c++ - 将 "this"从父类方法转换到子类方法是一个好的做法吗？

Question

我正在实现一个非常基本的基类来运行有限状态机。这就是我所做的...

/**
 * @class FSM
 * @brief This class is a base class for an object that wants to run a Finite State Machine.
 * That object, let's say States, would need to inherit from FSM<States>,
 * and its methods would then be available as state actions of the state machine.
 */
template<typename States>
class FSM
{
    public :
        /**
         * @typedef StateAction is an alias for a pointer to a method of States, only callable that can be a state of the FSM
         */
        typedef void(States::*StateAction)(void);

        /**
         * @fn FSM(States* states, StateAction initial_state_action = nullptr)
         * @brief Constructs a FSM object whose actions are of type StateAction
         */
        FSM(States* states, StateAction initial_state_action = nullptr)
        :
            m_states(states),
            m_state_action(initial_state_action),
            m_is_entering_state(true)
        {}

        /**
         * @fn bool update()
         * @brief Performs an iteration of the FSM
         * @returns true if and only if the FSM has a state to run
         */
        bool update()
        {
            if(m_states && m_state_action)
            {
                auto previous_action = m_state_action;
                (m_states->*m_state_action)();
                m_is_entering_state = (m_state_action != previous_action);
            }
            return m_state_action != nullptr;
        }

    protected :
        /**
         * @fn void setState(StateAction state_action)
         * @brief Change the state of the FSM
         * @param[in] state_action Method of States that implements the new state behavior (nullptr stops the FSM)
         */
        void setState(StateAction state_action)
        {
            m_state_action = state_action;
        }

        /**
         * @fn bool isEnteringState() const
         * @brief Tells whether the current state has just be entered or is looping, waiting for a transition
         * @returns] true if and only if the current state hasn't yet been executed since it has been changed
         */
        bool isEnteringState() const
        {
            return m_is_entering_state;
        }

    private :
        States*     m_states;            //!< Pointer to the child class that implements the States of the FSM
        StateAction m_state_action;      //!< Pointer to the method of States that implements the current state behavior
        bool        m_is_entering_state; //!< Tells whether the current state has just be entered or is looping, waiting for a transition
};

...以及它的用途:

class States : public FSM<States>
{
    public :
        States()
        :
            FSM<States>(this, &States::state1)
        {}

        void state1()
        {
            // Actions to perform when entering the state
            if(isEnteringState())    cout << "Entering ";

            // Actions to perform constantly (regulate a system, detect events, etc...)
            cout << __func__ << endl;

            // Transitions to other states
            bool is_event_detected = true;
            if(is_event_detected)    setState(&States::state2);
        }

        void state2()
        {
            if(isEnteringState())    cout << "Entering ";

            cout << __func__ << endl;

            bool is_exit_detected = true;
            if(is_exit_detected)    setState(nullptr);
        }
};

int main()
{
    States fsm;
    bool is_fsm_running = true;
    while(is_fsm_running)
    {
        is_fsm_running = fsm.update();
    }
    return EXIT_SUCCESS;
}

将m_states(指向实现状态的派生对象的指针)存储在FSM中的唯一目的是在update()中调用m_state_action，它是指向派生对象的方法的指针。相反，我想知道使用 static_cast 将 this (属于 FSM* 类型)转换为 States* 是否是一个好的做法？

(static_cast<States*>(this)->*m_state_action)();

行为定义明确吗？ (我知道它适用于 mingw 7.30)

另外，作为一个附带问题，使用作为其子级的类来模板化 FSM 看起来有点奇怪(类状态:公共(public) FSM)...我可以以不同的方式这样做吗？

Answer 1

是的，在两个条件下这是合法的:

1. States必须从 FSM<States> 公开且非虚拟地继承.

   您的设计已经需要这种使用模式，但最好添加 static_assert执行此要求。

2. 该对象实际上是 States 的实例.

   要预防的话，即使不是不可能，也是很困难的 struct BogusStates : FSM<TrueStates> {};

   你至少可以阻止任何人直接实例化FSM<States>给它一个protected构造函数。