std::ranges::views::common,std::ranges::common_view(3)

NAME
std::ranges::views::common,std::ranges::common_view - std::ranges::views::common,std::ranges::common_view

Synopsis
   Defined in header <ranges>
   template< ranges::view V >
       requires (not ranges::common_range<V> and
                 std::copyable<ranges::iterator_t<V>>) (1) (since C++20)
   class common_view
       : public ranges::view_interface<common_view<V>>
   namespace views {
       inline constexpr /* unspecified */ common = /* unspecified */; (2) (since C++20)
   }
   Call signature
   template< ranges::viewable_range R >
       requires /* see below */ (since C++20)
   constexpr ranges::view auto common( R&& r );
   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.

Member functions
   constructor constructs a common_view
   (C++20) (public member function)
   base returns a copy of the underlying (adapted) view
   (C++20) (public member function)
   begin returns an iterator to the beginning
   (C++20) (public member function)
   end returns an iterator to the end
   (C++20) (public member function)
   size returns the number of elements. Provided only if the underlying
   (C++20) (adapted) range satisfies sized_range.
                 (public member function)
         Inherited from std::ranges::view_interface
   empty returns whether the derived view is empty. Provided if it satisfies
   (C++20) sized_range or forward_range.
                 (public member function of std::ranges::view_interface<D>)
   cbegin returns a constant iterator to the beginning of the range.
   (C++23) (public member function of std::ranges::view_interface<D>)
   cend returns a sentinel for the constant iterator of the range.
   (C++23) (public member function of std::ranges::view_interface<D>)
   operator bool returns whether the derived view is not empty. Provided if
   (C++20) ranges::empty is applicable to it.
                 (public member function of std::ranges::view_interface<D>)
   data gets the address of derived view's data. Provided if its iterator type
   (C++20) satisfies contiguous_iterator.
                 (public member function of std::ranges::view_interface<D>)
   front returns the first element in the derived view. Provided if it
   (C++20) satisfies forward_range.
                 (public member function of std::ranges::view_interface<D>)
   back returns the last element in the derived view. Provided if it satisfies
   (C++20) bidirectional_range and common_range.
                 (public member function of std::ranges::view_interface<D>)
   operator[] returns the n^th element in the derived view. Provided if it satisfies
   (C++20) random_access_range.
                 (public member function of std::ranges::view_interface<D>)
   Deduction guides
   Helper templates
   template< class T >
   inline constexpr bool (since C++20)
   enable_borrowed_range<std::ranges::common_view<T>> =
       std::ranges::enable_borrowed_range<T>;
   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 it is a borrowed_range if its underlying
                       borrowed_range view is

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