std::variant<Types...>::operator=
- C++20
- C++17
// 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 */);
// 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.
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 inrhs
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 byrhs
is either nothrow copy constructible or not nothrow move constructible (as determined bystd::is_nothrow_copy_constructible
andstd::is_nothrow_move_constructible
, respectively), equivalent tothis->emplace<rhs.index()>(get<rhs.index()>(rhs))
.*this
may becomevalueless_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...
.
Move-assignment:
- If both
*this
andrhs
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
, assignsstd::get<j>(std::move(rhs))
to the value contained in*this
, withj
beingindex()
. 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 tothis->emplace<rhs.index()>(get<rhs.index()>(std::move(rhs)))
. If an exception is thrown byT_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...
.
Converting assignment.
- Determines the alternative type
T_j
that would be selected by overload resolution for the expressionF(std::forward<T>(t))
if there was an overload of imaginary functionF(T_i)
for everyT_i
fromTypes...
in scope at the same time, except that: An overloadF(T_i)
is only considered if the declarationT_i x[] = { std::forward<T>(t) };
is valid for some invented variablex
; - If
*this
already holds aT_j
, assignsstd::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 tothis->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
- May throw any exception thrown by assignment and copy/move initialization of any alternative.
- noexcept specification:
noexcept(((std::is_nothrow_move_constructible_v<Types> && std::is_nothrow_move_assignable_v<Types>) && ...))
- 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';
}
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 |