クラス内で引数を定義せずにフレンド関数にアクセスするにはどうすればよいでしょうか?

<code class="cpp">class A {
public:
    friend void fun(A a);  // Friend function that takes an argument of type A
    friend void fun2();  // Friend function without arguments
    friend void fun3();  // Friend function declaration
};

void fun(A a) { std::cout << "Im here" << std::endl; }
void fun3() { std::cout << "Im here3" << std::endl; }

int main() {
    fun(A()); // Works OK
    // fun2(); // Error: 'fun2' was not declared in this scope
    // A::fun2(); // Error: 'fun2' is not a member of 'A'
    fun3(); // Works OK
}</code>

<code class="cpp">void fun2() { std::cout << "Im here2" << std::endl; }</code>

<code class="cpp">fun2(); // Works OK</code>

<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(); // Leave it as a declaration

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