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Non-propagating cache (C++20) - cppreference.com

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template< class T > requires std::is_object_v<T> class /*non-propagating-cache*/;

 (since C++20)
(exposition only*)

Some range adaptors such as ranges::join_view and ranges::lazy_split_view conditionally store value (e.g. an iterator) which is specified in terms of an exposition-only class template non-propagating-cache.

The wrapper behaves exactly like std::optional<T>, except that:

  • it does not copy the value of the source when it is copy constructed or assigned to,
  • it resets the value of the source when it is moved-from,
  • it resets its value when it is assigned from, and
  • it additionally provides a member function template to enable an input view to temporarily cache values as it is iterated over.

The wrapper encapsulates a cache containing a value. Clearing cache is an operation equivalent to resetting a contained value. Such operation is performed when copying or moving a wrapper.

Template parameters

T - the type of the contained value, must be an object type

Member functions

Copy and move constructors

constexpr /*non-propagating-cache*/ ( const /*non-propagating-cache*/& ) noexcept {}

 (1) (since C++20)

constexpr /*non-propagating-cache*/ ( /*non-propagating-cache*/&& other ) noexcept { other.reset(); }

 (2) (since C++20)

1) The copy constructor has no effect.

2) The move constructor clears the cache of other.

Copy and move assignment operators

constexpr /*non-propagating-cache*/& operator=( const /*non-propagating-cache*/& other ) noexcept { if (std::addressof(other) != this) reset(); return *this; }

 (1) (since C++20)

constexpr /*non-propagating-cache*/& operator=( /*non-propagating-cache*/&& other ) noexcept { reset(); other.reset(); return *this; }

 (2) (since C++20)

1) The copy assignment operator clears the cache of *this.

2) The move assignment operator clears caches of both *this and other.

non-propagating-cache<T>::emplace-deref

template< class I > constexpr T& /*emplace-deref*/( const I& i );

 (since C++20)
(exposition only*)

Initializes the contained value by direct-initializing (but not direct-list-initializing) with *i. If *this already contains a value before the call, reset() is called.

Returns a reference to the new contained value.

The program is ill-formed unless the declaration T t(*i); is well-formed for some invented variable t. If *i is a prvalue of possibly cv-qualified T, then it is not required to be movable.

Members identical to std::optional

Member functions

constructs the optional object
(public member function of std::optional<T>) [edit]
destroys the contained value, if there is one
(public member function of std::optional<T>) [edit]
assigns contents
(public member function of std::optional<T>) [edit]
Observers
accesses the contained value
(public member function of std::optional<T>) [edit]
checks whether the object contains a value
(public member function of std::optional<T>) [edit]
Modifiers
destroys any contained value
(public member function of std::optional<T>) [edit]
constructs the contained value in-place
(public member function of std::optional<T>) [edit]

Notes

non-propagating-cache is used in implementations to cache the result of begin() to provide an amortized constant time complexity of the method.

See also