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Difference between revisions of "cpp/ranges/values view"

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< cpp‎ | ranges
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m (fmt, headers sorted, +newline)
 
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{{cpp/ranges/view title | values}}
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{{cpp/ranges/view title|values}}
 
{{cpp/ranges/navbar}}
 
{{cpp/ranges/navbar}}
  
 
{{dcl begin}}
 
{{dcl begin}}
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{{dcl header | ranges}}
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{{dcl header|ranges}}
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{{dcl | num=1 | since=c++20 | 1=
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{{dcl|num=1|since=c++20|1=
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template<class R>
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template< class R >
−
using values_view = ranges::elements_view<views::all_t<R>, 1>;
+
using values_view = ranges::elements_view<R, 1>;
 
}}
 
}}
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{{dcl | num=2 | since=c++20 | 1=
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{{dcl|num=2|since=c++20|1=
 
namespace views {
 
namespace views {
 
     inline constexpr auto values = ranges::elements<1>;
 
     inline constexpr auto values = ranges::elements<1>;
Line 17: Line 17:
 
Takes a {{lconcept|view}} of ''tuple-like'' values (e.g. {{lc|std::tuple}} or {{lc|std::pair}}), and produces a view with a ''value-type'' of the ''second'' element of the adapted view's value-type.
 
Takes a {{lconcept|view}} of ''tuple-like'' values (e.g. {{lc|std::tuple}} or {{lc|std::pair}}), and produces a view with a ''value-type'' of the ''second'' element of the adapted view's value-type.
  
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@1@ is an alias for {{c|ranges::elements_view<views​::​all_t<R>, 1>}}.
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@1@ An alias for {{c|ranges::elements_view<R, 1>}}.
  
−
@2@ [[cpp/ranges#Range adaptor objects|Range adaptor object]]. The expression {{c|views​::values(e)}} is ''expression-equivalent'' to {{c|values_view<views​::​all_t<decltype((e))>>{e} }} for any suitable subexpression {{c|e}}.
+
@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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{{cpp/expr-eq}}
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===Notes===
−
 
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{{lc|values_view}} can be useful for extracting ''values'' from associative containers, e.g.
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=== Notes ===
+
{{source|1=
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{{lc|values_view}} can be useful for extracting ''values'' from associative containers, e.g. {{c|for (auto const& value : std::views::values(map)) { /*...*/ } }}.
+
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 ===
+
===Example===
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{{example|code=
+
{{example
 +
|code=
 
#include <iostream>
 
#include <iostream>
−
#include <ranges>
 
 
#include <map>
 
#include <map>
 +
#include <ranges>
  
 
int main()
 
int main()
 
{
 
{
−
     std::map<char, int> map{ {'A', 1}, {'B', 2}, {'C', 3}, {'D', 4}, {'E', 5} };
+
     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); };
 
     auto odd = [](int x) { return 0 != (x & 1); };
−
 
+
     std::cout << "\nodd values in the map: ";
−
     std::cout << "Odd values in the map: ";
+
 
     for (int value : map {{!}} std::views::values {{!}} std::views::filter(odd))
 
     for (int value : map {{!}} std::views::values {{!}} std::views::filter(odd))
 
         std::cout << value << ' ';
 
         std::cout << value << ' ';
 +
    std::cout << '\n';
 
}
 
}
 
|output=
 
|output=
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Odd values in the map: 1 3 5
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pair::second values in the list: 11.1 22.2 33.3
 +
odd values in the map: 1 3 5
 
}}
 
}}
 +
 +
===Defect reports===
 +
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=3563|std=C++20|before={{tt|keys_view}} is unable to participate in CTAD due to its use of {{lc|views::all_t}}|after={{lc|views::all_t}} removed}}
 +
{{dr list end}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/ranges/dsc keys_view}}
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{{dsc inc|cpp/ranges/dsc keys_view}}
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{{dsc inc | cpp/ranges/dsc elements_view}}
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{{dsc inc|cpp/ranges/dsc elements_view}}
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{{dsc inc | cpp/numeric/valarray/dsc slice}}
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{{dsc inc|cpp/numeric/valarray/dsc slice}}
 
{{dsc end}}
 
{{dsc end}}
  
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 12:48, 20 October 2023

 
C++
 
Ranges library
Range adaptors
 
Defined in header <ranges>
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

[edit] 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

[edit] Example

Run this code
#include <iostream>
#include <map>
#include <ranges>
 
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 << ' ';
    std::cout << '\n';
}

Output: 

pair::second values in the list: 11.1 22.2 33.3
odd values in the map: 1 3 5

[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 3563 C++20 keys_view is unable to participate in CTAD due to its use of views::all_t views::all_t removed

[edit] 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)[edit]
(C++20)
 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)[edit]
BLAS-like slice of a valarray: starting index, length, stride
(class) [edit]
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