Why in one case I must write iterator_category without parentheses:

template<typename Iterator>
void my_advance(Iterator &iter, int n)
{
    if constexpr(std::is_same_v<
            typename std::iterator_traits<Iterator>::iterator_category,
            std::random_access_iterator_tag>)
        iter  = n;
    else
        for(int i = 0; i<n;   i,   iter);
}

and in enother case with parentheses:

template<typename Iterator, typename IterCategory>
void my_advance_helper(Iterator &iter, int n, IterCategory){
    for(int i = 0; i < n;   i,   iter);
}

template<typename Iterator>
void my_advance_helper(Iterator &iter, int n, std::random_access_iterator_tag){
    iter  = n;
}

template<typename Iterator>
void my_advance(Iterator &iter, int n)
{
    my_advance_helper(iter, n,
    typename std::iterator_traits<Iterator>::iterator_category());
}

If as i understand, iterator_traits::iterator_category is just a typedef. What do parentheses do in last case? Do they return actual value of iterator_category in this way? Seems pretty obvious, but I need some confirmation. Sorry for possibly stupidity of question =)

CodePudding user response：

If as i understand, iterator_traits::iterator_category is just a typedef

Correct.

What do parentheses do in last case?

That's syntax for value initialisation of a temporary object.