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std::ranges::views::common, 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:
  • views::all(e), if it is a well-formed expression and decltype((e)) models common_range;
  • common_view{e} otherwise.

Data members

Typical implementations of common_view hold only one non-static data member — the underlying view of type V.

Member functions

(C++20)
 constructs a common_view
(public member function)
(C++20)
 returns a copy of the underlying (adapted) view
(public member function)
(C++20)
 returns an iterator to the beginning
(public member function)
(C++20)
 returns an iterator to the end
(public member function)
(C++20)
 returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range.
(public member function)
Inherited from std::ranges::view_interface
(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>)
(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>)
(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>)
(C++20)
 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>)
(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>)
(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>)
(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>)

Deduction guides

Helper templates

template< class T >
inline constexpr bool enable_borrowed_range<std::ranges::common_view<T>> =
    std::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

#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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