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keys_view class (C++ Standard Library)
API reference for the Standard Template Library (STL) <ranges> keys_view class, which provides a view over the first index into each tuple-like value in a collection. It's useful for extracting keys from associative containers.
09/28/2022
ranges/std::keys_view
ranges/std::keys_view::base
ranges/std::keys_view::begin
ranges/std::keys_view::empty
ranges/std::keys_view::end
ranges/std::keys_view::size
ranges/std::keys_view::operator bool
ranges/std::keys_view::back
ranges/std::keys_view::front
ranges/std::keys_view::operator[]
std::ranges::keys_view [C++]
std::ranges::keys_view::base [C++]
std::ranges::keys_view::begin [C++]
std::ranges::keys_view::empty [C++]
std::ranges::keys_view::end [C++]
std::ranges::keys_view::size [C++]
std::ranges::keys_view::back [C++]
std::ranges::keys_view::front [C++]
std::ranges::keys_view::operator[] [C++]
std::ranges::keys_view::operator bool [C++]
C++

keys_view class (C++ Standard Library)

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

keys_view is an alias for elements_view<R, 0> and is useful for making of view of the keys from associative containers like std::map or std::unordered_map.

Syntax

template<input_range R>
using keys_view = ranges::elements_view<R, 0>;

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::keys
Underlying range Must satisfy input_range or higher
Element type Same as the type of the first tuple element of the underlying range
View iterator category random_access_range if the underlying range is contiguous, otherwise the same as the underlying 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

The following list of member functions refers to the keys_view class. Recall that this is an alias for an element_view class template instantiation.

Member functions Description
ConstructorsC++20 Construct a keys_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 view.
sizeC++20 Get the number of elements. The underlying range must satisfy sized_range.
Inherited from view_interface Description
backC++20 Get the last element.
emptyC++20 Test whether the keys_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 keys_view isn't empty.

Requirements

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

Namespace: std::ranges

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

Remarks

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

Constructors

Construct an instance of a keys_view.

1) constexpr keys_view(V base);
2) keys_view() requires std::default_initializable<V> = default;

Parameters

base
The underlying range.

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

Return value

A keys_view instance.

Remarks

The best way to create an keys_view is by using the keys 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 a keys_view from the specified view.
2) The default constructor creates an empty keys_view.

Example: keys_view

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

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 keys from the map
    for (const std::string& standards : std::views::keys(cpp_standards))
    {
        std::cout << standards << ' '; // C++03 C++11 C++14 C++17 C++98 C++20
    }
    std::cout << '\n';

    // ========== work with a range of 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 an keys_view from each pair
    for (const std::string& version : windows | std::views::keys)
    {
        std::cout << version << ' '; // Windows 1.0 Windows 2.0 Windows 3.0 ...
    }
}
C++03 C++11 C++14 C++17 C++98 c++20
Windows 1.0 Windows 2.0 Windows 3.0 Windows 3.1 Windows NT 3.1 Windows 95 Windows NT 4.0 Windows 95 Windows 98 Windows 1.0 Windows 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 keys_view.

1) constexpr auto begin() requires (!Simple_view<V>);
2) constexpr auto begin() const requires (Simple_view<V>) // or put another way, requires ranges::range<const V>;

Parameters

None.

Return value

An iterator pointing at the first element in the keys_view.

:::image type="content" source="media/begin-end-sentinel.png" alt-text="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 keys_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 keys_view:

:::image type="content" source="media/begin-end-sentinel.png" alt-text="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 keys_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
values_view
<ranges>
View classes