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std::optional::operator=

From cppreference.com
< cpp‎ | utility‎ | optional
 
 
 
 
optional& operator=( std::nullopt_t ) noexcept;
(1) (since C++17)
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)
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.
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.

Contents

[edit] Parameters

other - another optional object whose contained value to assign
value - value to assign to the contained value

[edit] Return value

*this

[edit] 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:  

[edit] 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.

[edit] Example

#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

[edit] 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

[edit] See also

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