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std::experimental::optional::operator=

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Technical specifications
Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals 2 TS)
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Concepts (concepts TS)
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Special mathematical functions (special math TR)
 
 
 
optional& operator=( std::experimental::nullopt_t );
(1) (library fundamentals TS)
optional& operator=( const optional& other );
(2) (library fundamentals TS)
optional& operator=( optional&& other );
(3) (library fundamentals TS)
template< class U >
optional& operator=( U&& value );
(4) (library fundamentals TS)

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.

Contents

[edit] 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).

[edit] Return value

*this

[edit] 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:  

[edit] Notes

An optional object op may be turned into an empty optional with both op = {}; and op = nullopt;.

[edit] Example

#include <experimental/optional>
#include <iostream>
int main()
{
    std::experimental::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] See also

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