Non-propagating cache (C++20)

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range    sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while concat_viewviews::concat (C++26)(C++26)	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

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

(constructor)	constructs the optional object (public member function of std::optional<T>)
(destructor)	destroys the contained value, if there is one (public member function of std::optional<T>)
operator=	assigns contents (public member function of std::optional<T>)
Observers
operator->operator*	accesses the contained value (public member function of std::optional<T>)
operator boolhas_value	checks whether the object contains a value (public member function of std::optional<T>)
Modifiers
reset	destroys any contained value (public member function of std::optional<T>)
emplace	constructs the contained value in-place (public member function of std::optional<T>)

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

ranges::join_viewviews::join (C++20)	a view consisting of the sequence obtained from flattening a view of ranges (class template) (range adaptor object)
ranges::join_with_viewviews::join_with (C++23)	a view consisting of the sequence obtained from flattening a view of ranges, with the delimiter in between elements (class template) (range adaptor object)
ranges::split_viewviews::split (C++20)	a view over the subranges obtained from splitting another view using a delimiter (class template) (range adaptor object)
ranges::lazy_split_viewviews::lazy_split (C++20)	a view over the subranges obtained from splitting another view using a delimiter (class template) (range adaptor object)
ranges::chunk_viewviews::chunk (C++23)	a range of views that are N-sized non-overlapping successive chunks of the elements of another view (class template) (range adaptor object)