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cppdraft/range/nonprop/cache.md

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