NAME

std::ranges::advance - std::ranges::advance

Synopsis

Defined in header <iterator>
Call signature
template< std::input_or_output_iterator I > (1) (since C++20)
constexpr void advance( I& i, std::iter_difference_t<I> n );
template< std::input_or_output_iterator I, std::sentinel_for<I> S
> (2) (since C++20)
constexpr void advance( I& i, S bound );
template< std::input_or_output_iterator I, std::sentinel_for<I> S
> (3) (since C++20)
constexpr std::iter_difference_t<I> advance( I& i,
std::iter_difference_t<I> n, S bound );

1) Increments given iterator i for n times.
2) Increments given iterator i until i == bound.
3) Increments given iterator i for n times, or until i == bound, whichever comes
first.

If n is negative, the iterator is decremented. In this case, I must model
std::bidirectional_iterator, and S must be the same type as I if bound is provided,
otherwise the behavior is undefined.

The function-like entities described on this page are niebloids, that is:

* Explicit template argument lists may not be specified when calling any of them.
* None of them is visible to argument-dependent lookup.
* When one of them is found by normal unqualified lookup for the name to the left
of the function-call operator, it inhibits argument-dependent lookup.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters

i - iterator to be advanced
bound - sentinel denoting the end of the range i is an iterator to
n - number of maximal increments of i

Return value

3) The difference between n and the actual distance i traversed.

Complexity

Linear.

However, if I additionally models std::random_access_iterator, or S models
std::sized_sentinel_for<I>, or I and S model std::assignable_from<I&, S>, complexity
is constant.

Notes

The behavior is undefined if the specified sequence of increments or decrements
would require that a non-incrementable iterator (such as the past-the-end iterator)
is incremented, or that a non-decrementable iterator (such as the front iterator or
the singular iterator) is decremented.

Possible implementation

struct advance_fn {
template<std::input_or_output_iterator I>
constexpr void operator()(I& i, std::iter_difference_t<I> n) const
{
if constexpr (std::random_access_iterator<I>) {
i += n;
}
else {
while (n > 0) {
--n;
++i;
}
if constexpr (std::bidirectional_iterator<I>) {
while (n < 0) {
++n;
--i;
}
}
}
}

template<std::input_or_output_iterator I, std::sentinel_for<I> S>
constexpr void operator()(I& i, S bound) const
{
if constexpr (std::assignable_from<I&, S>) {
i = std::move(bound);
}
else if constexpr (std::sized_sentinel_for<S, I>) {
(*this)(i, bound - i);
}
else {
while (i != bound) {
++i;
}
}
}

template<std::input_or_output_iterator I, std::sentinel_for<I> S>
constexpr std::iter_difference_t<I>
operator()(I& i, std::iter_difference_t<I> n, S bound) const
{
if constexpr (std::sized_sentinel_for<S, I>) {
// std::abs isn't constexpr until C++23
auto abs = [](const std::iter_difference_t<I> x) { return x < 0 ? -x : x; };

const auto dist = abs(n) - abs(bound - i);
if (dist < 0) {
(*this)(i, bound);
return -dist;
}

(*this)(i, n);
return 0;
}
else {
while (n > 0 && i != bound) {
--n;
++i;
}

if constexpr (std::bidirectional_iterator<I>) {
while (n < 0 && i != bound) {
++n;
--i;
}
}

return n;
}
}
};

inline constexpr auto advance = advance_fn();

Example

// Run this code

#include <iomanip>
#include <iostream>
#include <iterator>
#include <vector>

int main()
{
std::vector<int> v{ 3, 1, 4 };

auto vi = v.begin();

std::ranges::advance(vi, 2);
std::cout << "value: " << *vi << '\n';

{
std::ranges::advance(vi, v.end());
std::cout << std::boolalpha;
std::cout << "vi == v.end(): " << (vi == v.end()) << '\n';

std::ranges::advance(vi, -3);
std::cout << "value: " << *vi << '\n';

std::cout << "diff: " << std::ranges::advance(vi, 2, v.end()) << ", ";
std::cout << "value: " << *vi << '\n';

std::cout << "diff: " << std::ranges::advance(vi, 4, v.end()) << ", ";
std::cout << "vi == v.end(): " << (vi == v.end()) << '\n';
std::cout << std::noboolalpha;
}
}

Output:

value: 4
vi == v.end(): true
value: 3
diff: 0, value: 4
diff: 3, vi == v.end(): true

See also

ranges::next increment an iterator by a given distance or to a bound
(C++20) (niebloid)
ranges::prev decrement an iterator by a given distance or to a bound
(C++20) (niebloid)
ranges::distance returns the distance between an iterator and a sentinel, or between
(C++20) the beginning and end of a range
(niebloid)
advance advances an iterator by given distance
(function template)