std::ranges::views::cache_latest, std::ranges::cache_latest_view

 

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)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control 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 reserve_hint (C++26)   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    common_range	sized_range viewable_range    view	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	approximately_sized_range (C++26) 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 as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while common_viewviews::common views::counted to_input_viewviews::to_input (C++26)(C++26)	ref_view drop_viewviews::drop drop_while_viewviews::drop_while lazy_split_viewviews::lazy_split split_viewviews::split join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   concat_viewviews::concat (C++26)(C++26) cache_latest_viewviews::cache_latest (C++26)(C++26)	owning_view 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

 

std::ranges::cache_latest_view

Member functions

cache_latest_view::cache_latest_view

cache_latest_view::base

cache_latest_view::begin

cache_latest_view::end

cache_latest_view::size

Deduction guides

Iterator

Member functions

cache_latest_view::iterator::iterator

cache_latest_view::iterator::base

cache_latest_view::iterator::operator*

cache_latest_view::iterator::operator++ cache_latest_view::iterator::operator++(int)

Non-member functions

iter_move(cache_latest_view::iterator)

iter_swap(cache_latest_view::iterator)

Sentinel

Member functions

cache_latest_view::sentinel::sentinel

cache_latest_view::sentinel::base

Non-member functions

operator==(cache_latest_view::iterator, cache_latest_view::sentinel)

operator-(cache_latest_view::iterator, cache_latest_view::sentinel)

operator-(cache_latest_view::sentinel, cache_latest_view::iterator)

 

Defined in header <ranges>
template< ranges::input_range V >     requires ranges::view<V> class cache_latest_view     : public ranges::view_interface<cache_latest_view<V>>	(1)	(since C++26)
namespace views {     inline constexpr /* unspecified */ cache_latest = /* unspecified */; }	(2)	(since C++26)
Call signature
template< ranges::viewable_range R >     requires /* see below */ constexpr ranges::view auto cache_latest( R&& r );		(since C++26)

1) A range adaptor that caches the last-accessed element of its underlying view so that the element does not have to be recomputed on repeated access.

2) RangeAdaptorObject. The expression views::cache_latest(e) is expression-equivalent to cache_latest_view(e) for any suitable subexpression e.

cache_latest_view is an input_range-only that never models borrowed_range or common_range.

Nested types

Type	Definition
CacheT (private)	std::conditional_t<std::is_reference_v<ranges::range_reference_t<V>>,                    std::add_pointer_t<ranges::range_reference_t<V>>,                    ranges::range_reference_t<V>> (exposition-only member type*)

Data members

Member	Description
V base_ (private)	the underlying view (exposition-only member object*)
non-propagating-cache<CacheT> cache_ (private)	an object that caches the result of the last indirection of the current iterator (exposition-only member object*)

Member functions

(constructor)	constructs a cache_latest_view (public member function)
base	returns a copy of the underlying (adapted) view (public member function)
begin	returns an iterator to the beginning (public member function)
end	returns an iterator or a sentinel to the end (public member function)
size	returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range. (public member function)
Inherited from std::ranges::view_interface
empty	returns whether the derived view is empty, provided only if it satisfies sized_range or forward_range (public member function of std::ranges::view_interface<D>)
cbegin (C++23)	returns a constant iterator to the beginning of the range (public member function of std::ranges::view_interface<D>)
cend (C++23)	returns a sentinel for the constant iterator of the range (public member function of std::ranges::view_interface<D>)
operator bool	returns whether the derived view is not empty, provided only if ranges::empty is applicable to it (public member function of std::ranges::view_interface<D>)
data	gets the address of derived view's data, provided only if its iterator type satisfies contiguous_iterator (public member function of std::ranges::view_interface<D>)
front	returns the first element in the derived view, provided if it satisfies forward_range (public member function of std::ranges::view_interface<D>)
back	returns the last element in the derived view, provided only if it satisfies bidirectional_range and common_range (public member function of std::ranges::view_interface<D>)
operator[]	returns the nth element in the derived view, provided only if it satisfies random_access_range (public member function of std::ranges::view_interface<D>)

std::ranges::cache_latest_view::cache_latest_view

cache_latest_view() requires std::default_initializable<V> = default;	(1)	(since C++26)
constexpr explicit cache_latest_view( V base );	(2)	(since C++26)

1) Value-initializes base_ via its default member initializer (= V()).

2) Initializes base_ with std::move(base).

Parameters

base

-

a view

std::ranges::cache_latest_view::base

constexpr V base() const& requires std::copy_constructible<V>;	(1)	(since C++26)
constexpr V base() &&;	(2)	(since C++26)

1) Copy-constructs the result from the underlying view. Equivalent to return base_;.

2) Move-constructs the result from the underlying view. Equivalent to return std::move(base_);.

std::ranges::cache_latest_view::begin

constexpr auto begin();		(since C++26)

Equivalent to return /*iterator*/(*this);.

std::ranges::cache_latest_view::end

constexpr auto end();		(since C++26)

Equivalent to return /*sentinel*/(*this);.

std::ranges::cache_latest_view::size

constexpr auto size() requires ranges::sized_range<V>;	(1)	(since C++26)
constexpr auto size() const requires ranges::sized_range<const V>;	(2)	(since C++26)

1,2) Equivalent to return ranges::size(base_);

Deduction guides

template< class R > cache_latest_view( R&& ) -> cache_latest_view<views::all_t<R>>;		(since C++26)

Nested classes

iterator	the iterator type (exposition-only member class template*)
sentinel	the sentinel type (exposition-only member class template*)

Notes

cache_latest_view is useful if the computation of the element to produce is expensive.

Feature-test macro	Value	Std	Feature
__cpp_lib_ranges_cache_latest	202411L	(C++26)	std::ranges::cache_latest_view

Example

Run this code

#include <algorithm>
#include <print>
#include <ranges>
 
int main()
{
    const auto v = {1, 2, 3, 4, 5};
 
    auto square = [](int i)
    {
        std::print("transform: {} ", i);
        return i * i;
    };
 
    auto is_even = [](int i)
    {
        std::print("filter: {} ", i);
        return i % 2 == 0;
    };
 
    auto print = [](auto v)
    {
        std::ranges::for_each(v, [](int i){ std::println("Got: {}", i); });
        std::println();
    };
 
    std::println("Without caching latest (note recalculations):");
    auto even_squares = v
        | std::views::transform(square)
        | std::views::filter(is_even);
    print(even_squares);
 
    std::println("With caching latest:");
    auto fast_even_squares = v
        | std::views::transform(square)
        | std::views::cache_latest
        | std::views::filter(is_even);
    print(fast_even_squares);
}

Output:

Without caching latest (note recalculations):
transform: 1 filter: 1 transform: 2 filter: 4 transform: 2 Got: 4
transform: 3 filter: 9 transform: 4 filter: 16 transform: 4 Got: 16
transform: 5 filter: 25 
With caching latest:
transform: 1 filter: 1 transform: 2 filter: 4 Got: 4
transform: 3 filter: 9 transform: 4 filter: 16 Got: 16
transform: 5 filter: 25