C 클래스에 정의된 Friend 함수에 액세스하는 방법은 무엇입니까?

<code class="cpp">#include <iostream>

class A {
public:
    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() {
    std::cout << "I'm here3" << std::endl;
}

int main() {
    fun(A()); // Works
    fun2(); // Now works
    fun3(); // Works
}</code>

<code class="cpp">#include <iostream>

class A {
public:
    friend static void fun(A a);
    friend static void fun2();
    friend static void fun3();
};

static void fun(A a) { std::cout << "I'm here" << std::endl; }
static void fun2() { std::cout << "I'm here2" << std::endl; }
static void fun3() {
    std::cout << "I'm here3" << std::endl;
}

int main() {
    fun(A()); // Works
    A::fun2(); // Works
    fun3(); // Works
}</code>