std::optional::operator=

 

C++

Language

Standard library headers

Concepts

Utilities library

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Numerics library

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Regular expressions library (C++11)

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Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

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Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

Optional and any

optional (C++17)

any (C++17)

Pairs and tuples

pair

tuple (C++11)

apply (C++17)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

Type operations

declval (C++11)

as_const (C++17)

 

std::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator bool

optional::value

optional::value_or

Modifiers

optional::emplace

optional::swap

Non-member functions

operator==operator!=operator<operator<=operator>operator>=

make_optional

swap

Helper classes

hash

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

optional& operator=( std::nullopt_t );	(1)	(since C++17)
optional& operator=( const optional& other );	(2)	(since C++17)
optional& operator=( optional&& other );	(3)	(since C++17)
template< class U > optional& operator=( U&& value );	(4)	(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 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

1)

noexcept specification:  

noexcept

  

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 <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

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