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std::ranges:: ref_view

From cppreference.net
C++
Ranges library
Range adaptors
Defined in header <ranges>
template < ranges:: range R >

requires std:: is_object_v < R >
class ref_view

: public ranges:: view_interface < ref_view < R >>
(since C++20)

ref_view is a view of the elements of some other range . It wraps a reference to that range .

Contents

Data members

Member Description
R* r_ a pointer to the underlying range
( exposition-only member object* )

Member functions

constructs a ref_view that references to the given range
(public member function)
returns the references to the referenced range
(public member function)
returns the beginning iterator of the referenced range
(public member function)
returns the sentinel of the referenced range
(public member function)
checks whether the referenced range is empty
(public member function)
returns the size of the referenced sized_range
(public member function)
(C++26)
returns the approximate size of the referenced approximately_sized_range
(public member function)
returns the pointer to the beginning of the referenced contiguous_range
(public member function)
Inherited from std::ranges::view_interface
(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 only 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 only 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 only if it satisfies random_access_range
(public member function of std::ranges::view_interface<D> )

std::ranges::ref_view:: ref_view

template < /*different-from*/ < ref_view > T >

requires std:: convertible_to < T, R & > &&
requires { _FUN ( std:: declval < T > ( ) ) ; }

constexpr ref_view ( T && t ) ;
(since C++20)

Initializes r_ with std:: addressof ( static_cast < R & > ( std:: forward < T > ( t ) ) ) .

/*different-from*/ < T, U > is satisfied if and only if std:: remove_cvref_t < T > and std:: remove_cvref_t < U > are not the same type, and overloads of _FUN are declared as void _FUN ( R & ) ; void _FUN ( R && ) = delete ; .

Parameters

t - range to reference

std::ranges::ref_view:: base

constexpr R & base ( ) const ;
(since C++20)

Returns * r_ .

std::ranges::ref_view:: begin

constexpr ranges:: iterator_t < R > begin ( ) const ;
(since C++20)

Returns ranges:: begin ( * r_  ) .

std::ranges::ref_view:: end

constexpr ranges:: sentinel_t < R > end ( ) const ;
(since C++20)

Returns ranges:: end ( * r_  ) .

std::ranges::ref_view:: empty

constexpr bool empty ( ) const
requires requires { ranges:: empty ( * r_ ) ; } ;
(since C++20)

Returns ranges:: empty ( * r_  ) .

std::ranges::ref_view:: size

constexpr auto size ( ) const
requires ranges:: sized_range < R > ;
(since C++20)

Returns ranges:: size ( * r_  ) .

std::ranges::ref_view:: reserve_hint

constexpr auto size ( ) const
requires ranges :: approximately_sized_range < R > ;
(since C++26)

Returns ranges :: reserve_hint ( * r_  ) .

std::ranges::ref_view:: data

constexpr auto data ( ) const
requires ranges:: contiguous_range < R > ;
(since C++20)

Returns ranges:: data ( * r_  ) .

Deduction guides

template < class R >
ref_view ( R & ) - > ref_view < R > ;
(since C++20)

Helper templates

template < class T >
constexpr bool enable_borrowed_range < ranges :: ref_view < T >> = true ;
(since C++20)

This specialization of std::ranges::enable_borrowed_range makes ref_view satisfy borrowed_range .

Notes

Feature-test macro Value Std Feature
__cpp_lib_ranges_reserve_hint 202502L (C++26) ranges::approximately_sized_range and reserve_hint

Example

Run this code 
#include <iostream>
#include <ranges>
int main()
{
    const std::string s{"cosmos"};
    const std::ranges::take_view tv{s, 3};
    const std::ranges::ref_view rv{tv};
    std::cout
        << std::boolalpha
        << "call empty(): " << rv.empty() << '\n'
        << "call size() : " << rv.size() << '\n'
        << "call begin(): " << *rv.begin() << '\n'
        << "call end()  : " << *(rv.end() - 1) << '\n'
        << "call data() : " << rv.data() << '\n'
        << "call base() : " << rv.base().size() << '\n' // ~> tv.size()
        << "range-for   : ";
    for (const auto c : rv)
        std::cout << c;
    std::cout << '\n';
}

Output: 

call empty(): false
call size() : 3
call begin(): c
call end()  : s
call data() : cosmos
call base() : 3
range-for   : cos

Defect reports

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

DR Applied to Behavior as published Correct behavior
P2325R3 C++20 default constructor was provided as
view must be default_initializable 		removed along with the requirement

See also

(C++11)
CopyConstructible and CopyAssignable reference wrapper
(class template)
(C++20)
a view with unique ownership of some range
(class template)
(C++20)
a view that includes all elements of a range
(alias template) (range adaptor object)