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

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_value_t	range_reference_trange_rvalue_reference_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Semiregular wrapper
semiregular_box
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	basic_istream_viewistream_view
Adaptors
views::all_tviews::all common_viewviews::common join_viewviews::join split_viewviews::split	take_viewviews::take drop_viewviews::drop take_while_viewviews::take_while drop_while_viewviews::drop_while	transform_viewviews::transform reverse_viewviews::reverse filter_viewviews::filter ref_view	views::counted elements_viewviews::elements keys_viewviews::keys values_viewviews::values

 

std::ranges::common_view

Member functions

common_view::common_view

common_view::base

common_view::begin

common_view::end

common_view::size

Deduction guides

 

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) Converts a given view with different types for iterator/sentinel pair into a view with the same type for iterator/sentinel.

2) 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.

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.

Data members

std::ranges::common_view::base_

V base_ = V(); /* exposition-only */

the underlying view.

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)

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 common_range iterators (i.e. the types of iterators that represent the begin and the end of a sequence to be the same).

Example

Run this code

#include <iostream>
#include <numeric>
#include <ranges>
#include <string>
 
void print_sum(auto&& v) {
    auto cv { v | std::views::common };  // turns into a "common-range"
 
    using T = decltype(*cv.begin());  // needed to get initial value of the sum
 
    for (bool first{true}; auto const& e : cv)  // print range
        std::cout << (first ? first = false, "" : " + ") << e;
 
    std::cout << " = " << std::accumulate(cv.begin(), cv.end(), T{}) << '\n';
}
 
int main() {
    const auto v1 = {1, 2, 3, 4, 5};
    print_sum(v1);
 
    auto take3 = v1 | std::views::reverse | std::views::take(3);
    print_sum(take3); // 5 + 4 + 3
 
    using namespace std::literals;
    const auto v2 = { "[alpha]"s, "[beta]"s, "[gamma]"s };
    print_sum(v2);
}

Output:

1 + 2 + 3 + 4 + 5 = 15
5 + 4 + 3 = 12
[alpha] + [beta] + [gamma] = [alpha][beta][gamma]

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

See also

views::all_tviews::all (C++20)	a view that includes all elements of a range (alias template) (range adaptor object)
ranges::common_range (C++20)	specifies that a range has identical iterator and sentinel types (concept)