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

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< cpp‎ | ranges
 
 
Ranges library
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(C++23)

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Helper items
(until C++23)(C++23)


 
 
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.

Contents

[edit] Data members

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

[edit] Member functions

constructs a common_view
(public member function) [edit]
(C++20)
returns a copy of the underlying (adapted) view
(public member function) [edit]
(C++20)
returns an iterator to the beginning
(public member function) [edit]
(C++20)
returns an iterator to the end
(public member function) [edit]
(C++20)
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range.
(public member function) [edit]
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>) [edit]
(C++23)
returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>) [edit]
(C++23)
returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>) [edit]
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>) [edit]
(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>) [edit]
(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>) [edit]
(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>) [edit]
returns the nth element in the derived view. Provided if it satisfies random_access_range.
(public member function of std::ranges::view_interface<D>) [edit]

[edit] Deduction guides

[edit] Helper templates

template< class T >

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.

[edit] Notes

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

[edit] 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

[edit] 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

[edit] See also

specifies that a range has identical iterator and sentinel types
(concept) [edit]
adapts an iterator type and its sentinel into a common iterator type
(class template) [edit]