How to implement the imshowCompare() function without memory leak?

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I implemented the imshowCompare() function that switch the output and the original image by the left mouse button click in cv::imshow windows. This is a very useful function for testing. The code is as follows, but can not solve the problem of memory leak.

   void on_MouseHandle_Compare(int event, int x, int y, int flags, void* param);

   typedef struct {
    String winname;
    Mat src;
    Mat dst;
} strImshowCompare;

void imshowCompare(const String& winname, const Mat& src, const Mat& dst)
{
    // src, dst must be cloned, otherwise the external Mat may be modified.
    Mat newsrc = src.clone(); // Their memory will leak.
    Mat newdst = dst.clone(); // Their memory will leak.
    cv::imshow( winname, newdst);

   // struct strImshowCompare can't delete in this function! It must delete after the window is closed. But I don't know when the window is closed.
    strImshowCompare* str = new strImshowCompare; 
    str->winname = winname;
    str->src = newsrc;
    str->dst = newdst;

    cv::setMouseCallback(winname, on_MouseHandle_Compare, (void *)str);
}

void on_MouseHandle_Compare(int event, int x, int y, int flags, void* param)
{
    strImshowCompare* str = (strImshowCompare*) param;

    switch (event) {
        case cv::EVENT_LBUTTONDOWN: { 
            cv::imshow(str->winname, str->src);
            break;
        }
        case cv::EVENT_LBUTTONUP: {
            cv::imshow(str->winname,  str->dst);
            break;
        }
    }
}