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

< cpp‎ | ranges

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

(constructor) (C++20)	constructs a `common_view` (public member function)
base (C++20)	returns a copy of the underlying (adapted) view (public member function)
begin (C++20)	returns an iterator to the beginning (public member function)
end (C++20)	returns an iterator to the end (public member function)
size (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
empty (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>`)
cbegin (C++23)	returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`)
cend (C++23)	returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`)
operator bool (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>`)
data (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>`)
front (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>`)
back (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>`)
operator[] (C++20)	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 > 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

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