std::optional<T>::operator=

 

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std::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator booloptional::has_value

optional::value

optional::value_or

Iterators

optional::begin (C++26)

optional::end (C++26)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

Modifiers

optional::emplace

optional::swap

optional::reset

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

make_optional

swap(std::optional)

Deduction guides

Helper classes

hash<std::optional>

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

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.

• 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:

• T is not a scalar type;
• std::decay_t<U> is not T.

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
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

emplace

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