std::ranges::advance

 

C++

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Feature test macros (C++20)

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Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) is-integer-like is-signed-integer-like (C++20)(C++20)	sentinel_for (C++20) sized_sentinel_for (C++20) input_iterator (C++20) output_iterator (C++20) input_or_output_iterator (C++20)	forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20)	iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)	iterator (deprecated in C++17) iterator_traits incrementable_traits (C++20) indirectly_readable_traits (C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)	indirect_unary_predicate (C++20) indirect_binary_predicate (C++20)	indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20)	indirectly_copyable_storable (C++20)   indirectly_swappable (C++20) indirectly_comparable (C++20)	permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t (C++20)	projected (C++20)	projected_value_t (C++26)
Iterator adaptors
reverse_iterator make_reverse_iterator (C++14) move_iterator (C++11) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)	front_insert_iterator back_insert_iterator inserter insert_iterator front_inserter back_inserter move_sentinel (C++20)	common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) make_const_iterator (C++23) make_const_sentinel (C++23)
Stream iterators istream_iterator    ostream_iterator istreambuf_iterator ostreambuf_iterator	Iterator customization points ranges::iter_move (C++20) ranges::iter_swap (C++20)
Iterator operations advance distance prev (C++11)   next (C++11) ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)	Range access begincbegin (C++11)(C++14) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

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

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 cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

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 is not constexpr until C++23 auto abs = [](const std::iter_difference_t<I> x) { return x < 0 ? -x : x; };   if (const auto dist = abs(n) - abs(bound - i); 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 <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 << "1) value: " << *vi << '\n' << std::boolalpha;
 
    std::ranges::advance(vi, v.end());
    std::cout << "2) vi == v.end(): " << (vi == v.end()) << '\n';
 
    std::ranges::advance(vi, -3);
    std::cout << "3) value: " << *vi << '\n';
 
    std::cout << "4) diff: " << std::ranges::advance(vi, 2, v.end())
              << ", value: " << *vi << '\n';
 
    std::cout << "5) diff: " << std::ranges::advance(vi, 4, v.end())
              << ", vi == v.end(): " << (vi == v.end()) << '\n';
}

Output:

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

See also

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