[英]Create a non-constant pointer to member function for SDL event filtering
I'm playing with SDL, and I am trying to supply a function pointer to an event filter. 我正在使用SDL,并且试图提供指向事件过滤器的函数指针。 This works fine if I make the function a static member of
ObjectWithState
, but I'd like to have the callback function alter the state of the object. 如果将函数
ObjectWithState
的静态成员,则此方法ObjectWithState
,但我希望回调函数更改对象的状态。 I was hoping to do this perhaps using a functor, but I can't quite work it out. 我希望可以使用仿函数来完成此操作,但我无法完全解决。
Is there any C++11 trickery that I can use to make this work? 我可以使用任何C ++ 11技巧来实现此目的吗?
class ObjectWithState
{
int someState;
public:
int operator()(void* userData, SDL_Event *event)
{
return ++someState;
}
};
int main()
{
//boilerplate
ObjectWithState obj;
SDL_EventFilter f = &(obj.operator()); //ERROR -> cannot create non-constant pointer to member function
SDL_SetEventFilter( f, nullptr );
}
Use the userdata
parameter to point to your object, and dispatch through a static method to the non-static method: 使用
userdata
参数指向您的对象,并通过静态方法将其分配给非静态方法:
class ObjectWithState
{
int someState;
public:
int operator()(SDL_Event *event)
{
++someState
}
static int dispatch(void* userdata, SDL_Event* event)
{
return static_cast<ObjectWithState*>(userdata)->operator()(event);
}
};
int main()
{
//boilerplate
ObjectWithState obj;
SDL_SetEventFilter(&ObjectWithState::dispatch, &obj);
}
You can't assign pointer to member functions to C style function pointers. 您不能将指向成员函数的指针分配给C样式函数指针。 You have to use a free function or a static function, and then call whatever members you need inside that.
您必须使用自由函数或静态函数,然后在其中调用所需的任何成员。
Actually, std::bind
may allow you to do it. 实际上,
std::bind
可能允许您执行此操作。 Not entirely sure though. 虽然不完全确定。
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