std::optional<T>::optional
constexpr optional() noexcept; |
(1) | (since C++17) |
constexpr optional( std::nullopt_t ) noexcept; |
(2) | (since C++17) |
constexpr optional( const optional& other ); |
(3) | (since C++17) |
constexpr optional( optional&& other ) noexcept(/* see below */); |
(4) | (since C++17) |
template< class U > optional( const optional<U>& other ); |
(5) | (since C++17) (constexpr since C++20) (conditionally explicit) |
template< class U > optional( optional<U>&& other ); |
(6) | (since C++17) (constexpr since C++20) (conditionally explicit) |
template< class... Args > constexpr explicit optional( std::in_place_t, Args&&... args ); |
(7) | (since C++17) |
template< class U, class... Args > constexpr explicit optional( std::in_place_t, |
(8) | (since C++17) |
template< class U = std::remove_cv_t<T> > constexpr optional( U&& value ); |
(9) | (since C++17) (conditionally explicit) |
Constructs a new optional
object.
Contents |
[edit] 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 |
[edit] Effects
Overload | Initialization method | Initializer for the contained value | has_value() after construction
|
---|---|---|---|
(1) | N/A | - | false |
(2) | |||
(3) | Direct (non-list) | *other | other.has_value()
|
(4) | std::move(*other) | ||
(5) | *other | ||
(6) | std::move(*other) | ||
(7) | std::forward<Args>(args)... | true | |
(8) | ilist, std::forward<Args>(args)... | ||
(9) | std::forward<U>(value) |
[edit] Constraints and supplement information
- std::is_constructible_v<T, const U&> is true.
- If
T
is not (possibly cv-qualified) bool, the following 8 values are all false[1]:- 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>
- std::is_constructible_v<T, U> is true.
- If
T
is not (possibly cv-qualified) bool, the following 8 values are all false[1]:- 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
’s constructor selected for the initialization is a constexpr constructor, this constructor is also a constexpr constructor.T
’s constructor selected for the initialization is a constexpr constructor, this constructor is also a constexpr constructor.- std::is_constructible_v<T, U> is true.
- 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>.
- If
T
is (possibly cv-qualified) bool, std::decay_t<U>(until C++20)std::remove_cvref_t<U>(since C++20) is not a specialization ofstd::optional
.
T
’s constructor selected for the initialization is a constexpr constructor, this constructor is also a constexpr constructor.- ↑ 1.0 1.1 In other words,
T
is neither constructible nor convertible from any expression of type (possibly const-qualified) std::optional<U>
[edit] Exceptions
T
.T
. Has the following T
.[edit] Deduction guides
[edit] Notes
Before the resolution of LWG issue 3836, constructing an std::optional<bool> from std::optional<U> would select overload (9) instead of overloads (5,6) if U
is not bool. This is because overloads (5,6) did not participate in overload resolution if T
(bool in this case) can be constructed or converted from std::optional<U>, but std::optional::operator bool
makes the conversion possible for any U
.
As a result, the constructed std::optional<bool> always contains a value. That value is determined by whether the provided std::optional<U> object contains a value, rather than the bool value direct-initialized from the contained value:
std::optional<bool> op_false(false); std::optional<int> op_zero(0); std::optional<int> from_bool(op_false); // OK: contains 0 (initialized from false) std::optional<bool> from_int(op_zero); // DEFECT (LWG 3836): contains true because // op_zero contains a value, even if initializing // bool from that value gives false
Feature-test macro | Value | Std | Feature |
---|---|---|---|
__cpp_lib_optional |
202106L |
(C++20) (DR20) |
Fully constexpr (5,6) |
[edit] Example
#include <iostream> #include <optional> #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
[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 3836 | C++17 | when constructing an std::optional<bool> from std::optional<U>, the overload resolution would select overload (9) if U is not bool
|
always selects the converting copy/move constructor in this case |
LWG 3886 | C++17 | the default template argument of overload (9) was T
|
changed to std::remove_cv_t<T> |
P0602R4 | C++17 | copy/move constructors might not be trivial even if underlying constructor is trivial |
required to propagate triviality |
P2231R1 | C++20 | overloads (5,6) from another std::optional was not constexpr
|
made constexpr |
[edit] See also
(C++17) |
creates an optional object (function template) |