vector<cv::KeyPoint*> keypoints;
int N;
keypoints = vector<cv::KeyPoint*>(N, static_cast<cv::KeyPoint*>NULL); //开辟一块大小为Ｎ的内存，都指向空
std::fill(keypoints.begin(), keypoints.end(), static_cast<cv::KeyPoint*>(NULL)); //容器已指定了大小，直接全部填为空