49 lines
2.5 KiB
Markdown
49 lines
2.5 KiB
Markdown
[range.approximately.sized]
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# 25 Ranges library [[ranges]](./#ranges)
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## 25.4 Range requirements [[range.req]](range.req#range.approximately.sized)
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### 25.4.3 Approximately sized ranges [range.approximately.sized]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1427)
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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
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an approximation of the number of elements in the range
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can be determined in amortized constant time using ranges::reserve_hint[.](#1.sentence-1)
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[ð](#concept:approximately_sized_range)
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`template<class T>
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concept [approximately_sized_range](#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]") =
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[range](range.range#concept:range "25.4.2 Ranges [range.range]")<T> && requires(T& t) { ranges::reserve_hint(t); };
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`
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1440)
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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
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- [(2.1)](#2.1)
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ranges::reserve_hint(t) is amortized O(1),
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does not modify t, and
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has a value that
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is not negative and is representable in range_difference_t<T>, and
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- [(2.2)](#2.2)
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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
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regardless of the evaluation of ranges::begin(t)[.](#2.sentence-1)
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[*Note [1](#note-1)*:
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ranges::reserve_hint(t) is otherwise not required to be well-defined
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after evaluating ranges::begin(t)[.](#2.2.sentence-2)
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For example,
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it is possible for ranges::reserve_hint(t) to be well-defined for
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an [approximately_sized_range](#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]") whose
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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)
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â *end note*]
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