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)