Design Pattern --- Memento

class Originator;

class Memento
{
    // data.
    int m_x, m_y;

    friend class Originator;

private:
    Memento(int x, int y) : m_x(x), m_y(y) {}
};

class Originator
{
    // data.
    int m_x, m_y;

public:
    Originator(int x, int y) : m_x(x), m_y(y) {}
public:
    // Interface.
    void Move(int x, int y)
    { cout <<"move to: " <<(m_x = x) <<", " <<(m_y = y) <<endl; }

    Memento GetMemento() const
    { return Memento(m_x, m_y); }
    void SetMemento(Memento m)
    {
        m_x = m.m_x;
        m_y = m.m_y;

        cout <<"current position is: " <<m_x <<", " <<m_y <<endl;
    }
};

int main()
{
    Originator o(0, 0);

    Memento m1 = o.GetMemento();
    o.Move(1, -4);
    Memento m2 = o.GetMemento();

    o.SetMemento(m1);            // Back to 'm1'.
    o.SetMemento(m2);            // Back to 'm2'.

    return 0;
}

Memento 模式本质就是将某个复杂对象的当前状态单独封装为一个集合, 以便保存这个对象的状态.
还是那句话, 把易变的东西包装到类的外面.