std::ranges:: views:: chunk_by, std::ranges:: chunk_by_view

From cppreference.com
C++
Ranges library
Range adaptors
std::ranges::chunk_by_view
Defined in header <ranges>
template < ranges:: forward_range V, std:: indirect_binary_predicate < iterator_t < V > ,

ranges:: iterator_t < V >> Pred >
requires ranges:: view < V > && std:: is_object_v < Pred >
class chunk_by_view

: public ranges:: view_interface < chunk_by_view < V, Pred >>
(1) (since C++23)
namespace views {

inline constexpr /* unspecified */ chunk_by = /* unspecified */ ;

}
(2) (since C++23)
Call signature
template < ranges:: viewable_range R, class Pred >

requires /* see below */

constexpr ranges:: view auto chunk_by ( R && r, Pred && pred ) ;
(since C++23)
template < class Pred >
constexpr /*range adaptor closure*/ chunk_by ( Pred && pred ) ;
(since C++23)
1) chunk_by_view is a range adaptor that takes a view and an invocable object pred (the binary predicate), and produces a view of subranges (chunks), by splitting the underlying view between each pair of adjacent elements for which pred returns false . The first element of each such pair belongs to the previous chunk, and the second element belongs to the next chunk.
2) The name views :: chunk_by denotes a RangeAdaptorObject . Given a subexpression e and f , the expression views :: chunk_by ( e, f ) is expression-equivalent to chunk_by_view ( e, f ) .

chunk_by_view always models forward_range , and models bidirectional_range and/or common_range , if adapted view type models the corresponding concepts. chunk_by_view never models borrowed_range or sized_range .

Data members

Member Definition
V base_ the underlying view
( exposition-only member object* )
movable-box <Pred> pred_ an object that wraps the predicate used to split the elements of base_
( exposition-only member object* )
non-propagating-cache <iterator> begin_ an object that caches the iterator to the first element
( exposition-only member object* )

Member functions

constructs a chunk_by_view
(public member function)
returns a copy of the underlying (adapted) view
(public member function)
returns a reference to the stored predicate
(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 an iterator to the begin of the next subrange
( exposition-only member function* )
returns an iterator to the begin of the previous subrange
( exposition-only member function* )
Inherited from std::ranges::view_interface
returns whether the derived view is empty. Provided 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 if ranges::empty is applicable to it.
(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 if it satisfies bidirectional_range and common_range .
(public member function of std::ranges::view_interface<D> )

Deduction guides

Nested classes

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

Notes

In order to provide the amortized constant time complexity required by the range concept, the result of begin() is cached within the chunk_by_view object. If the underlying range is modified after the first call to begin() , subsequent uses of the chunk_by_view object might have unintuitive behavior.

Feature-test macro Value Std Feature
__cpp_lib_ranges_chunk_by 202202L (C++23) std::ranges::chunk_by_view

Example

Run this code 
#include <functional>
#include <iostream>
#include <ranges>
#include <string_view>
 
void print_chunks(auto view, std::string_view separator = ", ")
{
    for (auto const subrange : view)
    {
        std::cout << '[';
        for (std::string_view prefix; auto const& elem : subrange)
            std::cout << prefix << elem, prefix = separator;
        std::cout << "] ";
    }
    std::cout << '\n';
}
 
int main()
{
    std::initializer_list v1 = {1, 2, 3, 1, 2, 3, 3, 3, 1, 2, 3};
    auto fn1 = std::ranges::less{};
    auto view1 = v1 | std::views::chunk_by(fn1);
    print_chunks(view1);
 
    std::initializer_list v2 = {1, 2, 3, 4, 4, 0, 2, 3, 3, 3, 2, 1};
    auto fn2 = std::ranges::not_equal_to{};
    auto view2 = v2 | std::views::chunk_by(fn2);
    print_chunks(view2);
 
    std::string_view v3 = "__cpp_lib_ranges_chunk_by";
    auto fn3 = [](auto x, auto y) { return not(x == '_' or y == '_'); };
    auto view3 = v3 | std::views::chunk_by(fn3);
    print_chunks(view3, "");
 
    std::string_view v4 = "\u007a\u00df\u6c34\u{1f34c}"; // "zß水🍌"
    auto fn4 = [](auto, auto ß) { return 128 == ((128 + 64) & ß); };
    auto view4 = v4 | std::views::chunk_by(fn4);
    print_chunks(view4, "");
}

Output: 

[1, 2, 3] [1, 2, 3] [3] [3] [1, 2, 3] 
[1, 2, 3, 4] [4, 0, 2, 3] [3] [3, 2, 1] 
[_] [_] [cpp] [_] [lib] [_] [ranges] [_] [chunk] [_] [by]
[z] [ß] [水] [🍌]

References

  • C++23 standard (ISO/IEC 14882:2024):
  • 26.7.30 Chunk by view [range.chunk.by]

See also

(C++23)
a range of view s that are N -sized non-overlapping successive chunks of the elements of another view
(class template) (range adaptor object)
(C++23)
a view whose M th element is a view over the M th through (M + N - 1) th elements of another view
(class template) (range adaptor object)
(C++23)
a view consisting of elements of another view , advancing over N elements at a time
(class template) (range adaptor object)
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