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()>(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

// 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