Ranges library (since C++20)

The ranges library is an extension and generalization of the algorithms and iterator libraries that makes them more powerful by making them composable and less error-prone.

The library creates and manipulates range views, lightweight objects that indirectly represent iterable sequences (ranges). Ranges are an abstraction on top of

• [begin, end) – iterator pairs, e.g. ranges made by implicit conversion from containers. All algorithms that take iterator pairs now have overloads that accept ranges (e.g. ranges::sort)
• begin + [0, size) – counted sequences, e.g. range returned by views::counted
• [begin, predicate) – conditionally-terminated sequences, e.g. range returned by views::take_while
• [begin, ..) – unbounded sequences, e.g. range returned by views::iota

The ranges library includes range algorithms, which are applied to ranges eagerly, and range adaptors, which are applied to views lazily. Adaptors can be composed into pipelines, so that their actions take place as the view is iterated.

namespace std {
    namespace views = ranges::views;
}

The namespace alias std::views is provided as a shorthand for std::ranges::views.

Defined in namespace std::ranges

Range access

Defined in header <ranges>

Defined in header <iterator>

ranges::begin :badge[C++20] returns an iterator to the beginning of a range (customization point object)

ranges::end :badge[C++20] returns a sentinel indicating the end of a range (customization point object)

ranges::cbegin :badge[C++20] returns an iterator to the beginning of a read-only range (customization point object)

ranges::cend :badge[C++20] returns a sentinel indicating the end of a read-only range (customization point object)

ranges::rbegin :badge[C++20] returns a reverse iterator to a range (customization point object)

ranges::rend :badge[C++20] returns a reverse end iterator to a range (customization point object)

ranges::crbegin :badge[C++20] returns a reverse iterator to a read-only range (customization point object)

ranges::crend :badge[C++20] returns a reverse end iterator to a read-only range (customization point object)

ranges::reserve_hint :badge[C++26] returns an integer equal to the reserve hint given by a range (customization point object)

ranges::size :badge[C++20] returns an integer equal to the size of a range (customization point object)

ranges::ssize :badge[C++20] returns a signed integer equal to the size of a range (customization point object)

ranges::empty :badge[C++20] checks whether a range is empty (customization point object)

ranges::data :badge[C++20] obtains a pointer to the beginning of a contiguous range (customization point object)

ranges::cdata :badge[C++20] obtains a pointer to the beginning of a read-only contiguous range (customization point object)

Range primitives

Defined in header <ranges>

ranges::iterator_t ranges::const_iterator_t ranges::sentinel_t ranges::const_sentinel_t obtains iterator and sentinel types of a range (alias template)

ranges::range_difference_t ranges::range_size_t ranges::range_value_t obtains size, difference, and value types of a range (alias template)

ranges::range_reference_t ranges::range_const_reference_t ranges::range_rvalue_reference_t ranges::range_common_reference_t obtains reference types of a range (alias template)

Dangling iterator handling

Defined in header <ranges>

ranges::dangling a placeholder type indicating that an iterator or a subrange should not be returned since it would be dangling (class)

ranges::borrowed_iterator_t ranges::borrowed_subrange_t obtains iterator type or subrange type of a borrowed_range (alias template)

Other utilities

Defined in header <ranges>

ranges::elements_of tags a range to be treated as a sequence rather than a single value (class template)

Range concepts

Defined in header <ranges>

ranges::range specifies that a type is a range, that is, it provides a begin iterator and an end sentinel (concept)

ranges::borrowed_range specifies that a type is a range and iterators obtained from an expression of it can be safely returned without danger of dangling (concept)

ranges::approximately_sized_range specifies that a range can estimate its size in constant time (concept)

ranges::sized_range specifies that a range knows its size in constant time (concept)

ranges::view specifies that a range is a view, that is, it has constant time copy/move/assignment (concept)

ranges::input_range specifies a range whose iterator type satisfies input_iterator (concept)

ranges::output_range specifies a range whose iterator type satisfies output_iterator (concept)

ranges::forward_range specifies a range whose iterator type satisfies forward_iterator (concept)

ranges::bidirectional_range specifies a range whose iterator type satisfies bidirectional_iterator (concept)

ranges::random_access_range specifies a range whose iterator type satisfies random_access_iterator (concept)

ranges::contiguous_range specifies a range whose iterator type satisfies contiguous_iterator (concept)

ranges::common_range specifies that a range has identical iterator and sentinel types (concept)

ranges::viewable_range specifies the requirements for a range to be safely convertible to a view (concept)

ranges::constant_range specifies that a range has read-only elements (concept)

Range conversions

Defined in header <ranges>

ranges::to constructs a new non-view object from an input range (function template)

