std::ranges:: views::single, std::ranges::
Defined in header <ranges>
|
||
(1) | ||
template
<
std::copy_constructible T >
requires std::is_object_v<T> |
(since C++20) (until C++23) |
|
template
<
std::move_constructible T >
requires std::is_object_v<T> |
(since C++23) | |
namespace views {
inline constexpr /* unspecified */ single = /* unspecified */; |
(2) | (since C++20) |
Call signature |
||
template
<
class T >
requires /* see below */ |
(since C++20) | |
view
that contains exactly one element of a specified value.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
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...>
-
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
constructs a single_view (public member function) |
|
returns a pointer to the element (public member function) |
|
returns a pointer past the element (public member function) |
|
[static]
|
returns false (public static member function) |
[static]
|
returns 1 (public static member function) |
returns a pointer to the element (public member function) |
|
Inherited from std::ranges::view_interface |
|
(C++23)
|
returns a constant iterator to the beginning of the range. (public member function of std::ranges::view_interface<D> ) |
(C++23)
|
returns a sentinel for the constant iterator of the range. (public member function of std::ranges::view_interface<D> ) |
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> )
|
|
returns the first element in the derived view. Provided if it satisfies forward_range . (public member function of std::ranges::view_interface<D> )
|
|
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> )
|
|
returns the n
th 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...> |
(4) | (since C++20) |
Constructs a single_view
.
value_
, which value-initializes its contained value.value_
with t.value_
with std::move(t)
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
#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
(C++17)
|
a wrapper that may or may not hold an object (class template) |
(C++20)
|
an empty view with no elements(class template) (variable template) |
(C++20)
|
a view over the subranges obtained from splitting another view using a delimiter(class template) (range adaptor object) |