[range.sized]

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

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

### 25.4.4 Sized ranges [range.sized]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1467)

The [sized_range](#concept:sized_range "25.4.4 Sized ranges [range.sized]") concept
refines [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]") with
the requirement that the number of elements in the range can be determined
in amortized constant time using ranges​::​size[.](#1.sentence-1)

[🔗](#concept:sized_range)

`template<class T>
  concept [sized_range](#concept:sized_range "25.4.4 Sized ranges [range.sized]") =
    [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")<T> && requires(T& t) { ranges::size(t); };
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1480)

Given an lvalue t of type remove_reference_t<T>, T models [sized_range](#concept:sized_range "25.4.4 Sized ranges [range.sized]") only if

- [(2.1)](#2.1)

ranges​::​size(t) is amortized O(1), does not modify t,
and is equal to ranges​::​distance(​ranges​::​begin(t), ranges​::​end(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​::​size(t) is well-defined regardless of the evaluation ofranges​::​begin(t)[.](#2.sentence-1)
  [*Note [1](#note-1)*:
  ranges​::​size(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​::​size(t) to be well-defined
for a [sized_range](#concept:sized_range "25.4.4 Sized ranges [range.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*]

[🔗](#lib:disable_sized_range)

`template<class>
  constexpr bool disable_sized_range = false;
`

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1508)

*Remarks*: Pursuant to [[namespace.std]](namespace.std "16.4.5.2.1 Namespace std"),
users may specialize disable_sized_range for cv-unqualified program-defined types[.](#3.sentence-1)

Such specializations shall
be usable in constant expressions ([[expr.const]](expr.const "7.7 Constant expressions")) and
have type const bool[.](#3.sentence-2)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1517)

[*Note [2](#note-2)*:

disable_sized_range allows use of [range](range.range#concept:range "25.4.2 Ranges [range.range]") types with the library
that satisfy but do not in fact model [sized_range](#concept:sized_range "25.4.4 Sized ranges [range.sized]")[.](#4.sentence-1)

— *end note*]