0) : x(v) {}
    friend void foo(A);  // foo can access A::x
};

Why/when to use

Symmetric operators that shouldn’t be members (e.g., operator+, operator<<) but need private access.
Cooperating classes where a utility needs to inspect internals of multiple classes.
Non-member utilities that conceptually don’t belong to the class interface but require privileged access.

Key properties & rules

A friend is not a member of the class.
Friendship is granted, not taken: it’s declared inside the class.
Friendship is not inherited and not transitive (friends of my friend aren’t my friends).
Access is limited to what’s declared as friend; use sparingly to avoid eroding encapsulation.
You can friend:
- a free function
- a member function of another class
- an entire class (friend class X;)



#include <iostream>
using namespace std;

class Beta; // forward declaration

class Alpha {
private:
    int a;
public:
    Alpha(int v = 0) : a(v) {}
    // Declare free function 'sum' as friend
    friend int sum(const Alpha&, const Beta&);
};

class Beta {
private:
    int b;
public:
    Beta(int v = 0) : b(v) {}
    // Declare the same free function as friend
    friend int sum(const Alpha&, const Beta&);
};

// Friend function definition (not a member of either class)
int sum(const Alpha& x, const Beta& y) {
    // Can access x.a and y.b even though they are private
    return x.a + y.b;
}

int main() {
    Alpha A(35);
    Beta  B(25);

    cout << "Sum = " << sum(A, B) << '\n';
    return 0;
}