elements_view class (C++ Standard Library)

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

Syntax

template<input_range V, size_t N>
class elements_view : public view_interface<elements_view<V, N>>;

Template parameters

N
The index of the element to select for the view.

V
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::elements
Underlying range	Must satisfy input_range or higher
Element type	Same as the type of the indexed 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 elements_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 in this view. The underlying range must satisfy sized_range.
Inherited from view_interface	Description
backC++20	Get the last element.
emptyC++20	Test whether the elements_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 elements_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 elements_view are std::tuple, std::pair, and std::array.

Constructors

Construct an instance of a elements_view.

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

Parameters

base
The underlying range.

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

Return value

An elements_view instance.

Remarks

The best way to create an elements_view is by using the elements 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 elements_view from the specified view.
2) Default construct an elements_view.

Example: elements_view

// requires /std:c++20 or later
#include <array>
#include <iostream>
#include <map>
#include <ranges>
#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}
    };

    // create an elements_view of all the string elements (<1>) from each tuple
    for (int const year : std::views::elements<1>(cpp_standards))
    {
        std::cout << year << ' '; // 2003 2011 2014 2017 1988 2020
    }

    std::cout << '\n';

    // Another way to call the range adaptor using pipe (|) syntax
    for (auto&& name : cpp_standards | std::views::elements<0>)
    {
        std::cout << name << ' '; // C++03 C++11 C++14 C++17 C++98 C++20
    }
    std::cout << '\n';

    // ========== working with arrays

    std::array<std::array<int, 4>, 3> arr = { {{0,1,2,3}, {4,5,6,7}, {8,9,10,11}} };
    for (int& fourth : arr | std::views::elements<3>)
    {
        std::cout << fourth << ' '; // 3 7 11
    }
    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 98", 1998},
        {"Windows 2000", 2000}
    };

    for (int year : std::views::elements<1>(windows))
    {
        std::cout << year << ' '; // 1985 1987 1990 1992 1993 1995 1996 1998 2000
    }
}

2003 2011 2014 2017 1988 2020
C++03 C++11 C++14 C++17 C++98 c++20
3 7 11
1985 1987 1990 1992 1993 1995 1996 1998 2000

base

Gets a copy of the underlying range.

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

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

Parameters

None.

Return value

The underlying range.

begin

Get an iterator to the first element in the elements_view.

1) constexpr auto begin() requires (!Simple_view<V>);
2) constexpr auto begin() const requires range<const V>;

Parameters

None.

Return value

An iterator pointing at the first element in the elements_view.

end

Get the sentinel at the end of the elements_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 elements_view:

size

Get the number of elements in the view.

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 elements_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

keys_view
values_view
View classes
<ranges>