std::optional<T>::operator=

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C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
std::optional
 
optional& operator=( std::nullopt_t ) noexcept;
 (1) (since C++17)
(constexpr since C++20)
constexpr optional& operator=( const optional& other );
 (2) (since C++17)
constexpr optional& operator=( optional&& other ) noexcept(/* see below */);
 (3) (since C++17)
template< class U = T >
optional& operator=( U&& value );
 (4) (since C++17)
(constexpr since C++20)
template< class U >
optional& operator=( const optional<U>& other );
 (5) (since C++17)
(constexpr since C++20)
template< class U >
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.
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:
5,6) Assigns the state of other.

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.

Feature-test macro Value Std Feature
__cpp_lib_optional 202106L (C++20)
(DR20)		 Fully constexpr (1), (4-6)

Example

Run this code
#include <iostream>
#include <optional>
 
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
P0602R4 C++17 copy/move assignment operator may not be trivial
even if underlying operations are trivial 		required to propagate triviality
P2231R1 C++20 converting assignment operators (1) and (4-6) were not constexpr
while the required operations can be in C++20 		made constexpr

See also

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