std::ranges:: views:: common, std::ranges:: common_view

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C++
Ranges library
Range adaptors
std::ranges::common_view
Defined in header <ranges>
template < ranges:: view V >

requires ( not ranges:: common_range < V > and
std:: copyable < ranges:: iterator_t < V >> )
class common_view

: public ranges:: view_interface < common_view < V >>
(1) (since C++20)
namespace views {

inline constexpr /* unspecified */ common = /* unspecified */ ;

}
(2) (since C++20)
Call signature
template < ranges:: viewable_range R >

requires /* see below */

constexpr ranges:: view auto common ( R && r ) ;
(since C++20)
1) Adapts a given view with different types for iterator/sentinel pair into a view that is also a common_range . A common_view always has the same iterator/sentinel type.
2) RangeAdaptorObject . Let e be a subexpression. Then the expression views :: common ( e ) is expression-equivalent to:

Data members

Member name Definition
base_ (private) The underlying view of type V .
( exposition-only member object* )

Member functions

constructs a common_view
(public member function)
returns a copy of the underlying (adapted) view
(public member function)
returns an iterator to the beginning
(public member function)
returns an iterator to the end
(public member function)
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range .
(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> )
gets the address of derived view's data. Provided if its iterator type satisfies contiguous_iterator .
(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> )

Deduction guides

Helper templates

template < class T >

constexpr bool enable_borrowed_range < std :: ranges :: common_view < T >> =

ranges:: enable_borrowed_range < T > ;
(since C++20)

This specialization of std :: ranges:: enable_borrowed_range makes common_view satisfy borrowed_range when the underlying view satisfies it.

Notes

common_view can be useful for working with legacy algorithms that expect the iterator and sentinel are of the same type.

Example

Run this code 
#include <iostream>
#include <iterator>
#include <list>
#include <numeric>
#include <ranges>
 
int main()
{
    auto v1 = {1, 2, 3, 4, 5};
    auto i1 = std::counted_iterator{v1.begin(), std::ssize(v1)};
    auto r1 = std::ranges::subrange{i1, std::default_sentinel};
//  auto e1 = std::accumulate(r1.begin(), r1.end(), 0); // error: "common range" required
    auto c1 = std::ranges::common_view{r1};
    std::cout << "accumulate: " << std::accumulate(c1.begin(), c1.end(), 0) << '\n';
 
    // inherited from ranges::view_interface:
    std::cout << "c1.front(): " << c1.front() << '\n';
    std::cout << "c1.back(): " << c1.back() << '\n';
    std::cout << "c1.data(): " << c1.data() << '\n';
    std::cout << "c1[0]: " << c1[0] << '\n';
 
    auto v2 = std::list{1, 2, 3, 4, 5};
    auto i2 = std::counted_iterator{v2.begin(), std::ssize(v2)};
    auto r2 = std::ranges::subrange{i2, std::default_sentinel};
//  auto e2 = std::accumulate(r2.begin(), r2.end(), 0); // error: "common range" required
    auto c2 = std::ranges::common_view{ r2 };
    std::cout << "accumulate: " << std::accumulate(c2.begin(), c2.end(), 0) << '\n';
 
    // inherited from ranges::view_interface:
    std::cout << "c2.front(): " << c2.front() << '\n';
//  auto e3 = c2.back(); // error: "bidirectional range" required
//  auto e4 = c2.data(); // error: "contiguous range" required
//  auto e5 = c2[0];     // error: "random access range" required
}

Possible output: 

accumulate: 15
c1.front(): 1
c1.back(): 5
c1.data(): 0x7f19937f00d0
c1[0]: 1
accumulate: 15
c2.front(): 1

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 3494 C++20 common_view was never a borrowed_range it is a borrowed_range if its underlying view is

See also

(C++20)
specifies that a range has identical iterator and sentinel types
(concept)
(C++20)
adapts an iterator type and its sentinel into a common iterator type
(class template)
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