朋友的方法

方法可以声明为朋友和功能：

class Accesser {
public:
    void private_accesser();
};

class PrivateHolder {
public:
    PrivateHolder(int val) : private_value(val) {}
    friend void Accesser::private_accesser();
private:
    int private_value;
};

void Accesser::private_accesser() {
    PrivateHolder ph(10);
    // OK: this method is declares as friend.
    std::cout << ph.private_value << std::endl;
}