Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | ranges
 
 
 
std::ranges::common_view
 
Defined in header <ranges>
template< ranges::view V >

    requires (!ranges::common_range<V> && 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)
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) Range adaptor object. 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] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] Data members

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

[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 forward_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>

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.

[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 <numeric>
#include <ranges>
#include <list>
 
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: "contiguous range" required
//  auto e4 = c2.data(); // error: "contiguous range" required
//  auto e5 = c2[0];     // error: "contiguous range" required
}

Possible output:

accumulate: 15
c1.front(): 1
c1.back(): 5
c1.data(): 0x400ec0
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]