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cppdraft/range/approximately/sized.md

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+[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*]