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

From cppreference.com
< cpp‎ | utility‎ | variant

constexpr variant& operator=( const variant& rhs ); | (1) | (since C++17)
constexpr variant& operator=( variant&& rhs ) noexcept(/* see below */); | (2) | (since C++17)
template< class T > variant& operator=( T&& t ) noexcept(/* see below */); | (3) | (since C++17) (constexpr since C++20)

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>)

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_variant` | `202106L` | (C++20) (DR) | Fully constexpr `std::variant` (3)

### 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
--- | --- | --- | ---
LWG 3024 | C++17 | copy assignment operator doesn't participate in overload resolution if any member type is not copyable | defined as deleted instead
LWG 3585 | C++17 | converting assignment was sometimes unexpectedly ill-formed because there was no available move assignment | made well-formed
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 (3) was not constexpr while the required operations can be constexpr in C++20 | made constexpr

### See also

emplace | constructs a value in the `variant`, in place (public member function)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/variant/operator%3D&oldid=172844"