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) |
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, |
(7) | (since C++17) |
template < class U = value_type > 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> is false. It is a trivial constructor if std::is_trivially_copy_constructible_v<T> is true.3) Move 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 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 trivial constructor if std::is_trivially_move_constructible_v<T> is true.4) Converting copy constructor: If
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>
5) Converting move constructor: If
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>
6) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type
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 true7) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type
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 true8) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type
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.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
Run this code
#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 |
See also
(C++17) |
creates an optional object (function template) |