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

 

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Language support Type support (basic types, RTTI) Library feature-test macros(C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support(C++20) Contract support(C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation(C++20) bitset hash (C++11) Relational operators(deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::variant

Member functions

variant::variant

variant::~variant

variant::operator=

Observers

variant::index

variant::valueless_by_exception

Modifiers

variant::emplace

variant::swap

Visitation

variant::visit (C++26)

Non-member functions

visit(std::variant)

holds_alternative

get(std::variant)

get_if

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

swap(std::variant)

Helper classes

monostate

bad_variant_access

variant_size

variant_alternative

hash<std::variant>

Helper objects

variant_npos

 

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";// OKstd::variant<std::string, std::string> v2; v2 ="abc";// Errorstd::variant<std::string, bool> v3; v3 ="abc";// OK, chooses string; bool is not a candidatestd::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 constexprstd::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)>;ifconstexpr(std::is_same_v<T, int>) os << arg;elseifconstexpr(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)