std::optional<T>::operator= - cppreference.com
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    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 = std::remove_cv_t<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.
    • 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 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:
    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.

    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
    LWG 3886 C++17 the default template argument of overload (6) was T changed to std::remove_cv_t<T>
    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