std::experimental::optional<T>::operator=

optional& operator=( std::experimental::nullopt_t ) noexcept;	(1)	(library fundamentals TS)
optional& operator=( const optional& other );	(2)	(library fundamentals TS)
optional& operator=( optional&& other ) noexcept(/* see below */);	(3)	(library fundamentals TS)
template< class U > optional& operator=( U&& value );	(4)	(library fundamentals TS)

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 val->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.

4) Decay-only 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::is_same<std::decay_t<U>, T>::value is true.

Parameters

other	-	another `optional` object whose contained value to assign
value	-	value to assign to the contained value
Type requirements
- `T` must meet the requirements of CopyAssignable and CopyConstructible in order to use overload (2).
- `T` must meet the requirements of MoveAssignable and MoveConstructible in order to use overload (3).

Return value

*this

Exceptions

2-4) 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) ) 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 declaration:

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;.

Example

Run this code

#include <experimental/optional>
#include <iostream>
int main()
{
    std::experimental::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

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals 2 TS)
Extensions for parallelism (parallelism TS)
Extensions for parallelism 2 (parallelism TS v2)
Extensions for concurrency (concurrency TS)
Concepts (concepts TS)
Ranges (ranges TS)
Special mathematical functions (special math TR)

experimental::optional
experimental::any
experimental::basic_string_view
experimental::sample
experimental::shared_ptr
experimental::weak_ptr
experimental::apply
experimental::invocation_type experimental::raw_invocation_type
experimental::search
experimental::default_searcher experimental::make_default_searcher
experimental::boyer_moore_searcher experimental::make_boyer_moore_searcher
experimental::boyer_moore_horspool_searcher experimental::make_boyer_moore_horspool_searcher
type-erased and polymorphic allocators
variable templates for type traits