The second version (template function), has the same behavior, but takes a specific function to perform the comparison operation in charge of determining the insertion points. The comparison function has to perform weak strict ordering (which basically means the comparison operation has to be transitive and irreflexive). The merging is performed using two iterators: one to iterate through x and another one to keep the insertion point in the vector object; During the iteration of x, if the current element in x compares less than the element at the current insertion point in the list object, the element is removed from x and inserted into that location, otherwise the insertion point is advanced. This operation is repeated until either end is reached, in which moment the remaining elements of x (if any) are moved to the end of the list object and the function returns (this last operation is performed in constant time). Common quick sort algorithm.