std::ranges::advance - cppreference.com (original) (raw)

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 algorithm function objects (informally known as 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.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Notes
• 5 Possible implementation
• 6 Example
• 7 See also

[edit] Parameters

[edit] Return value

1. The difference between n and the actual distance i traversed.

[edit] 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.

[edit] 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.

[edit] Possible implementation

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

[edit] Example

#include #include #include   int main() { std::vector 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

[edit] See also

| | increment an iterator by a given distance or to a bound(algorithm function object)[edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | decrement an iterator by a given distance or to a bound(algorithm function object)[edit] | | | returns the distance between an iterator and a sentinel, or between the beginning and end of a range(algorithm function object)[edit] | | | advances an iterator by given distance (function template) [edit] |