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      std::optional<T>::operator=

      From cppreference.com
      < cpp‎ | utility‎ | optional
       
      C++
       
      Utilities library
       
      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 ValueStdFeature
      __cpp_lib_optional202106L(C++20)
      (DR20)      		Fully constexpr(1), (4-6)

      [edit]Example

      Run this code
      #include <iostream>#include <optional>   int main(){std::optional<constchar*> 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 3886C++17 the default template argument of overload (6) was Tchanged to std::remove_cv_t<T>
      P0602R4C++17 copy/move assignment operator may not be trivial
      even if underlying operations are trivial       		required to propagate triviality
      P2231R1C++20 overloads (1,4-6) were not constexprmade constexpr

      [edit]See also

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