71 lines
2.6 KiB
Markdown
71 lines
2.6 KiB
Markdown
[range.nonprop.cache]
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# 25 Ranges library [[ranges]](./#ranges)
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## 25.7 Range adaptors [[range.adaptors]](range.adaptors#range.nonprop.cache)
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### 25.7.4 Non-propagating cache [range.nonprop.cache]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L4371)
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Some types in [[range.adaptors]](range.adaptors "25.7 Range adaptors") are specified in terms of
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an exposition-only class template *non-propagating-cache*[.](#1.sentence-1)
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*non-propagating-cache*<T> behaves exactly like optional<T> with the following differences:
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- [(1.1)](#1.1)
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*non-propagating-cache*<T> constrains its type parameter T with is_object_v<T>.
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- [(1.2)](#1.2)
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The copy constructor is equivalent to:constexpr *non-propagating-cache*(const *non-propagating-cache*&) noexcept {}
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- [(1.3)](#1.3)
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The move constructor is equivalent to:constexpr *non-propagating-cache*(*non-propagating-cache*&& other) noexcept { other.reset();}
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- [(1.4)](#1.4)
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The copy assignment operator is equivalent to:constexpr *non-propagating-cache*& operator=(const *non-propagating-cache*& other) noexcept {if (addressof(other) != this) reset(); return *this;}
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- [(1.5)](#1.5)
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The move assignment operator is equivalent to:constexpr *non-propagating-cache*& operator=(*non-propagating-cache*&& other) noexcept { reset();
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other.reset(); return *this;}
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- [(1.6)](#1.6)
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*non-propagating-cache*<T> has an additional member function template
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specified as follows:
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[ð](#1.6.itemdecl:1)
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`template<class I>
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constexpr T& emplace-deref(const I& i); // exposition only
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`
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*Mandates*: The declaration T t(*i); is well-formed
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for some invented variable t[.](#1.6.sentence-2)
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[*Note [1](#note-1)*:
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If *i is a prvalue of type cv T,
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there is no requirement that it is movable ([[dcl.init.general]](dcl.init.general "9.5.1 General"))[.](#1.6.sentence-3)
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â *end note*]
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*Effects*: Calls reset()[.](#1.6.sentence-4)
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Then direct-non-list-initializes the contained value with *i[.](#1.6.sentence-5)
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*Postconditions*: *this contains a value[.](#1.6.sentence-6)
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*Returns*: A reference to the new contained value[.](#1.6.sentence-7)
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*Throws*: Any exception thrown by the initialization of the contained value[.](#1.6.sentence-8)
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*Remarks*: If an exception is thrown during the initialization of T,*this does not contain a value, and
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the previous value (if any) has been destroyed[.](#1.6.sentence-9)
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L4449)
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[*Note [2](#note-2)*:
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*non-propagating-cache* enables an input view
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to temporarily cache values as it is iterated over[.](#2.sentence-1)
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â *end note*]
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