constexpr optional() noexcept; constexpr optional( std::nullopt_t ) noexcept; | (1) | (since C++17) |
constexpr optional( const optional& other ); | (2) | (since C++17) |
constexpr optional( optional&& other ) noexcept(/* see below */); | (3) | (since C++17) |
template < class U > optional( const optional<U>& other ); | (4) | (since C++17) (conditionally explicit) |
template < class U > optional( optional<U>&& other ); | (5) | (since C++17) (conditionally explicit) |
template< class... Args > constexpr explicit optional( std::in_place_t, Args&&... args ); | (6) | (since C++17) |
template< class U, class... Args > constexpr explicit optional( std::in_place_t, std::initializer_list<U> ilist, Args&&... args ); | (7) | (since C++17) |
template < class U = value_type > constexpr optional( U&& value ); | (8) | (since C++17) (conditionally explicit) |
Constructs a new optional
object.
other
contains a value, initializes the contained value as if direct-initializing (but not direct-list-initializing) an object of type T
with the expression *other
. If other
does not contain a value, constructs an object that does not contain a value. This constructor is defined as deleted if std::is_copy_constructible_v<T>
is false
. It is a constexpr constructor if std::is_trivially_copy_constructible_v<T>
is true
.other
contains a value, initializes the contained value as if direct-initializing (but not direct-list-initializing) an object of type T
with the expression std::move(*other)
and does not make other
empty: a moved-from optional still contains a value, but the value itself is moved from. If other
does not contain a value, constructs an object that does not contain a value. This constructor does not participate in overload resolution unless std::is_move_constructible_v<T>
is true
. It is a constexpr constructor if std::is_trivially_move_constructible_v<T>
is true
.other
doesn't contain a value, constructs an optional object that does not contain a value. Otherwise, constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type T
with the expression *other
. This constructor does not participate in overload resolution unless the following conditions are met: std::is_constructible_v<T, const U&>
is true
. T
is not constructible or convertible from any expression of type (possibly const
) std::optional<U>
, i.e., the following 8 type traits are all false
: std::is_constructible_v<T, std::optional<U>&>
std::is_constructible_v<T, const std::optional<U>&>
std::is_constructible_v<T, std::optional<U>&&>
std::is_constructible_v<T, const std::optional<U>&&>
std::is_convertible_v<std::optional<U>&, T>
std::is_convertible_v<const std::optional<U>&, T>
std::is_convertible_v<std::optional<U>&&, T>
std::is_convertible_v<const std::optional<U>&&, T>
other
doesn't contain a value, constructs an optional object that does not contain a value. Otherwise, constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type T
with the expression std::move(*other)
. This constructor does not participate in overload resolution unless the following conditions are met: std::is_constructible_v<T, U&&>
is true
. T
is not constructible or convertible from any expression of type (possibly const
) std::optional<U>
, i.e., the following 8 type traits are all false
: std::is_constructible_v<T, std::optional<U>&>
std::is_constructible_v<T, const std::optional<U>&>
std::is_constructible_v<T, std::optional<U>&&>
std::is_constructible_v<T, const std::optional<U>&&>
std::is_convertible_v<std::optional<U>&, T>
std::is_convertible_v<const std::optional<U>&, T>
std::is_convertible_v<std::optional<U>&&, T>
std::is_convertible_v<const std::optional<U>&&, T>
T
from the arguments std::forward<Args>(args)...
. If the selected constructor of T
is a constexpr
constructor, this constructor is a constexpr
constructor. The function does not participate in the overload resolution unless std::is_constructible_v<T, Args...>
is true
T
from the arguments ilist, std::forward<Args>(args)...
. If the selected constructor of T
is a constexpr
constructor, this constructor is a constexpr
constructor. The function does not participate in the overload resolution unless std::is_constructible_v<T, std::initializer_list<U>&, Args&&...>
is true
T
(where T
= value_type
) with the expression std::forward<U>(value)
. If the selected constructor of T
is a constexpr
constructor, this constructor is a constexpr
constructor. This constructor does not participate in overload resolution unless std::is_constructible_v<T, U&&>
is true
and std::decay_t<U>
(until C++20)std::remove_cvref_t<U>
(since C++20) is neither std::in_place_t
nor std::optional<T>
. This constructor is explicit
if and only if std::is_convertible_v<U&&, T>
is false
.other | - | another optional object whose contained value is copied |
value | - | value with which to initialize the contained value |
args... | - | arguments with which to initialize the contained value |
ilist | - | initializer list with which to initialize the contained value |
T
.T
. Has the following noexcept
specification: noexcept(std::is_nothrow_move_constructible<T>::value)
T
.#include <optional> #include <iostream> #include <string> int main() { std::optional<int> o1, // empty o2 = 1, // init from rvalue o3 = o2; // copy-constructor // calls std::string( initializer_list<CharT> ) constructor std::optional<std::string> o4(std::in_place, {'a', 'b', 'c'}); // calls std::string( size_type count, CharT ch ) constructor std::optional<std::string> o5(std::in_place, 3, 'A'); // Move-constructed from std::string using deduction guide to pick the type std::optional o6(std::string{"deduction"}); std::cout << *o2 << ' ' << *o3 << ' ' << *o4 << ' ' << *o5 << ' ' << *o6 << '\n'; }
Output:
1 1 abc AAA deduction
(C++17) | creates an optional object (function template) |
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