Help in understanding the following code?

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I was trying to store my mouse positions in a vector and then call the vector elements to display my points. I stumbled upon this thread in stackoverflow.com (http://stackoverflow.com/questions/24586013/opencv-storing-the-coordinates-of-the-points-clicked-by-mouse-left-button[^]

I couldn't understand the following lines of code and would really appreciate if someone can explain me the meaning of the code.

C++

void onMouse(int evt, int x, int y, int flags, void* param) {
    if(evt == CV_EVENT_LBUTTONDOWN) {
        std::vector<cv::Point>* ptPtr = (std::vector<cv::Point>*)param;  // Especially this line
        ptPtr->push_back(cv::Point(x,y));
    }
}

[Your code was corrupted, so I fixed it back to the original]

Posted 17-Sep-14 16:32pm

Rebecca1995

Updated 17-Sep-14 16:57pm

George Jonsson

v4

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barneyman 17-Sep-14 22:58pm

void* is generally used as a mechanism to pass arbitrary data around ... the param in this case is actually a memory pointer to a vector of cv::points

The function is taking that pointer, materialising/casting it to ptPtr and then adding a new point to the back of the vector

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George Jonsson · Accepted Answer · 2014-09-17T17:08:00

Basically what happens in this line

C++

std::vector<cv::point xmlns:cv="#unknown">* ptPtr = (std::vector<cv::point>*)param;</cv::point></cv::point>

is that the input argument param is being type cast into a pointer of type std::vector<cv::Point>
Then the coordinates (x,y) is added to the array/vector.

Here you can read more about std::vector[^]