std::optional<T>::optional
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) |
(4) | ||
template < class U > optional( const optional<U>& other ); |
(since C++17) (until C++20) (conditionally explicit) |
|
template < class U > constexpr optional( const optional<U>& other ); |
(since C++20) (conditionally explicit) |
|
(5) | ||
template < class U > optional( optional<U>&& other ); |
(since C++17) (until C++20) (conditionally explicit) |
|
template < class U > constexpr optional( optional<U>&& other ); |
(since C++20) (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 = T > constexpr optional( U&& value ); |
(8) | (since C++17) (conditionally explicit) |
Constructs a new optional
object.
1) Constructs an object that
does not contain a value.
2) Copy constructor: If
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>
isfalse
. - It is a trivial constructor if
std::is_trivially_copy_constructible_v<T>
istrue
.
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>
istrue
. - It is a trivial constructor if
std::is_trivially_move_constructible_v<T>
istrue
.
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&>
istrue
. -
T
is not constructible or convertible from any expression of type (possiblyconst
)std::optional<U>
, i.e., the following 8 type traits are allfalse
:-
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>
-
-
- This constructor is
explicit
if and only ifstd::is_convertible_v<const U&, T>
isfalse
.
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&&>
istrue
. -
T
is not constructible or convertible from any expression of type (possiblyconst
)std::optional<U>
, i.e., the following 8 type traits are allfalse
:-
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>
-
-
- This constructor is
explicit
if and only ifstd::is_convertible_v<U&&, T>
isfalse
.
T
from the arguments
std::forward<Args>(args)...
.
- If the selected constructor of
T
is aconstexpr
constructor, this constructor is aconstexpr
constructor. - The function does not participate in the overload resolution unless
std::is_constructible_v<T, Args...>
istrue
T
from the arguments
ilist, std::forward<Args>(args)...
.
- If the selected constructor of
T
is aconstexpr
constructor, this constructor is aconstexpr
constructor. - The function does not participate in the overload resolution unless
std::is_constructible_v<T, std::initializer_list<U>&, Args&&...>
istrue
T
with the expression
std::forward<U>(value)
.
- If the selected constructor of
T
is aconstexpr
constructor, this constructor is aconstexpr
constructor. - This constructor does not participate in overload resolution unless
std::is_constructible_v<T, U&&>
istrue
andstd::decay_t<U>
(until C++20)std::remove_cvref_t<U>
(since C++20) is neitherstd::in_place_t
norstd::optional<T>
. - This constructor is
explicit
if and only ifstd::is_convertible_v<U&&, T>
isfalse
.
Parameters
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 |
Exceptions
2) Throws any exception thrown by the constructor of
T
.
3) Throws any exception thrown by the constructor of
.
T
. Has the following
noexcept
specification:
noexcept(std::is_nothrow_move_constructible<T>::value)
4-8) Throws any exception thrown by the constructor of
T
.
Deduction guides
Example
#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
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
P0602R4 | C++17 | copy/move constructors may not be trivial even if underlying constructor is trivial | required to propagate triviality |
P2231R1 | C++20 | converting constructors from another optional was not constexprwhile the required operations can be in C++20 |
made constexpr |
See also
(C++17)
|
creates an optional object (function template) |
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