How to Access Friend Functions Defined in a Class in C ?

Access Friend Functions Defined in Class

In C , friend functions are declared within classes but can access private and protected members of the class. Consider the following code snippet:

<code class="cpp">class A {
    friend void fun(A a);
    friend void fun2();
    friend void fun3();
};

void fun3() {
    std::cout << "I'm here3" << std::endl;
}</code>

Accessing the fun(A) function works fine as it has a parameter of type A, allowing Argument-Dependent Lookup to locate it. However, there is an issue with accessing the global functions fun2() and fun3().

The declaration of fun2 within the class makes it a friend function but does not declare it in the global scope. Consequently, when accessing fun2() outside the class:

• fun2(); results in an error.
• A::fun2(); also results in an error.

To resolve this, the correct approach is to define all friend functions outside the class and make them explicit friends of the class:

<code class="cpp">class A {
    friend void fun(A a);
    friend void fun2();
    friend void fun3();
};

void fun(A a) { std::cout << "I'm here" << std::endl; }
void fun2() { std::cout << "I'm here2" << std::endl; }
void fun3();</code>

Defining friend functions this way allows them to be accessed both inside and outside the class.

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