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

From cppreference.com
C++
Ranges library
Range adaptors
std::ranges::repeat_view
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 .
3) Determines whether a type is integer-like and has a usable difference type .

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. 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

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

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 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> )
returns the n th 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) 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

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 ...

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

(C++20)
a view consisting of a sequence generated by repeatedly incrementing an initial value
(class template) (customization point object)
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