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std::optional<T>::operator=

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< cpp‎ | utility‎ | optional
 
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 >
optional& operator=( const optional<U>& other );
 (4) (since C++17)
(constexpr since C++20)
template< class U >
optional& operator=( optional<U>&& other );
 (5) (since C++17)
(constexpr since C++20)
template< class U = std::remove_cv_t<T> >
optional& operator=( U&& value );
 (6) (since C++17)
(constexpr since C++20)

Replaces contents of *this with the contents of other.

1) If *this contains a value, calls val ->T::~T() to destroy the contained value; otherwise no effect. *this does not contain a value after this call.
2-5) Assigns the state of other. has_value() returns other.has_value() after this call.
Effect *this contains a value *this does not contain a value
other contains a value
  • for overloads (2,4), assigns *other to the contained value
  • for overloads (3,5), assigns std::move(*other) to the contained value
  • for overloads (2,4), direct-non-list-initializes the contained value with *other
  • for overloads (3,5), direct-non-list-initializes the contained value with std::move(*other)
other does not contain a value destroys the contained value by calling val ->T::~T() no effect
2) If std::is_copy_constructible_v<T> or std::is_copy_assignable_v<T> is false, the assignment operator is defined as deleted.
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, the assignment operator is trivial.
3) This overload participates in overload resolution only if std::is_move_constructible_v<T> and std::is_move_assignable_v<T> are both true.
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, the assignment operator is trivial.
4,5) These overloads participate in overload resolution only if all following conditions are satisfied:
6) If *this contains a value, assigns std::forward<U>(value) to the contained value; otherwise direct-non-list-initializes the contained value with std::forward<U>(value). *this contains a value after this call.
This overload participates in overload resolution only if all following conditions are satisfied:
  1. ↑ In other words, T is not constructible, convertible, or assignable from any expression of type (possibly const-qualified) std::optional<U>

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 (2-5)) 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>)

[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.

Feature-test macro Value Std Feature
__cpp_lib_optional 202106L (C++20)
(DR20)		 Fully constexpr (1), (4-6)

[edit] 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

[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
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 overloads (1,4-6) were not constexpr made constexpr

[edit] See also

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