为一个vector 类型的变量做初始化

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