Views

Defined in header <ranges>

ranges::view_interface helper class template for defining a view, using the curiously recurring template pattern (class template)

ranges::subrange combines an iterator-sentinel pair into a view (class template)

Range factories

Defined in header <ranges>

Defined in namespace std::ranges

ranges::empty_view views::empty an empty view with no elements (class template) (variable template)

ranges::single_view views::single a view that contains a single element of a specified value (class template) (customization point object)

ranges::iota_view views::iota a view consisting of a sequence generated by repeatedly incrementing an initial value (class template) (customization point object)

ranges::repeat_view views::repeat a view consisting of a generated sequence by repeatedly producing the same value (class template) (customization point object)

ranges::basic_istream_view views::istream a view consisting of the elements obtained by successive application of operator>> on the associated input stream (class template) (customization point object)

Range adaptors

Defined in header <ranges>

Defined in namespace std::ranges

ranges::range_adaptor_closure helper base class template for defining a range adaptor closure object (class template)

views::all_t views::all a view that includes all elements of a range (alias template) (range adaptor object)

ranges::ref_view a view of the elements of some other range (class template)

ranges::owning_view a view with unique ownership of some range (class template)

ranges::as_rvalue_view views::as_rvalue a view of a sequence that casts each element to an rvalue (class template) (range adaptor object)

ranges::filter_view views::filter a view that consists of the elements of a range that satisfies a predicate (class template) (range adaptor object)

ranges::transform_view views::transform a view of a sequence that applies a transformation function to each element (class template) (range adaptor object)

ranges::take_view views::take a view consisting of the first N elements of another view (class template) (range adaptor object)

ranges::take_while_view views::take_while a view consisting of the initial elements of another view, until the first element on which a predicate returns false (class template) (range adaptor object)

ranges::drop_view views::drop a view consisting of elements of another view, skipping the first N elements (class template) (range adaptor object)

ranges::drop_while_view views::drop_while a view consisting of the elements of another view, skipping the initial subsequence of elements until the first element where the predicate returns false (class template) (range adaptor object)

ranges::join_view views::join a view consisting of the sequence obtained from flattening a view of ranges (class template) (range adaptor object)

ranges::join_with_view views::join_with 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::lazy_split_view views::lazy_split a view over the subranges obtained from splitting another view using a delimiter (class template) (range adaptor object)

ranges::split_view views::split a view over the subranges obtained from splitting another view using a delimiter (class template) (range adaptor object)

ranges::concat_view views::concat a view consisting of concatenation of the adapted views (class template) (customization point object)

views::counted creates a subrange from an iterator and a count (customization point object)

ranges::common_view views::common converts a view into a common_range (class template) (range adaptor object)

ranges::reverse_view views::reverse a view that iterates over the elements of another bidirectional view in reverse order (class template) (range adaptor object)

ranges::as_const_view views::as_const converts a view into a constant_range (class template) (range adaptor object)

ranges::elements_view views::elements takes a view consisting of tuple-like values and a number N and produces a view of Nth element of each tuple (class template) (range adaptor object)

ranges::keys_view views::keys takes a view consisting of pair-like values and produces a view of the first elements of each pair (class template) (range adaptor object)

ranges::values_view views::values takes a view consisting of pair-like values and produces a view of the second elements of each pair (class template) (range adaptor object)

ranges::enumerate_view views::enumerate a view that maps each element of adapted sequence to a tuple of both the element’s position and its value (class template) (range adaptor object)

ranges::zip_view views::zip a view consisting of tuples of references to corresponding elements of the adapted views (class template) (customization point object)

ranges::zip_transform_view views::zip_transform a view consisting of results of application of a transformation function to corresponding elements of the adapted views (class template) (customization point object)

ranges::adjacent_view views::adjacent a view consisting of tuples of references to adjacent elements of the adapted view (class template) (range adaptor object)

ranges::adjacent_transform_view views::adjacent_transform a view consisting of results of application of a transformation function to adjacent elements of the adapted view (class template) (range adaptor object)

ranges::chunk_view views::chunk a range of views that are N-sized non-overlapping successive chunks of the elements of another view (class template) (range adaptor object)

ranges::slide_view views::slide 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)

ranges::chunk_by_view views::chunk_by splits the view into subranges between each pair of adjacent elements for which the given predicate returns false (class template) (range adaptor object)

ranges::stride_view views::stride a view consisting of elements of another view, advancing over N elements at a time (class template) (range adaptor object)

ranges::cartesian_product_view views::cartesian_product a view consisting of tuples of results calculated by the n-ary cartesian product of the adapted views (class template) (customization point object)

ranges::cache_latest_view views::cache_latest a view that caches the last-accessed element of its underlying sequence (class template) (range adaptor object)

ranges::to_input_view views::to_input converts a view into a range that is input_range-only and non-common_range (class template) (range adaptor object)

Range generators C++23

Defined in header <generator>

Defined in namespace std

generator A view that represents synchronous coroutine generator (class template)

Helper items

Range adaptor objects

See RangeAdaptorObject (RAO).

