如何对基类中的派生类函数加好友？

#include <iostream>
using namespace std;

class Base {
private:
    void privateFunc() {
        cout << "This is a private function in Base class." << endl;
    }

protected:
    void protectedFunc() {
        cout << "This is a protected function in Base class." << endl;
    }

public:
    void publicFunc() {
        cout << "This is a public function in Base class." << endl;
    }

    friend class Derived; // 将Derived类声明为Base类的友元
};

class Derived : public Base {
public:
    void callBaseFunc() {
        publicFunc(); // 可以直接调用基类的公有函数
        protectedFunc(); // 可以直接调用基类的受保护函数
        // privateFunc(); // 无法直接调用基类的私有函数，会编译错误
    }
};

int main() {
    Derived derived;
    derived.callBaseFunc();

    return 0;
}