[range.approximately.sized]

# 25 Ranges library [[ranges]](./#ranges)

## 25.4 Range requirements [[range.req]](range.req#range.approximately.sized)

### 25.4.3 Approximately sized ranges [range.approximately.sized]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1427)

The [approximately_sized_range](#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]") concept refines [range](range.range#concept:range "25.4.2 Ranges [range.range]") with the requirement that
an approximation of the number of elements in the range
can be determined in amortized constant time using ranges​::​reserve_hint[.](#1.sentence-1)

[🔗](#concept:approximately_sized_range)

`template<class T>
  concept [approximately_sized_range](#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]") =
    [range](range.range#concept:range "25.4.2 Ranges [range.range]")<T> && requires(T& t) { ranges::reserve_hint(t); };
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1440)

Given an lvalue t of type remove_reference_t<T>,T models [approximately_sized_range](#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]") only if

- [(2.1)](#2.1)

ranges​::​reserve_hint(t) is amortized O(1),
does not modify t, and
has a value that
is not negative and is representable in range_difference_t<T>, and

- [(2.2)](#2.2)

if iterator_t<T> models [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11 Concept forward_­iterator [iterator.concept.forward]"),ranges​::​reserve_hint(t) is well-defined
regardless of the evaluation of ranges​::​begin(t)[.](#2.sentence-1)
  [*Note [1](#note-1)*:
  ranges​::​reserve_hint(t) is otherwise not required to be well-defined
after evaluating ranges​::​begin(t)[.](#2.2.sentence-2)
  For example,
it is possible for ranges​::​reserve_hint(t) to be well-defined for
an [approximately_sized_range](#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]") whose
iterator type does not model [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11 Concept forward_­iterator [iterator.concept.forward]") only if evaluated before the first call to ranges​::​begin(t)[.](#2.2.sentence-3)
 — *end note*]