std::ranges::views::single, std::ranges::single_view
<ranges>
requires std::is_object_v <T>
class single_view
(until C++23)
requires std::is_object_v <T>
class single_view
inline constexpr /* unspecified */ single = /* unspecified */;
requires /* see below */
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.
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
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
(public member function of
std::ranges::view_interface<D>
) [edit]
(public member function of
std::ranges::view_interface<D>
) [edit]
(public member function of
std::ranges::view_interface<D>
) [edit]
forward_range
(public member function of
std::ranges::view_interface<D>
) [edit]
bidirectional_range
and common_range
(public member function of
std::ranges::view_interface<D>
) [edit]
n
th 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
(until C++23)
requires std::copy_constructible <T>;
requires std::constructible_from <T, Args...>
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 const T* begin() const noexcept;
Equivalent to return data();.
std::ranges::single_view::end
constexpr const T* end() const noexcept;
Equivalent to return data() + 1;.
std::ranges::single_view::empty
Equivalent to return false;.
std::ranges::single_view::size
Equivalent to return 1;.
Makes single_view
model /*tiny-range*/ as required by split_view
.
std::ranges::single_view::data
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
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
#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 |