std::ranges::views::single, std::ranges::single_view

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_reference_trange_rvalue_reference_t	range_value_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	basic_istream_viewistream_view
Adaptors
views::all_tviews::all common_viewviews::common filter_viewviews::filter reverse_viewviews::reverse	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform	join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split ref_view views::counted
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

Defined in header <ranges>
template< std::copy_constructible T >   requires std::is_object_v<T> class single_view : public ranges::view_interface<single_view<T>>	(1)	(since C++20)
namespace views {     inline constexpr /*unspecified*/ single = /*unspecified*/; }	(2)	(since C++20)

1) Produces a view that contains exactly one element of a specified value.

2) The expression views::single(e) is expression-equivalent to single_view<std::decay_t<decltype((e))>>(e) for any suitable subexpression e.

Typical implementations of single_view store a single member of type copyable-box<T>. For description purposes, this member is hereinafter called value_.

The lifetime of the element is bound to the parent single_view. Copying single_view makes a copy of the element.

Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

Customization point objects

The name views::single denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __single_fn.

All instances of __single_fn are equal. The effects of invoking different instances of type __single_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, views::single can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to views::single above, __single_fn models std::invocable<__single_fn, Args...>, std::invocable<const __single_fn, Args...>, std::invocable<__single_fn&, Args...>, and std::invocable<const __single_fn&, Args...>. Otherwise, no function call operator of __single_fn participates in overload resolution.

Member functions

(constructor) (C++20)	constructs a single_view (public member function)
begin (C++20)	returns a pointer to the element (public member function)
end (C++20)	returns a pointer past the element (public member function)
size [static] (C++20)	returns 1 (one) (public static member function)
data (C++20)	returns a pointer to the element (public member function)
Inherited from std::ranges::view_interface
empty (C++20)	Returns whether the derived view is empty. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
operator bool (C++20)	Returns whether the derived view is not empty. Provided if ranges::empty is applicable to it. (public member function of std::ranges::view_interface<D>)
front (C++20)	Returns the first element in the derived view. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
back (C++20)	Returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range. (public member function of std::ranges::view_interface<D>)
operator[] (C++20)	Returns the nth element in the derived view. Provided if it satisfies random_access_range. (public member function of std::ranges::view_interface<D>)

std::ranges::single_view::single_view

single_view() requires std::default_initializable<T> = default;	(1)	(since C++20)
constexpr explicit single_view( const T& t );	(2)	(since C++20)
constexpr explicit single_view( T&& t );	(3)	(since C++20)
template< class... Args >   requires std::constructible_from<T, Args...> constexpr explicit single_view( std::in_place_t, Args&&... args );	(4)	(since C++20)

Constructs a single_view.

1) Default initializes value_, which value-initializes its contained value.

2) Initializes value_ with t.

3) Initializes value_ with std::move(t).

4) Initializes value_ as if by value_{std::in_place, std::forward<Args>(args)...}.

std::ranges::single_view::begin

constexpr T* begin() noexcept; constexpr const T* begin() const noexcept;		(since C++20)

Equivalent to return data();.

std::ranges::single_view::end

constexpr T* end() noexcept; constexpr const T* end() const noexcept;		(since C++20)

Equivalent to return data() + 1;.

std::ranges::single_view::size

static constexpr std::size_t size() noexcept;		(since C++20)

Equivalent to return 1;.

This is a static function. This makes single_view model /*tiny-range*/ as required by split_view.

std::ranges::single_view::data

constexpr T* data() noexcept; constexpr const T* data() const noexcept;		(since C++20)

Returns a pointer to the contained value of value_. The behavior is undefined if value_ does not contains a value.

Deduction guides

template<class T> single_view(T) -> single_view<T>;		(since C++20)

Notes

For a single_view, the inherited empty member function always returns false, and the inherited operator bool conversion function always returns true.

Example

Run this code

#include <iostream>
#include <iomanip>
#include <ranges>
#include <string>
#include <tuple>
 
int main()
{
    constexpr std::ranges::single_view sv1{3.1415}; // uses (const T&) constructor
    static_assert(sv1);
    static_assert(not sv1.empty());
 
    std::cout << "1) *sv1.data(): " << *sv1.data() << '\n'
              << "2) *sv1.begin(): " << *sv1.begin() << '\n'
              << "3)  sv1.size(): " << sv1.size() << '\n'
              << "4)  distance: " << std::distance(sv1.begin(), sv1.end()) << '\n';
 
    std::string str{"C++20"};
    std::cout << "5)  str = " << std::quoted(str) << '\n';
    std::ranges::single_view sv2{std::move(str)}; // uses (T&&) constructor
    std::cout << "6) *sv2.data(): " << quoted(*sv2.data()) << '\n'
              << "7)  str = " << quoted(str) << '\n';
 
    std::ranges::single_view<std::tuple<int, double, std::string>>
        sv3{std::in_place, 42, 3.14, "😄"}; // uses (std::in_place_t, Args&&... args)
 
    std::cout << "8)  sv3 holds a tuple: { "
              << std::get<0>(sv3[0]) << ", "
              << std::get<1>(sv3[0]) << ", "
              << std::get<2>(sv3[0]) << " }\n";
}

Output:

1) *sv1.data(): 3.1415
2) *sv1.begin(): 3.1415
3)  sv1.size(): 1
4)  distance: 1
5)  str = "C++20"
6) *sv2.data(): "C++20"
7)  str = ""
8)  sv3 holds a tuple: { 42, 3.14, 😄 }

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 3428	C++20	single_view was convertible from std::in_place_t	the constructor is made explicit
P2367R0	C++20	deduction guides for single_view failed to decay the argument; views::single copied but not wrapped a single_view	a decaying guide provided; made always wrapping

See also

ranges::empty_viewviews::empty (C++20)	an empty view with no elements (class template) (variable template)
ranges::split_viewviews::split (C++20)	a view over the subranges obtained from splitting another view using a delimiter (class template) (range adaptor object)