继承和模板，奇怪的行为

Question

我正在尝试开发一个基于事件的状态机几个小时，但我无法确定为什么下面的模板类不匹配。基本上，State 发布事件，StateMachine 监听它们。这是一个片段：

#include <iostream>
#include <vector>

// Interface for event listening
template <typename Event, typename Sender>
class EventListener
{
public:
    virtual ~EventListener() = default;
    virtual void onEvent(const Sender* sender, const Event& data) = 0;
};

// Abstract class for event publishing
template <typename Event>
class EventPublisher
{
public:
    typedef EventListener<Event, EventPublisher> Listener;
    virtual ~EventPublisher() = default;

    // Queues an event listener
    void attach(Listener* listener) {
        listeners.push_back(listener);
    }

protected:
    // Publishes an event among all registered listeners
    void publish(const Event& e) {
        for (Listener* listener : listeners) {
            listener->onEvent(this, e);
        }
    }

private:
    std::vector<Listener*> listeners;
};

// Concrete publisher & listener
class StateEvent {};
class StateEventPublisher : public EventPublisher<StateEvent> {};
class StateEventListener : public EventListener<StateEvent, StateEventPublisher> {};

class State : public StateEventPublisher {
public:
    void foo() {
        publish(StateEvent());
    }
};

class StateMachine final : public StateEventListener {
private:
    void onEvent(const StateEventPublisher* sender, const StateEvent& e) override {}
};

int main()
{
    State state;
    StateMachine machine; // Is a StateEventListener, which is a EventListener<StateEvent, StateEventPublisher>, whereas StateEventPublisher is a EventPublisher<StateEvent>

    state.attach(&machine); // Incompatible with EventListener<StateEvent, EventPublisher<StateEvent>>*
    state.foo();

    return 0;
}

如果机器是 StateEventListener，即 EventListener<StateEvent, StateEventPublisher>，而 StateEventPublisher 是 EventPublisher<StateEvent>，为什么机器不能附加到状态？！我做错了什么？！

Answer 1

在 EventPublisher<StateEvent> 声明中 EventPublisher 期望监听器是 EventListener<StateEvent, EventPublisher<StateEvent>> 而你提供的监听器是 EventListener<StateEvent, StateEventPublisher> 这是不相关的类型（即使 StateEventPublisher 继承来自 EventPublisher<StateEvent>).

有多种方法可以解决这个问题，例如使用 CRTP：

// Interface for event listening
template <typename Event, typename Sender>
class EventListener
{
public:
    virtual ~EventListener() = default;
    virtual void onEvent(const Sender* sender, const Event& data) = 0;
};

// Abstract class for event publishing
template <typename Event, typename Sender>
class EventPublisher
{
public:
    typedef EventListener<Event, Sender> Listener;
    virtual ~EventPublisher() = default;

    // Queues an event listener
    void attach(Listener* listener) {
        listeners.push_back(listener);
    }

protected:
    // Publishes an event among all registered listeners
    void publish(const Event& e) {
        for (Listener* listener : listeners) {
            listener->onEvent(static_cast<Sender *>(this), e);
        }
    }

private:
    std::vector<Listener*> listeners;
};

// Concrete publisher & listener
class StateEvent {};
class StateEventPublisher : public EventPublisher<StateEvent, StateEventPublisher> {};
class StateEventListener : public EventListener<StateEvent, StateEventPublisher> {};

class State : public StateEventPublisher {
public:
    void foo() {
        publish(StateEvent());
    }
};

class StateMachine final : public StateEventListener {
private:
    void onEvent(const StateEventPublisher* sender, const StateEvent& e) override {}
};

int main()
{
    State state;
    StateMachine machine; // Is a StateEventListener, which is a EventListener<StateEvent, StateEventPublisher>, whereas StateEventPublisher is a EventPublisher<StateEvent>

    state.attach(&machine); // Incompatible with EventListener<StateEvent, EventPublisher<StateEvent>>*
    state.foo();

    return 0;
}

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继承和模板，奇怪的行为

Inheritance and templates, a weird behaviour

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