std::variant<Types...>::operator=

C++20

// 1)
constexpr variant& operator=( const variant& rhs );
// 2)
constexpr variant& operator=( variant&& rhs ) noexcept(/* see below */);
// 3)
template< class T >
constexpr variant& operator=( T&& t ) noexcept(/* see below */);

C++17

// 1)
constexpr variant& operator=( const variant& rhs );
// 2)
constexpr variant& operator=( variant&& rhs ) noexcept(/* see below */);
// 3)
template< class T >
variant& operator=( T&& t ) noexcept(/* 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()>(get<rhs.index()>(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::get<j>(std::move(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()>(get<rhs.index()>(std::move(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

#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 Result

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
LWG 3024	C++17	copy assignment operator doesn't participate in overload resolution if any member type is not copyable	defined as deleted instead
P0602R4	C++17	copy/move assignment may not be trivial even if underlying operations are trivial	required to propagate triviality
P0608R3	C++17	converting assignment blindly assembles an overload set, leading to unintended conversions	narrowing and boolean conversions not considered
P2231R1	C++20	converting assignment was not constexpr while the required operations can be in C++20	made constexpr
LWG 3585	C++17	converting assignment was sometimes unexpectedly ill-formed because there was no available move assignment	made well-formed