Would calling the default parameter with `new` cause memory leak?

Question

I am trying to write a function that returns the center point of biggest detected object. No problem here. The issue is that I also want to return the foreground mask from parameters, if the user wants to use it.

In order to handle this situation, my solution is as following:

cv::Point detect(const cv::Mat &img, cv::Mat &mask = *new cv::Mat());

If mask parameter is specified, it can be used in main. Does RAII works in the other case, or would there be a memory leak?

Note: I know I can overload the function by 2 lines, but I want to know is it possible to do it properly by using default parameters, and not using pointers (input type is strict).

Another note: If cv::noArray() or a similar function can be used, that is totally okay.

Example usage:

char ch = 0;
while (ch != 27) // ESC is pressed
{
    cap >> img;
    if (img.empty())
        break;
    cv::Mat mask;
    cv::Point pt = detect(img, mask);
    // or pt = detect(img);
    cv::imshow("original", img);
    cv::imshow("foreground", mask);
    ch = cv::waitKey(1);
}

Answer 1

Yes, that introduces a memory leak. You might instead consider using a tuple return value:

std::tuple<cv::Point, cv::Mat> detect(const cv::Mat &img);

or making mask a pointer:

cv::Point detect(const cv::Mat &img, cv::Mat *mask = nullptr);

You could also try to keep the same signature with a cast. This is ugly but it might work:

cv::Point detect(const cv::Mat &img, cv::Mat &mask = const_cast<cv::Mat&>(static_cast<cv::Mat const&>(cv::Mat())));

Answer 2

Ugly, but no memory leak:

cv::Point detect(const cv::Mat &img, cv::Mat &mask = *std::make_unique<cv::Mat>());

If the user does not provide a mask, one will be created. detect will write the mask back, which is then immediately destroyed. Using an overload will be more efficient as it can skip the needless update.

Alternatively, use cv::Mat* mask = nullptr. I don't know what you mean by "(input type is strict)." but for an experienced C++ programmer the cv::Mat* mask = nullptr pattern is clear: optional out parameter, no ownership transfer implied.