Range adaptor closure objects

See RangeAdaptorClosureObject (RACO).

Customization point objects

See Customization point object (CPO).

Assignable wrapper

Some range adaptors wrap their elements or function objects with the *copyable-box* (until C++23) *movable-box* (since C++23) . The wrapper augments the wrapped object with assignability when needed.

Non-propagating cache

Some range adaptors are specified in terms of an exposition-only class template *non-propagating-cache*, which behaves almost like std::optional<T> (see description for differences).

Conditionally-const type

template< bool Const, class T >
using /*maybe-const*/ = std::conditional_t<Const, const T, T>;

The alias template /*maybe-const*/ is a shorthand used to conditionally apply a const qualifier to the type T.

Integer-like type helper templates

template< /*is-integer-like*/ T >
using /*make-signed-like-t*/<T> = /* see description */;

(1)

template< /*is-integer-like*/ T >
using /*make-unsigned-like-t*/<T> = /* see description */;

(2)

template< /*is-integer-like*/ T >
/*make-unsigned-like-t*/<T> /*to-unsigned-like*/( T t )
{
    return static_cast</*make-unsigned-like-t*/<T>>(t);
}

(3)

1. For an integer-like type T:

   • If T is an integer type, /*make-signed-like-t*/<T> is std::make_signed_t<T>.
   • Otherwise, /*make-signed-like-t*/<T> is a corresponding unspecified signed-integer-like type of the same width as T.

2. For an integer-like type T:

   • If T is an integer type, /*make-unsigned-like-t*/<T> is std::make_unsigned_t<T>.
   • Otherwise, /*make-signed-like-t*/<T> is a corresponding unspecified unsigned-integer-like type of the same width as T.

3. Explicitly converts t to /*make-unsigned-like-t*/<T>.

Customization point object helpers

template< ranges::input_range R >
constexpr auto& /*possibly-const-range*/(R& r) noexcept
{
    if constexpr (ranges::input_range<const R>)
        return const_cast<const R&>(r);
    else
        return r;
}

(1)

template< class T >
constexpr auto /*as-const-pointer*/( const T* p ) noexcept
{
    return p;
}

(2)

Some range access customization point objects are specified in terms of these exposition-only function templates.

1. /*possibly-const-range*/ returns the const-qualified version of r if const R models input_range; otherwise, returns r without any casting.
2. /*as-const-pointer*/ returns a pointer to object of constant type.

Range adaptor helpers

template< class F, class Tuple >
constexpr auto /*tuple-transform*/( F&& f, Tuple&& tuple )
{
    return std::apply([]<class... Ts>(Ts&&... args)
    {
        return std::tuple<std::invoke_result_t<F&, Ts>...>
            (std::invoke(f, std::forward<Ts>(args))...);
    }, std::forward<Tuple>(tuple));
}

(1)

template< class F, class Tuple >
constexpr void /*tuple-for-each*/( F&& f, Tuple&& tuple )
{
    std::apply([]<class... Ts>(Ts&&... args)
    {
        (static_cast<void>(std::invoke(f, std::forward<Ts>(args))), ...);
    }, std::forward<Tuple>(tuple));
}

(2)

template< class T >
constexpr T& /*as-lvalue*/( T&& t )
{
    return static_cast<T&>(t);
}

(3)

Some range adaptors are specified in terms of these exposition-only function templates.

1. /*tuple-transform*/ returns a new tuple constructed by applying f to each element of tuple.
2. /*tuple-for-each*/ applies f to each element of tuple and returns nothing.
3. /*as-lvalue*/ forwards rvalue t as lvalue.

Helper concepts

Following exposition-only concepts are used for several types, but they are not parts of the interface of standard library.

template< class R >
concept /*simple-view*/ =
    ranges::view<R> && ranges::range<const R> &&
    std::same_as<ranges::iterator_t<R>, ranges::iterator_t<const R>> &&
    std::same_as<ranges::sentinel_t<R>, ranges::sentinel_t<const R>>;

(1)

template< class I >
concept /*has-arrow*/ =
    ranges::input_iterator<I> &&
    (std::is_pointer_v<I> || requires(const I i) { i.operator->(); });

(2)

template< class T, class U >
concept /*different-from*/ =
    !std::same_as<std::remove_cvref_t<T>, std::remove_cvref_t<U>>;

