[英]Inheritance and templates, a weird behaviour
我現在試圖開發一個基於事件的狀態機幾個小時,但是我無法確定為什么下面的模板類不匹配。 基本上, State
發布事件, StateMachine
監聽事件。 這是一個片段:
#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;
}
如果計算機是StateEventListener
,它是EventListener<StateEvent, StateEventPublisher>
,而StateEventPublisher
是EventPublisher<StateEvent>
,為什么不能將計算機附加到狀態? 我究竟做錯了什么?!
在EventPublisher<StateEvent>
聲明中, EventPublisher
期望偵聽器為EventListener<StateEvent, EventPublisher<StateEvent>>
而您提供的偵聽器為EventListener<StateEvent, StateEventPublisher>
,這是無關的類型(即使StateEventPublisher
繼承自EventPublisher<StateEvent>
)。
可能有幾種解決方法,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;
}
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.