std::variant::operator=(3)

NAME
std::variant::operator= - std::variant::operator=

Synopsis
   constexpr variant& operator=( const variant& rhs ); (1) (since C++17)
   constexpr variant& operator=( variant&& rhs ) (2) (since C++17)
   noexcept(/* see below */);
   template< class T > (since C++17)
   variant& operator=( T&& t ) noexcept(/* see below (until C++20)
   */); (3)
   template< class T >
   constexpr variant& operator=( T&& t ) noexcept(/* (since C++20)
   see below */);
   Assigns a new value to an existing variant object.
   1) Copy-assignment:
     * If both *this and rhs are valueless by exception, does nothing.
     * Otherwise, if rhs is valueless, but *this is not, destroys the value contained
       in *this and makes it valueless.
     * Otherwise, if rhs holds the same alternative as *this, assigns the value
       contained in rhs to the value contained in *this. If an exception is thrown,
       *this does not become valueless: the value depends on the exception safety
       guarantee of the alternative's copy assignment.
     * Otherwise, if the alternative held by rhs is either nothrow copy constructible
       or not nothrow move constructible (as determined by
       std::is_nothrow_copy_constructible and std::is_nothrow_move_constructible,
       respectively), equivalent to
       this->emplace<rhs.index()>(*std::get_if<rhs.index()>(std::addressof(rhs))).
       *this may become valueless_by_exception if an exception is thrown on the
       copy-construction inside emplace.
     * Otherwise, equivalent to this->operator=(variant(rhs)).
   This overload is defined as deleted unless std::is_copy_constructible_v<T_i> and
   std::is_copy_assignable_v<T_i> are both true for all T_i in Types.... This overload
   is trivial if
   std::is_trivially_copy_constructible_v<T_i>,std::is_trivially_copy_assignable_v<T_i>
   and std::is_trivially_destructible_v<T_i> are all true for all T_i in Types....
   2) Move-assignment:
     * If both *this and rhs are valueless by exception, does nothing.
     * Otherwise, if rhs is valueless, but *this is not, destroys the value contained
       in *this and makes it valueless.
     * Otherwise, if rhs holds the same alternative as *this, assigns
       std::move(*std::get_if<j>(std::addressof(rhs))) to the value contained in *this,
       with j being index(). If an exception is thrown, *this does not become
       valueless: the value depends on the exception safety guarantee of the
       alternative's move assignment.
     * Otherwise (if rhs and *this hold different alternatives), equivalent to
       this->emplace<rhs.index()>(std::move(*std::get_if<rhs.index()>(std::addressof(rhs)))).
       If an exception is thrown by T_i's move constructor, *this becomes
       valueless_by_exception.
   This overload participates in overload resolution only if
   std::is_move_constructible_v<T_i> and std::is_move_assignable_v<T_i> are both true
   for all T_i in Types.... This overload is trivial if
   std::is_trivially_move_constructible_v<T_i>,
   std::is_trivially_move_assignable_v<T_i>, and std::is_trivially_destructible_v<T_i>
   are all true for all T_i in Types....
   3) Converting assignment.
     * Determines the alternative type T_j that would be selected by overload
       resolution for the expression F(std::forward<T>(t)) if there was an overload of
       imaginary function F(T_i) for every T_i from Types... in scope at the same time,
       except that:
     * An overload F(T_i) is only considered if the declaration T_i x[] = {
       std::forward<T>(t) }; is valid for some invented variable x;
     * If *this already holds a T_j, assigns std::forward<T>(t) to the value contained
       in *this. If an exception is thrown, *this does not become valueless: the value
       depends on the exception safety guarantee of the assignment called.
     * Otherwise, if std::is_nothrow_constructible_v<T_j, T> ||
       !std::is_nothrow_move_constructible_v<T_j> is true, equivalent to
       this->emplace<j>(std::forward<T>(t)). *this may become valueless_by_exception if
       an exception is thrown on the initialization inside emplace.
     * Otherwise, equivalent to this->emplace<j>(T_j(std::forward<T>(t))).
   This overload participates in overload resolution only if
   std::decay_t<T>
   (until C++20)
   std::remove_cvref_t<T>
   (since C++20) is not the same type as variant and std::is_assignable_v<T_j&, T> is
   true and std::is_constructible_v<T_j, T> is true and the expression
   F(std::forward<T>(t)) (with F being the above-mentioned set of imaginary functions)
   is well formed.
 std::variant<std::string> v1;
 v1 = "abc"; // OK
 std::variant<std::string, std::string> v2;
 v2 = "abc"; // Error
 std::variant <std::string, bool> v3;
 v3 = "abc"; // OK, chooses string; bool is not a candidate
 std::variant<float, long, double> v4; //holds float
 v4 = 0; // OK, holds long; float and double are not candidates

