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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General 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)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

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	constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while concat_viewviews::concat (C++26)(C++26)	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

Defined in header <ranges>
	(1)
template< std::copy_constructible T >     requires std::is_object_v<T> class single_view     : public ranges::view_interface<single_view<T>>			(since C++20) (until C++23)
template< std::move_constructible T >     requires std::is_object_v<T> class single_view     : public ranges::view_interface<single_view<T>>			(since C++23)
namespace views {     inline constexpr /* unspecified */ single = /* unspecified */; }		(2)	(since C++20)
Call signature
template< class T >     requires /* see below */ constexpr /* see below */ single( T&& t );			(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.

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

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.

Data members

Member	Definition
copyable-box <T> value_ (until C++23)	the single element of the view (exposition-only member object*)
movable-box <T> value_ (since C++23)	the single element of the view (exposition-only member object*)

Member functions

(constructor)	constructs a single_view (public member function)
begin	returns a pointer to the element (public member function)
end	returns a pointer past the element (public member function)
empty [static]	returns false (public static member function)
size [static]	returns 1 (public static member function)
data	returns a pointer to the element (public member function)
Inherited from std::ranges::view_interface
cbegin (C++23)	returns a constant iterator to the beginning of the range. (public member function of std::ranges::view_interface<D>)
cend (C++23)	returns a sentinel for the constant iterator of the range. (public member function of std::ranges::view_interface<D>)
operator bool	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	returns the first element in the derived view. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
back	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[]	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)
	(2)
constexpr explicit single_view( const T& t );			(since C++20) (until C++23)
constexpr explicit single_view( const T& t )     requires std::copy_constructible<T>;			(since C++23)
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::empty

static constexpr bool empty() noexcept;		(since C++20)

Equivalent to return false;.

std::ranges::single_view::size

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

Equivalent to return 1;.

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 <iomanip>
#include <iostream>
#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(): " << std::quoted(*sv2.data()) << '\n'
              << "7)  str = " << std::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
LWG 4035	C++20	single_view did not provide the member function empty()	provides empty()
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

optional (C++17)	a wrapper that may or may not hold an object (class template)
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)