c++ - 打印 vector 的内容

Question

我想执行一个非常简单的任务，但由于我无法弄清楚的错误而无法执行。我想使用以下代码将检测到的特征的内容保存到 vector 到 txt 文件中

Ptr<FeatureDetector> feature_detector = FeatureDetector::create("SIFT");
vector<KeyPoint> keypoints;

feature_detector->detect(img, keypoints);

for(unsigned int i = 0; i < keypoints.size(); i++)
{
    ofstream outf("vector.txt", ios::app);
    outf<<"value at "<< i << " = " << keypoints.at<KeyPoint>(i)<<endl;
}

但是我遇到了以下错误:

std::vector<_Ty>::at': function call missing argument list; use '&std::vector<_Ty>::at' to create a pointer to member

我检查了我的语法，没有发现任何错误。

编辑:在此之前，我想打印出矩阵的内容，这种格式非常适合它，这里是我用来打印矩阵内容的代码:

for(int x = 0;x < dst.rows ; x++)
{
    for( int y = 0; y < dst.cols; y++)
    {
        ofstream outf("Sample.txt", ios::app);
        outf<<"value at "<< x << "  " << y << " = " << dst.at<float>(x,y)<<endl;
    }
}

其中dst是由float数据类型组成的矩阵

Answer 1

尝试将代码更改为以下:

 ofstream outf("vector.txt", ios::app);  // you don't want to open file again and again
 for(unsigned int i = 0; i < keypoints.size(); i++)
 {
     outf<<"value at "<< i << " = " << keypoints.at(i)<<endl;
 }
 outf.close(); 

如@jogojapan 所述，为 OpenCV KeyPoint 重载运算符<<(..)。

std::ostream& operator<<(std::ostream& out,const KeyPoint& keypoint)
{
  // add stream keypoint member by yourself here
  out << keypoint.size;
  out << keypoint.angle;
  out << keypoint.response;
  out << keypoint.octave;
  out << keypoint.class_id;
  return out;
}