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

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C++
Ranges library
Range adaptors
std::ranges::cache_latest_view
Member functions
Deduction guides
Iterator
Member functions
Non-member functions
Sentinel
Member functions
Non-member functions
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 .

Contents

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

constructs a cache_latest_view
(public member function)
returns a copy of the underlying (adapted) view
(public member function)
returns an iterator to the beginning
(public member function)
returns an iterator or a sentinel to the end
(public member function)
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range .
(public member function)
Inherited from std::ranges::view_interface
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> )
(C++23)
returns a constant iterator to the beginning of the range
(public member function of std::ranges::view_interface<D> )
(C++23)
returns a sentinel for the constant iterator of the range
(public member function of std::ranges::view_interface<D> )
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> )
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> )
returns the first element in the derived view, provided if it satisfies forward_range
(public member function of std::ranges::view_interface<D> )
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> )
returns the n th 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

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