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

From cppreference.com
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::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 ;

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

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 "