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std::ranges::views::repeat, std::ranges::repeat_view - cppreference.com

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Defined in header <ranges> 

template< std::move_constructible W,
          std::semiregular Bound = std::unreachable_sentinel_t >
    requires (std::is_object_v<W> && std::same_as<W, std::remove_cv_t<W>> &&
             (/*integer-like-with-usable-difference-type*/<Bound> ||
              std::same_as<Bound, std::unreachable_sentinel_t>))
class repeat_view : public ranges::view_interface<repeat_view<W, Bound>>
(1) (since C++23) 
namespace views {
    inline constexpr /* unspecified */ repeat = /* unspecified */;
}
(2) (since C++23)

Call signature

 
template< class W >
    requires /* see below */
constexpr /* see below */ repeat( W&& value );
(since C++23) 
template< class W, class Bound >
    requires /* see below */
constexpr /* see below */ repeat( W&& value, Bound&& bound );
(since C++23)

Helper concepts

 
concept /*integer-like-with-usable-difference-type*/ =
    /*is-signed-integer-like*/<T> ||
    (/*is-integer-like*/ <T> && std::weakly_incrementable<T>)
(3) (exposition only*)

1) A range factory that generates a sequence of elements by repeatedly producing the same value. Can be either bounded or unbounded (infinite).

2) views::repeat(e) and views::repeat(e, f) are expression-equivalent to repeat_view<std::decay_t<decltype((E))>>(e) and repeat_view(e, f) respectively for any suitable subexpressions e and f.

repeat_view models random_access_range. If Bound is not std::unreachable_sentinel_t, repeat_view also models sized_range and common_range.

Customization point objects

The name views::repeat denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

Data members

Member Definition
movable-box <W> value_ the repeating element of the view
(exposition-only member object*)
Bound bound_ the sentinel value
(exposition-only member object*)

Member functions

creates a repeat_view
(public member function)
obtains the beginning iterator of a repeat_view
(public member function)
obtains the sentinel denoting the end of a repeat_view
(public member function)
obtains the size of a repeat_view if it is sized
(public member function)
Inherited from 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>) [edit]

(C++23)

 returns a constant iterator to the beginning of the range
(public member function of std::ranges::view_interface<D>) [edit]

(C++23)

 returns a sentinel for the constant iterator of the range
(public member function of std::ranges::view_interface<D>) [edit]
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>) [edit]
returns the first element in the derived view, provided if it satisfies forward_range
(public member function of std::ranges::view_interface<D>) [edit]
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>) [edit]
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>) [edit]

std::ranges::repeat_view::repeat_view

repeat_view() requires std::default_initializable<W> = default;
(1) (since C++23) 
constexpr explicit repeat_view( const W& value, Bound bound = Bound() );
(2) (since C++23) 
constexpr explicit repeat_view( W&& value, Bound bound = Bound() );
(3) (since C++23) 
template < class... WArgs, class... BoundArgs >
    requires std::constructible_from<W, WArgs...>
          && std::constructible_from<Bound, BoundArgs...>
constexpr explicit
    repeat( std::piecewise_construct_t, std::tuple<WArgs...> value_args,
            std::tuple<BoundArgs...> bound_args = std::tuple<>{} );
(4) (since C++23)

1) Default-initializes value_ and value-initializes bound_ .

2) Initializes value_ with value and initializes bound_ with bound.

If Bound is not std::unreachable_sentinel_t and bool(bound >= 0) is false, the behavior is undefined.

3) Initializes value_ with std::move(value) and initializes bound_ with bound.

If Bound is not std::unreachable_sentinel_t and bool(bound >= 0) is false, the behavior is undefined.

4) Initializes value_ with std::make_from_tuple<T>(std::move(value_args)) and bound_ with std::make_from_tuple<Bound>(std::move(bound_args)).

If Bound is not std::unreachable_sentinel_t and bool(bound >= 0) is false, the behavior is undefined.

Parameters

value - the value to be repeatedly produced
bound - the bound
value_args - the tuple containing the initializers of value_
bound_args - the tuple containing the initializers of bound_

std::ranges::repeat_view::begin

constexpr /*iterator*/ begin() const;
(since C++23)

Returns iterator (std::addressof(*value_ )).

std::ranges::repeat_view::end

constexpr /*iterator*/ end() const
    requires (!std::same_as<Bound, std::unreachable_sentinel_t>);
(1) (since C++23) 
constexpr std::unreachable_sentinel_t end() const;
(2) (since C++23)

1) Returns iterator (std::addressof(*value_ ), bound_ ).

2) Returns std::unreachable_sentinel.

std::ranges::repeat_view::size

constexpr auto size() const
    requires (!std::same_as<Bound, std::unreachable_sentinel_t>);
(since C++23)

Returns to-unsigned-like (bound_ ).

Deduction guides

template< class W, class Bound = std::unreachable_sentinel_t >
repeat_view( W, Bound = Bound() ) -> repeat_view<W, Bound>;
(since C++23)

Nested classes

the iterator type
(exposition-only member class*)

Notes

Feature-test macro Value Std Feature
__cpp_lib_ranges_repeat 202207L (C++23) std::ranges::repeat_view

Example

#include <iostream>
#include <ranges>
#include <string_view>
using namespace std::literals;

int main()
{
    // bounded overload
    for (auto s : std::views::repeat("C++"sv, 3))
        std::cout << s << ' ';
    std::cout << '\n';
    
    // unbounded overload
    for (auto s : std::views::repeat("I know that you know that"sv)
                | std::views::take(3))
        std::cout << s << ' ';
    std::cout << "...\n";
}

Output: 

C++ C++ C++
I know that you know that I know that you know that I know that you know that ...

Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 4053 C++20 unary calls to views::repeat did not decay the argument decay the argument
LWG 4054 C++20 calling views::repeat with a repeat_view
did not create a nested repeat_view 		creates a nested
repeat_view

See also