(3)

template< class R >
concept /*range-with-movable-references*/ =
    ranges::input_range<R> &&
    std::move_constructible<ranges::range_reference_t<R>> &&
    std::move_constructible<ranges::range_rvalue_reference_t<R>>;

(4)

template< bool C, class... Views >
concept /*all-random-access*/ =
    (ranges::random_access_range
    <std::conditional_t<C, const Views, Views>> && ...);

(5)

template< bool C, class... Views >
concept /*all-bidirectional*/ =
    (ranges::bidirectional_range
    <std::conditional_t<C, const Views, Views>> && ...);

(6)

template< bool C, class... Views >
concept /*all-forward*/ =
    (ranges::forward_range
    <std::conditional_t<C, const Views, Views>> && ...);

(7)

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_generator	202207L	(since C++23)	std::generator – synchronous coroutine generator for ranges
__cpp_lib_ranges	201911L	(since C++20)	Ranges library and constrained algorithms
	202106L	(since C++23) (DR20)	Non-default-initializable views
	202110L	(since C++23) (DR20)	Views with ownership
	202202L	(since C++23)	ranges::range_adaptor_closure
	202207L	(since C++23)	Relaxing range adaptors to allow for move-only types
	202211L	(since C++23)	Removing “poison pills” P2602 overloads in ranges::begin etc
	202302L	(since C++23)	Relaxing ranges to allow certain projections
	202406L	(since C++26) (DR20)	Removing the common reference requirement from the indirectly invocable concepts
__cpp_lib_ranges_as_const	202207L	(since C++23)	std::const_iterator, ranges::as_const_view
__cpp_lib_ranges_as_rvalue	202207L	(since C++23)	ranges::as_rvalue_view
__cpp_lib_ranges_cache_latest	202411L	(since C++26)	ranges::cache_latest_view
__cpp_lib_ranges_cartesian_product	202207L	(since C++23)	ranges::cartesian_product_view
__cpp_lib_ranges_chunk	202202L	(since C++23)	ranges::chunk_view
__cpp_lib_ranges_chunk_by	202202L	(since C++23)	ranges::chunk_by_view
__cpp_lib_ranges_concat	202403L	(since C++26)	ranges::concat_view
__cpp_lib_ranges_enumerate	202302L	(since C++23)	ranges::enumerate_view
__cpp_lib_ranges_join_with	202202L	(since C++23)	ranges::join_with_view
__cpp_lib_ranges_repeat	202207L	(since C++23)	ranges::repeat_view
__cpp_lib_ranges_reserve_hint	202502L	(since C++26)	ranges::reserve_hint and ranges::approximately_sized_range
__cpp_lib_ranges_slide	202202L	(since C++23)	ranges::slide_view
__cpp_lib_ranges_stride	202207L	(since C++23)	ranges::stride_view
__cpp_lib_ranges_to_container	202202L	(since C++23)	ranges::to
__cpp_lib_ranges_to_input	202502L	(since C++26)	ranges::to_input_view
__cpp_lib_ranges_zip	202110L	(since C++23)	ranges::zip_view, ranges::zip_transform_view, ranges::adjacent_view, ranges::adjacent_transform_view

Example

#include <iostream>
#include <ranges>

int main()
{
    auto const ints = {0, 1, 2, 3, 4, 5};
    auto even = [](int i) { return 0 == i % 2; };
    auto square = [](int i) { return i * i; };

    // the "pipe" syntax of composing the views:
    for (int i : ints | std::views::filter(even) | std::views::transform(square))
        std::cout << i << ' ';

    std::cout << '\n';

    // a traditional "functional" composing syntax:
    for (int i : std::views::transform(std::views::filter(ints, even), square))
        std::cout << i << ' ';
}

Output:

0 4 16
0 4 16

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

LWG 3509 (C++20)

Link	https://cplusplus.github.io/LWG/issues/3509.html
Applied to	C++20
Behavior as published	was unclear how range adaptor objects bound trailing arguments
Correct behavior	they are bound by value

LWG 3948 (C++23)

Link	https://cplusplus.github.io/LWG/issues/3948.html
Applied to	C++23
Behavior as published	*possibly-const-range* and *as-const-pointer* were not declared noexcept
Correct behavior	declared noexcept

LWG 4027 (C++23)

Link	https://cplusplus.github.io/LWG/issues/4027.html
Applied to	C++23
Behavior as published	*possibly-const-range* would not add const-qualification for ranges that has already modeled constant_range
Correct behavior	adds const-qualification for such ranges

LWG 4112 (C++20)

Link	https://cplusplus.github.io/LWG/issues/4112.html
Applied to	C++20
Behavior as published	*has-arrow* did not require i to be const-qualified
Correct behavior	requires

See also

• Iterator library
• Constrained algorithms