# std::ranges::views::repeat, std::ranges::repeat_view

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< cpp‎ | ranges

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>> && (/*is-integer-like*/<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)

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 (has the same effect as) repeat_view(e) and repeat_view(e, f) respectively for any suitable subexpressions e and f.

### Customization point objects

The name `views::repeat` denotes a customization point object, which is a const function object of a literal `semiregular` class type. For exposition purposes, the cv-unqualified version of its type is denoted as `__repeat_fn`.

All instances of `__repeat_fn` are equal. The effects of invoking different instances of type `__repeat_fn` on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, `views::repeat` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval<Args>()... meet the requirements for arguments to `views::repeat` above, `__repeat_fn` models

- std::invocable<__repeat_fn, Args...>,
- std::invocable<const __repeat_fn, Args...>,
- std::invocable<__repeat_fn&, Args...>, and
- std::invocable<const __repeat_fn&, Args...>.

Otherwise, no function call operator of `__repeat_fn` participates in overload resolution.

### Data members

Typical implementation of `repeat_view` holds two non-static data members:

- `value_` (exposition only*) - The beginning value of wrapped type /*movable-box*/<W>, and
- `bound_` (exposition only*) - The sentinel value of type `Bound`.

### Member functions

(constructor) (C++23) | creates a `repeat_view` (public member function)
begin (C++23) | obtains the beginning iterator of a `repeat_view` (public member function)
end (C++23) | obtains the sentinel denoting the end of a `repeat_view` (public member function)
size (C++23) | obtains the size of a `repeat_view` if it is sized (public member function)
##### Inherited from std::ranges::view_interface
empty (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>`)
cbegin (C++23) | returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`)
cend (C++23) | returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`)
operator bool (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>`)
front (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>`)
back (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>`)
operator[] (C++20) | returns the nth element in the derived view. Provided if it satisfies `random_access_range`. (public member function of `std::ranges::view_interface<D>`)

## 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) Value-initializes `value_` and `bound_` via their default member initializers (= W() and = Bound()).
2) Initializes `value_` with value and initializes `bound_` with bound. The behavior is undefined if `Bound` is not std::unreachable_sentinel_t and bool(bound >= 0) is false.
3) Initializes `value_` with std::move(value) and initializes `bound_` with bound. The behavior is undefined if `Bound` is not std::unreachable_sentinel_t and bool(bound >= 0) is false.
4) Initializes `value_` and `bound_` through piecewise construction.

### Parameters

value | - | the value to be repeatedly produced
bound | - | the bound

## std::ranges::repeat_view::begin

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

Returns an iterator initialized with 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 an iterator initialized with std::addressof(*value_) and `bound_`.
2) Returns an 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 the size of the view if the view is bounded. Equivalent to return /*to-unsigned-like*/(bound_);.

The exposition-only function template `to-unsigned-like` converts its argument (which must be integer-like) to the corresponding unsigned version of the argument type.

### Deduction guides

template< class W, class Bound > repeat_view( W, Bound ) -> repeat_view<W, Bound>; |  | (since C++23)

### Nested classes

iterator (C++23) | the iterator type (exposition-only member class*)

### Notes

If Bound is not std::unreachable_sentinel_t, the `repeat_view` models `sized_range` and `common_range`.

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

### Example

Run this code

    #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 ...

### See also

ranges::iota_view views::iota (C++20) | a `view` consisting of a sequence generated by repeatedly incrementing an initial value (class template) (customization point object)

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