From cppreference.com
optional&operator=(std::nullopt_t)noexcept; (1)(since C++17)
(constexpr since C++20)constexproptional&operator=(constoptional&other); (2)(since C++17)constexproptional&operator=(optional&&other)noexcept(/* see below */); (3) (since C++17)template<classU=std::remove_cv_t<T>>optional&operator=(U&&value); (4)(since C++17)
(constexpr since C++20)template<classU>optional&operator=(constoptional<U>&other); (5)(since C++17)
(constexpr since C++20)template<classU>optional&operator=(optional<U>&&other); (6)(since C++17)
(constexpr since C++20)Replaces contents of *this with the contents of other.
1) If *this contains a value before the call, the contained value is destroyed by calling its destructor as if by value().T::~T(). *this does not contain a value after this call.
2,3) Assigns the state of other.
If both *this and other do not contain a value, the function has no effect.
If *this contains a value, but other does not, then the contained value is destroyed by calling its destructor. *this does not contain a value after the call.
If other contains a value, then depending on whether *this contains a value, the contained value is either
or assigned from *other(2) or std::move(*other)(3). Note that a moved-from optional still contains a value.
Overload (2) is deleted when either std::is_copy_constructible_v<T> or std::is_copy_assignable_v<T> is false. It is trivial if std::is_trivially_copy_constructible_v<T>, std::is_trivially_copy_assignable_v<T> and std::is_trivially_destructible_v<T> are all true.
Overload (3) does not participate in overload resolution when either std::is_move_constructible_v<T> or std::is_move_assignable_v<T> is false. It is trivial if std::is_trivially_move_constructible_v<T>, std::is_trivially_move_assignable_v<T> and std::is_trivially_destructible_v<T> are all true.
4) Perfect-forwarded assignment: depending on whether *this contains a value before the call, the contained value is either direct-initialized from std::forward<U>(value) or assigned from std::forward<U>(value). The function does not participate in overload resolution unless std::decay_t<U>(until C++20)std::remove_cvref_t<U>(since C++20) is not std::optional<T>, std::is_constructible_v<T,U> is true, std::is_assignable_v<T&,U> is true, and at least one of the following is true:
T is not a
;
std::decay_t<U> is not T.
5,6) Assigns the state of other.
If both *this and other do not contain a value, the function has no effect.
If *this contains a value, but other does not, then the contained value is destroyed by calling its destructor. *this does not contain a value after the call.
If other contains a value, then depending on whether *this contains a value, the contained value is either
or assigned from *other(5) or std::move(*other)(6). Note that a moved-from optional still contains a value.
These overloads do not participate in overload resolution unless the following conditions are met: T is not constructible, convertible, or assignable from any expression of type (possibly const) std::optional<U>, i.e., the following 12 type traits are all false: std::is_constructible_v<T,std::optional<U>&>
std::is_constructible_v<T,conststd::optional<U>&>
std::is_constructible_v<T,std::optional<U>&&>
std::is_constructible_v<T,conststd::optional<U>&&>
std::is_convertible_v<std::optional<U>&,T>
std::is_convertible_v<conststd::optional<U>&,T>
std::is_convertible_v<std::optional<U>&&,T>
std::is_convertible_v<conststd::optional<U>&&,T>
std::is_assignable_v<T&,std::optional<U>&>
std::is_assignable_v<T&,conststd::optional<U>&>
std::is_assignable_v<T&,std::optional<U>&&>
std::is_assignable_v<T&,conststd::optional<U>&&>
For overload (5), std::is_constructible_v<T,constU&> and std::is_assignable_v<T&,constU&> are both true.
For overload (6), std::is_constructible_v<T,U> and std::is_assignable_v<T&,U> are both true.
Parameters
other - another optional object whose contained value to assign value - value to assign to the contained value Return value
*this
Exceptions
2-6) Throws any exception thrown by the constructor or assignment operator of T. If an exception is thrown, the initialization state of *this (and of other in case of (
) and (
)) is unchanged, i.e. if the object contained a value, it still contains a value, and the other way round. The contents of value and the contained values of *this and other depend on the exception safety guarantees of the operation from which the exception originates (copy-constructor, move-assignment, etc.).
3) Has following
specification:
noexcept(std::is_nothrow_move_assignable_v<T>&&std::is_nothrow_move_constructible_v<T>)Notes
An optional object op may be turned into an empty optional with both op={}; and op=nullopt;. The first expression constructs an empty optional object with {} and assigns it to op.
macro ValueStdFeature
(C++20)
(DR20)Fully constexpr(
), (
)Example
Run this code
#include<iostream>#include<optional>intmain(){std::optional<constchar*>s1="abc",s2;// constructors2=s1;// assignments1="def";// decaying assignment (U = char[4], T = const char*)std::cout<<*s2<<' '<<*s1<<'\n';}Output:
abc def Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++17 the default template argument of overload (
) was Tchanged to std::remove_cv_t<T>
C++17 copy/move assignment operator may not be trivial
even if underlying operations are trivial required to propagate triviality
C++20 converting assignment operators (
) and (
) were not constexpr
while the required operations can be in C++20 made constexprSee also
constructs the contained value in-place
(public member function)