std::optional::operator= - cppreference.com (original) (raw)

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](../optional.html#val "cpp/utility/optional")_ ->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

4,5) These overloads participate in overload resolution only if all following conditions are satisfied:

  1. 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.
  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.).

[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

#include #include   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:

[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 trivialeven 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] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |