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std::ranges::views::chunk_by, 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

Typical implementations of chunk_by_view hold three non-static data members:

  • the underlying view of type V (shown here as base_ for exposition only),
  • a wrapper that wraps the predicate used to split the elements of base_ of type movable-box<Pred> (shown here as pred_ for exposition only), where movable-box is a wrapper class template that always satisfies movable,
  • an object of std::optional-like type (shown here as begin_ for exposition only) that caches an iterator to the first element.

Member functions

(C++23)
 constructs a chunk_by_view
(public member function)
(C++23)
 returns a copy of the underlying (adapted) view
(public member function)
(C++23)
 returns a reference to the stored predicate
(public member function)
(C++23)
 returns an iterator to the beginning
(public member function)
(C++23)
 returns an iterator or a sentinel to the end
(public member function)
(C++23)
 returns an iterator to the begin of the next subrange
(exposition-only member function)
(C++23)
 returns an iterator to the begin of the previous subrange
(exposition-only member function)
Inherited from std::ranges::view_interface
(C++20)
 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>)
(C++20)
 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>)
(C++20)
 returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
(C++20)
 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

(C++23)
 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 Comment
__cpp_lib_ranges_chunk_by 202202L (C++23) std::ranges::chunk_by_view

Example

A link to test: Compiler Explorer.

#include <functional>
#include <iostream>
#include <ranges>
#include <string_view>
#include <vector>
 
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()
{
    {
        auto v = std::vector{1, 2, 3, 1, 2, 3, 3, 3, 1, 2, 3};
        auto fun = std::ranges::less{};
        auto view = v | std::views::chunk_by(fun);
        print_chunks(view);
    }
    {
        auto v = std::vector{1, 2, 3, 4, 4, 0, 2, 3, 3, 3, 2, 1};
        auto fun = std::not_fn(std::ranges::equal_to{}); // or ranges::not_equal_to
        auto view = v | std::views::chunk_by(fun);
        print_chunks(view);
    }
    {
        std::string v = "__cpp_lib_ranges_chunk_by";
        auto fun = [](char x, char y) { return not(x == '_' or y == '_'); };
        auto view = v | std::views::chunk_by(fun);
        print_chunks(view, "");
    }
}

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]

References

  • C++23 standard (ISO/IEC 14882:2023):
    • 26.7.30 Chunk by view [range.chunk.by]

See also

(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)
(C++23)
 a view whose Mth element is a view over the Mth 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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