矢量標超出範圍C++誤差

Question

當我運行下面的代碼我得到這個例外矢量下標超出範圍，該代碼成功地工作沒有例外的圖像上，但是當我改變的圖像發生異常。

Mat bw; 
inRange(output1, Scalar(low_h, low_s, low_v), Scalar(high_h, high_s, high_v), bw); 
vector<vector<Point> > contours; 
findContours(bw.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE); 
Mat dst = Mat::zeros(input_image.size(), input_image.type()); 

for(unsigned int i=0;i<contours.size();i++) 
{ 
    cout << "# of contour points: " << contours[i].size() << endl ; 
    for(unsigned int j=0;j<contours[i].size();j++) 
    { 
     cout << "Point(x,y)=" << contours[i][j]<< endl; 
    } 
    cout << " Area: " << cv::contourArea(cv::Mat(contours[i], false)) << endl; 
    std::vector<std::vector<cv::Point> >::iterator itc= contours.begin(); 
    while (itc!=contours.end()) { 
     if (contours[i].size()>500 || contours[i].size()<20) 
     itc= contours.erase(itc); 
    else 
     ++itc; 
    } 

Answer 1

問題是，當您從矢量contours中刪除一個元素時，它會減小矢量的大小。該指數i從0到contours.size() - 1，其中contours.size()在外部循環計算。在內部循環中，當從contours中刪除元素時，contours的大小將會減小，但i未更新以反映輪廓的縮小大小。這就是爲什麼你矢量下標超出範圍錯誤的。

for(unsigned int i=0;i<contours.size();i++) 
{ 
    // something else ... 

    std::vector<std::vector<cv::Point> >::iterator itc= contours.begin(); 
    while (itc!=contours.end()) {  
     if (contours[i].size()>500 || contours[i].size()<20) { 
      itc= contours.erase(itc); 
      // Now contours will have one less element 
      // However index i has not been updated and so 
      // contours[i] will eventually index out of range in 
      // one of the subsequent iterations of this inner loop 
     } 
     else { 
      ++itc; 
     } 
    } 
}