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values_view class (C++ Standard Library)

  • Artikkeli

A view consisting of the second index into each tuple-like value in a collection. For example, given a range of std::tuple<string, int> values, create a view consisting of all the int elements from each tuple.

values_view is an alias for elements_view<R, 1> and is useful for creating a view of the values in associative containers such as std::unordered_map.

Syntax

template<input_range R>
using values_view = ranges::elements_view<R, 1>;

Template parameters

R
The type of the underlying range. This type must satisfy ranges::input_range.

View characteristics

For a description of the following entries, see View class characteristics

Characteristic Description
Range adaptor views::values
Underlying range Must satisfy forward_range or higher
Element type Same as the type of the second tuple element
View iterator category forward_range, bidirectional_range, or random_access_range
Sized Only if the underlying range satisfies sized_range
Is const-iterable Only if the underlying range satisfies const-iterable
Common range Only if the underlying range satisfies common_range
Borrowed range Only if the underlying range satisfies borrowed_range

Members

Member functions Description
ConstructorsC++20 Construct a values_view.
baseC++20 Get the underlying range.
beginC++20 Get an iterator to the first element.
endC++20 Get the sentinel at the end of the values_view.
sizeC++20 Get the number of elements.
Inherited from view_interface Description
backC++20 Get the last element.
emptyC++20 Test whether the values_view is empty.
frontC++20 Get the first element.
operator[]C++20 Get the element at the specified position.
operator boolC++20 Test whether the values_view isn't empty.

Requirements

Header: <ranges> (since C++20)

Namespace: std::ranges

Compiler Option: /std:c++20 or later is required.

Remarks

Tuple-like types that you can use with values_view are std::tuple, std::pair, and std::array.

Constructors

Construct an instance of a values_view.

1) constexpr values_view(R base);
2) values_view() requires default_initializable<V> = default;

Parameters

base
The underlying range of tuple-like types.

For information about the template parameter type, see Template parameters.

Return value

An values_view instance.

Remarks

The best way to create an values_view is by using the values range adaptor. Range adaptors are the intended way to create view classes. The view types are exposed in case you want to create your own custom view type.

  1. Create an values_view from the specified view.
  2. The default constructor creates a default-constructed values_view.

Example: values_view

// requires /std:c++20 or later
#include <ranges>
#include <iostream>
#include <map>
#include <string>
#include <utility>
#include <vector>

int main()
{
    // ========== work with a std::map

    std::map<std::string, int> cpp_standards
    {
        {"C++98", 1988},
        {"C++03", 2003},
        {"C++11", 2011},
        {"C++14", 2014},
        {"C++17", 2017},
        {"C++20", 2020}
    };

    // Extract all of the values from the map
    for (int years : std::views::values(cpp_standards))
    {
        std::cout << years << ' '; // 2003 2011 2014 2017 1988 2020
    }
    std::cout << '\n';

    // ========== work with a std::pair

    std::vector<std::pair<std::string, int>> windows
    {
        {"Windows 1.0", 1985},
        {"Windows 2.0", 1987},
        {"Windows 3.0", 1990},
        {"Windows 3.1", 1992},
        {"Windows NT 3.1", 1993},
        {"Windows 95", 1995},
        {"Windows NT 4.0", 1996},
        {"Windows 95", 1995},
        {"Windows 98", 1998},
        {"Windows 1.0", 1985},
        {"Windows 2000", 2000}
    };

    // Another way to call the range adaptor using '|': create a keys_view from each pair
    for (int years : windows | std::views::values)
    {
        std::cout << years << ' '; // 1985 1987 1990 1992 ...
    }
}

2003 2011 2014 2017 1988 2020
1985 1987 1990 1992 1993 1995 1996 1995 1998 1985 2000

base

Gets a copy of the underlying view.

// Uses a copy constructor to return the underlying view
constexpr V base() const& requires std::copy_constructible<V>;

// Uses a move constructor to return the underlying view
constexpr V base() &&;

Parameters

None

Return value

The underlying view.

begin

Get an iterator to the first element in the values_view.

// returns a non-const iterator
1) constexpr auto begin() requires (!Simple_view<V>);

// returns a const iterator
2) constexpr auto begin() const requires ranges::range<const V>;

Parameters

None.

Return value

An iterator pointing at the first element in the values_view.

Picture of a vector with the elements 10, 20, and 30. The first element contains 10 and is labeled begin(). The last element contains 30 and is labeled 'last element'. An imaginary box after the last element indicates the sentinel and is labeled end().

end

Get the sentinel at the end of the values_view

1) constexpr auto end() requires (!Simple_view<V> && !ranges::common_range<V>);
2) constexpr auto end() requires (!Simple_view<V> && ranges::common_range<V>);
3) constexpr auto end() const requires ranges::range<const V>;
4) constexpr auto end() const requires ranges::common_range<const V>;

Parameters

None.

Return value

The sentinel that follows the last element in the values_view

Picture of a vector with the elements 10, 20, and 30. The first element contains 10 and is labeled begin(). The last element contains 30 and is labeled 'last element'. An imaginary box after the last element indicates the sentinel and is labeled end().

size

Get the number of elements.

constexpr auto size() requires sized_range<V>
constexpr auto size() const requires sized_range<const V>

Parameters

None.

Return value

The number of elements in the values_view.

Remarks

The size of the view is only available if the underlying range is a sized_range, or in other words, bounded.

See also

elements_view
keys_view
<ranges>
View classes