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std::optional::operator=

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

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.
  • 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.
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> 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 the following
noexcept specification:  
noexcept(std::is_nothrow_move_assignable<T>::value && std::is_nothrow_move_constructible<T>::value)

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

See also

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