Difference between revisions of "cpp/ranges/values view"
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@1@ An alias for {{c|ranges::elements_view<R, 1>}}. | @1@ An alias for {{c|ranges::elements_view<R, 1>}}. | ||
| − | @2@ {{named req|RangeAdaptorObject}} (and also {{named req|RangeAdaptorClosureObject}}). The expression {{c|views::values(e)}} is | + | @2@ {{named req|RangeAdaptorObject}} (and also {{named req|RangeAdaptorClosureObject}}). The expression {{c|views::values(e)}} is [[cpp/language/expressions#Expression-equivalence|expression-equivalent]] to {{c|values_view<views::all_t<decltype((e))>>{e} }} for any suitable subexpression {{c|e}}. |
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===Notes=== | ===Notes=== | ||
Revision as of 14:38, 27 April 2023
| Defined in header <ranges>
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| template< class R > using values_view = ranges::elements_view<R, 1>; |
(1) | (since C++20) |
| namespace views { inline constexpr auto values = ranges::elements<1>; |
(2) | (since C++20) |
Takes a view of tuple-like values (e.g. std::tuple or std::pair), and produces a view with a value-type of the second element of the adapted view's value-type.
1) An alias for ranges::elements_view<R, 1>.
2) RangeAdaptorObject (and also RangeAdaptorClosureObject). The expression views::values(e) is expression-equivalent to values_view<views::all_t<decltype((e))>>{e} for any suitable subexpression e.
Contents |
Notes
values_view can be useful for extracting values from associative containers, e.g.
std::map<int, std::string> map{{1, "alpha"}, {2, "beta"}}; for (auto const& value : std::views::values(map)) std::cout << value << ' '; // prints: alpha beta
Example
Run this code
#include <iostream> #include <ranges> #include <map> int main() { const auto list = { std::pair{1, 11.1}, {2, 22.2}, {3, 33.3} }; std::cout << "pair::second values in the list: "; for (double value : list | std::views::values) std::cout << value << ' '; std::map<char, int> map{ {'A', 1}, {'B', 2}, {'C', 3}, {'D', 4}, {'E', 5} }; auto odd = [](int x) { return 0 != (x & 1); }; std::cout << "\nodd values in the map: "; for (int value : map | std::views::values | std::views::filter(odd)) std::cout << value << ' '; }
Output:
pair::second values in the list: 11.1 22.2 33.3 odd values in the map: 1 3 5
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 3563 | C++20 | keys_view is unable to participate in CTAD due to its use of views::all_t
|
views::all_t removed |
See also
| (C++20) |
takes a view consisting of pair-like values and produces a view of the first elements of each pair(class template) (range adaptor object) |
takes a view consisting of tuple-like values and a number N and produces a view of Nth element of each tuple(class template) (range adaptor object) | |
| BLAS-like slice of a valarray: starting index, length, stride (class) |
