97 lines
3.2 KiB
Markdown
97 lines
3.2 KiB
Markdown
[unord.map.elem]
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# 23 Containers library [[containers]](./#containers)
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## 23.5 Unordered associative containers [[unord]](unord#map.elem)
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### 23.5.3 Class template unordered_map [[unord.map]](unord.map#elem)
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#### 23.5.3.3 Element access [unord.map.elem]
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[ð](#lib:unordered_map,operator%5b%5d)
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`constexpr mapped_type& operator[](const key_type& k);
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`
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13803)
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*Effects*: Equivalent to: return try_emplace(k).first->second;
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[ð](#lib:unordered_map,operator%5b%5d_)
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`constexpr mapped_type& operator[](key_type&& k);
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`
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13815)
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*Effects*: Equivalent to: return try_emplace(std::move(k)).first->second;
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[ð](#lib:unordered_map,operator%5b%5d__)
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`template<class K> constexpr mapped_type& operator[](K&& k);
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`
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13827)
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*Constraints*: The [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]")*s* Hash::is_transparent andPred::is_transparent are valid and denote types[.](#3.sentence-1)
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13832)
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*Effects*: Equivalent to: return try_emplace(std::forward<K>(k)).first->second;
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[ð](#lib:unordered_map,at)
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`constexpr mapped_type& at(const key_type& k);
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constexpr const mapped_type& at(const key_type& k) const;
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`
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13845)
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*Returns*: A reference to x.second, where x is the (unique) element whose key is equivalent to k[.](#5.sentence-1)
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13849)
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*Throws*: An exception object of type out_of_range if no such element is present[.](#6.sentence-1)
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[ð](#lib:unordered_map,at_)
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`template<class K> constexpr mapped_type& at(const K& k);
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template<class K> constexpr const mapped_type& at(const K& k) const;
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`
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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13862)
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*Constraints*: The [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]")*s* Hash::is_transparent andPred::is_transparent are valid and denote types[.](#7.sentence-1)
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[8](#8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13867)
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*Preconditions*: The expression find(k) is well-formed and has well-defined behavior[.](#8.sentence-1)
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[9](#9)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13871)
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*Returns*: A reference to find(k)->second[.](#9.sentence-1)
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[10](#10)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13875)
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*Throws*: An exception object of type out_of_range if find(k) == end() is true[.](#10.sentence-1)
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