about findcountour

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what kind of function is findcontour function? For example when this function is used eg: findContours( fgMask, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) ); This function takes the return type i.e contours, hierarchy as an argument? What is this technique called in c++? i'm asking cas i am also trying to make a function similar to this but have no idea how this is done.