Memento - 备忘录模式

class Memento
{
public:
    void setState(State* state) { m_state = state; }
    State* state() const { return m_state; }
private:
    friend class Viewer;
    Memento() { }
private:
    State* m_state;
};

class Viewer
{
public:
    void drawGraphics();
    Memento* createMemento();
    void setMemento(Memento* memento);
};
Memento* Viewer::createMemento()
{
    State* state = getSelfState();
    Memento* memento = new Memento();
    memento->setState(state);
    return memento;
}
void setMemento(Memento* memento)
{
    setSelfState(memento->state());
}

class Command
{
public:
    void execute();
    void unExecute();
private:
    Viewer* m_viewer;
    std::vector<Memento*> m_mementos;
};
void Command::execute()
{
    m_viewer->drawGraphics();
    m_mementos.push_back(m_viewer->createMemento());
}
void Command::unExecute()
{
    if(!m_memento.isEmpty())
    {
        m_viewer->setMemento(m_mementos.back());
        m_mementso.pop_back();
    }
}