A friend declaration for a non-member function inside a class grants that functon access to the class's private and protected members. However, such a declaration does not serve as a full function declaration in the surrounding scope. If the function is used before its actual definition or a separate declaration appears, the compiler will generate an error.

#include <iostream>
#include <string>
using namespace std;

class Person {
    friend void fun();
public:
    Person(string name, int age) : name_(name), age_(age) {
        fun(); // Error: 'fun' not declared in this scope
    }
private:
    string name_;
    int age_;
};

void fun() {
    cout << "Friend of Person" << endl;
}

Too resolve this, either define fun() before the class or provide a separate declaration before its use:

class Person {
    friend void fun();
    void fun(); // Member function overload
public:
    Person(string name, int age) : name_(name), age_(age) {
        fun(); // Calls member function, not the friend
    }
private:
    string name_;
    int age_;
};

void fun() {
    cout << "Friend of Person" << endl;
}

If a non-member function is used outside the class before its definition—even if it was declared as a friend—a separate forward declaration is required:

class Person {
    friend void fun();
    void fun();
public:
    Person(string name, int age) : name_(name), age_(age) {}
private:
    string name_;
    int age_;
};

void func() {
    fun(); // Requires prior declaration of ::fun
}

void fun() {
    cout << "Friend of Person" << endl;
}

This code fails unless void fun(); is declared before func().

Even when a friend function is defined inline within the class, the same rule applies—it is not automatically visible in the enclosing namespace unless a matching declaration exists:

class Person {
    friend void fun() {
        cout << "Friend of Person" << endl;
    }
    void fun();
public:
    Person(string name, int age) : name_(name), age_(age) {
        fun(); // Calls member function, NOT the friend
    }
private:
    string name_;
    int age_;
};

However, once properly declared, the inline-defined friend can be called from outside:

class Person {
    friend void fun() {
        cout << "Friend of Person" << endl;
    }
    void fun();
public:
    Person(string name, int age) : name_(name), age_(age) {}
private:
    string name_;
    int age_;
};

void fun(); // Required for external visibility

void func() {
    fun(); // OK
}

int main() {
    func();
    return 0;
}