[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*]