NAME¶

std::optional::operator= - std::optional::operator=

Synopsis¶

(since
optional& operator=( std::nullopt_t ) C++17)
noexcept; (until
C++20)
constexpr optional& operator=( (since
std::nullopt_t ) noexcept; C++20)
constexpr optional& operator=( const (2) (since
optional& other ); C++17)
constexpr optional& operator=( optional&& (3) (since
other ) noexcept(/* see below */); C++17)
(since
template< class U = T > C++17)
optional& operator=( U&& value ); (until
C++20)
template< class U = T > (1) (since
constexpr optional& operator=( U&& value ); C++20)
template< class U > (since
optional& operator=( const optional<U>& C++17)
other ); (until
(4) C++20)
template< class U > (since
constexpr optional& operator=( const C++20)
optional<U>& other ); (5)
(since
template< class U > C++17)
optional& operator=( optional<U>&& other ); (until
(6) C++20)
template< class U > (since
constexpr optional& operator=( optional<U>&& C++20)
other );

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 direct-initialized or assigned from *other (2) or
std::move(*other) (3). Note that a moved-from optional still contains a value.
* Overload (2) is defined as deleted unless std::is_copy_constructible_v<T> and
std::is_copy_assignable_v<T> are both true. 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 unless
std::is_move_constructible_v<T> and std::is_move_assignable_v<T> are both true.
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 scalar type;
* 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 direct-initialized 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, 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_assignable_v<T&, std::optional<U>&>
* std::is_assignable_v<T&, const std::optional<U>&>
* std::is_assignable_v<T&, std::optional<U>&&>
* std::is_assignable_v<T&, const std::optional<U>&&>.

* For overload (5), std::is_constructible_v<T, const U&> and
std::is_assignable_v<T&, const U&> 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
(2-3) and (5-6) ) 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
noexcept 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.

Example¶

// Run this code

#include <optional>
#include <iostream>
int main()
{
std::optional<const char*> s1 = "abc", s2; // constructor
s2 = s1; // assignment
s1 = "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
copy/move assignment operator may not be required to propagate
P0602R4 C++17 trivial triviality
even if underlying operations are trivial
converting assignment operators were not
P2231R1 C++20 constexpr made constexpr
while the required operations can be in
C++20

See also¶

emplace constructs the contained value in-place
(public member function)