Parameters
   rhs - another variant
   t - a value convertible to one of the variant's alternatives

Return value
   *this

Exceptions
   1) May throw any exception thrown by assignment and copy/move initialization of any
   alternative.
   2)
   noexcept specification:
   noexcept(((std::is_nothrow_move_constructible_v<Types> &&
              std::is_nothrow_move_assignable_v<Types>) && ...))
   3)
   noexcept specification:
   noexcept(std::is_nothrow_assignable_v<T_j&, T> &&
   std::is_nothrow_constructible_v<T_j, T>)

Example
// Run this code
 #include <iomanip>
 #include <iostream>
 #include <string>
 #include <type_traits>
 #include <variant>
 std::ostream& operator<<(std::ostream& os, std::variant<int, std::string> const& va)
 {
     os << ": { ";
     std::visit([&](auto&& arg)
     {
         using T = std::decay_t<decltype(arg)>;
         if constexpr (std::is_same_v<T, int>)
             os << arg;
         else if constexpr (std::is_same_v<T, std::string>)
             os << std::quoted(arg);
     }, va);
     return os << " };\n";
 }
 int main()
 {
     std::variant<int, std::string> a{2017}, b{"CppCon"};
     std::cout << "a" << a << "b" << b << '\n';
     std::cout << "(1) operator=( const variant& rhs )\n";
     a = b;
     std::cout << "a" << a << "b" << b << '\n';
     std::cout << "(2) operator=( variant&& rhs )\n";
     a = std::move(b);
     std::cout << "a" << a << "b" << b << '\n';
     std::cout << "(3) operator=( T&& t ), where T is int\n";
     a = 2019;
     std::cout << "a" << a << '\n';
     std::cout << "(3) operator=( T&& t ), where T is std::string\n";
     std::string s{"CppNow"};
     std::cout << "s: " << std::quoted(s) << '\n';
     a = std::move(s);
     std::cout << "a" << a << "s: " << std::quoted(s) << '\n';
 }

Possible output:
 a: { 2017 };
 b: { "CppCon" };
 (1) operator=( const variant& rhs )
 a: { "CppCon" };
 b: { "CppCon" };
 (2) operator=( variant&& rhs )
 a: { "CppCon" };
 b: { "" };
 (3) operator=( T&& t ), where T is int
 a: { 2019 };
 (3) operator=( T&& t ), where T is std::string
 s: "CppNow"
 a: { "CppNow" };
 s: ""
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
                       copy assignment operator doesn't
   LWG 3024 C++17 participate in overload resolution defined as deleted instead
                       if any member type is not copyable
                       copy/move assignment may not be
   P0602R4 C++17 trivial required to propagate
                       even if underlying operations are triviality
                       trivial
                       converting assignment blindly narrowing and boolean
   P0608R3 C++17 assembles an overload set, conversions
                       leading to unintended conversions not considered
                       converting assignment was not
   P2231R1 C++20 constexpr made constexpr
                       while the required operations can be
                       in C++20
                       converting assignment was sometimes
   LWG 3585 C++17 unexpectedly ill-formed made well-formed
                       because there was no available move
                       assignment

See also
   emplace constructs a value in the variant, in place
           (public member function)

Category:
     * conditionally noexcept