std::ranges::views::single, std::ranges::single_view
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C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Ranges library
(until C++23)
(exposition-only member object*)
(exposition-only member object*)
returns a constant iterator to the beginning of the range
(public member function of
returns a sentinel for the constant iterator of the range
(public member function of
(until C++23)
(C++23)(C++23)
single_viewviews::single
(C++23)(C++23)
(C++23)(C++23)
(C++26)(C++26)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)
(C++23)(C++23)
(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)
Defined in header
<ranges>
(1)
template< std::copy_constructible T >
(since C++20) requires std::is_object_v <T>
class single_view
(until C++23)
template< std::move_constructible T >
(since C++23)
requires std::is_object_v <T>
class single_view
namespace views {
(2)
(since C++20)
inline constexpr /* unspecified */ single = /* unspecified */;
Call signature
template< class T >
(since C++20)
requires /* see below */
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.
Contents
Customization point objects
The name views::single denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.
[edit] 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*)
[edit] Member functions
Inherited from std::ranges::view_interface
(C++23)
(public member function of
std::ranges::view_interface<D>) [edit]
(C++23)
(public member function of
std::ranges::view_interface<D>) [edit]
returns whether the derived view is not empty, provided only if ranges::empty is applicable to it
(public member function of
(public member function of
std::ranges::view_interface<D>) [edit]
returns the first element in the derived view, provided if it satisfies
(public member function of
forward_range (public member function of
std::ranges::view_interface<D>) [edit]
returns the last element in the derived view, provided only if it satisfies
(public member function of
bidirectional_range and common_range (public member function of
std::ranges::view_interface<D>) [edit]
returns the
(public member function of
nth element in the derived view, provided only if it satisfies random_access_range (public member function of
std::ranges::view_interface<D>) [edit]
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)
requires std::copy_constructible <T>;
constexpr explicit single_view( T&& t );
(3)
(since C++20)
template< class... Args >
(4)
(since C++20)
requires std::constructible_from <T, Args...>
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).
std::ranges::single_view::begin
constexpr T* begin() noexcept;
constexpr const T* begin() const noexcept;
(since C++20)
constexpr const T* begin() const noexcept;
Equivalent to return data();.
std::ranges::single_view::end
constexpr T* end() noexcept;
constexpr const T* end() const noexcept;
(since C++20)
constexpr const T* end() const noexcept;
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)
constexpr const T* data() const noexcept;
Returns a pointer to the contained value of value_. The behavior is undefined if value_ does not contains a value.
[edit] Deduction guides
template< class T >
single_view( T ) -> single_view<T>;
(since C++20)
single_view( T ) -> single_view<T>;
[edit] Notes
For a single_view, the inherited empty member function always returns false, and the inherited operator bool conversion function always returns true.
[edit] 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, 😄 }[edit] 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 |