Convert between cv::mat and Qimage would make the program crash

[stereomatching]

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#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"

#include <QtGui>

inline QImage const mat_to_qimage_ref(cv::Mat &mat)
{
  return QImage((unsigned char*)(mat.data), mat.cols, mat.rows, mat.step, QImage::Format_RGB32);
}
    
int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  QImage img("lena2.jpg");
  cv::Mat mat(img.height(), img.width(), CV_8UC4, img.bits(), img.bytesPerLine());
  QImage img = mat_to_qimage_ref(mat); //#1
  //QImage img2((unsigned char*)(mat.data), mat.cols, mat.rows, mat.step, QImage::Format_RGB32); #2

  QLabel label;
  label.setPixmap(QPixmap::fromImage(img)); //crash at here
  label.show();

  return a.exec&#40;&#41;;
}

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(#2) is ok, but #1 will occur undefined behavior?(my case is crash)

Besides, if you use it as the codes below, It is okay

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cv::Mat img = cv::imread("lena2.jpg");
QLabel label;
label.setPixmap(QPixmap::fromImage(mat_to_qimage_ref(img)));
label.show();
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Don't know what is happening, something related to cycle dependency?