std::variant<Types...>::variant
Constructs a new variant object.
Declarations
// 1)
constexpr variant() noexcept(/* see below */);
// 2)
constexpr variant( const variant& other );
// 3)
constexpr variant( variant&& other ) noexcept(/* see below */);
// 4)
template< class T >
constexpr variant( T&& t ) noexcept(/* see below */);
// 5)
template< class T,
class... Args >
constexpr explicit variant( std::in_place_type_t<T>,
Args&&... args );
// 6)
template< class T,
class U,
class... Args >
constexpr explicit variant( std::in_place_type_t<T>,
std::initializer_list<U> il,
Args&&... args );
// 7)
template< std::size_t I,
class... Args >
constexpr explicit variant( std::in_place_index_t<I>,
Args&&... args );
// 8)
template< std::size_t I,
class U,
class... Args >
constexpr explicit variant( std::in_place_index_t<I>,
std::initializer_list<U> il,
Args&&... args );
Default constructor. Constructs a variant holding the value-initialized value of the first alternative (index()
is zero).
- This constructor is constexpr if and only if the value initialization of the alternative type
T_0
would satisfy the requirements for a constexpr function. - This overload participates in overload resolution only if
std::is_default_constructible_v<T_0>
is true.
Copy constructor. If other is not valueless_by_exception, constructs a variant holding the same alternative as other and direct-initializes
the contained value with std::get<other.index()>(other)
. Otherwise, initializes a valueless_by_exception variant.
- This constructor is defined as deleted unless
std::is_copy_constructible_v<T_i>
is true for allT_i
inTypes...
. - It is trivial if
std::is_trivially_copy_constructible_v<T_i>
is true for allT_i
inTypes...
.
Move constructor. If other is not valueless_by_exception, constructs a variant holding the same alternative as other and direct-initializes
the contained value with std::get<other.index()>(std::move(other))
. Otherwise, initializes a valueless_by_exception variant.
- This overload participates in overload resolution only if
std::is_move_constructible_v<T_i>
is true for allT_i
inTypes...
. - It is trivial if
std::is_trivially_move_constructible_v<T_i>
is true for allT_i
inTypes...
.
Converting constructor. Constructs a variant holding 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 declarationT_i x[] = { std::forward<T>(t) };
is valid for some invented variablex
.
Direct-initializes the contained value as if by direct non-list-initialization from std::forward<T>(t)
.
- This overload participates in overload resolution only if
sizeof...(Types) > 0
,std::decay_t<T>
(until C++20) /std::remove_cvref_t<T>
(since C++20) is neither the same type as variant, nor a specialization ofstd::in_place_type_t
, nor a specialization ofstd::in_place_index_t
,std::is_constructible_v<T_j, T>
is true,- and the expression
F(std::forward<T>(t))
(withF
being the above-mentioned set of imaginary functions) is well formed.
- This constructor is a constexpr constructor if
T_j
's selected constructor is a constexpr constructor.
std::variant<std::string> v("abc"); // OK
std::variant<std::string, std::string> w("abc"); // ill-formed
std::variant<std::string, const char*> x("abc"); // OK, chooses const char*
std::variant<std::string, bool> y("abc"); // OK, chooses string; bool is not a candidate
std::variant<float, long, double> z = 0; // OK, holds long
// float and double are not candidates
Constructs a variant with the specified alternative T
and initializes the contained value with the arguments std::forward<Args>(args)...
.
- If
T
's selected constructor is a constexpr constructor, this constructor is also a constexpr constructor. - This overload participates in overload resolution only if there is exactly one occurrence of
T
inTypes...
andstd::is_constructible_v<T, Args...>
is true.
Constructs a variant with the specified alternative T
and initializes the contained value with the arguments il, std::forward<Args>(args)...
.
- If
T
's selected constructor is a constexpr constructor, this constructor is also a constexpr constructor. - This overload participates in overload resolution only if there is exactly one occurrence of
T
inTypes...
andstd::is_constructible_v<T, initializer_list<U>&, Args...>
is true.
Constructs a variant with the alternative T_i
specified by the index I and initializes the contained value with the arguments std::forward<Args>(args)...
.
- If
T_i
's selected constructor is a constexpr constructor, this constructor is also a constexpr constructor. - This overload participates in overload resolution only if I < sizeof...(Types) and
std::is_constructible_v<T_i, Args...>
is true.
Constructs a variant with the alternative T_i
specified by the index I and initializes the contained value with the arguments il, std::forward<Args>(args)...
.
- If
T_i
's selected constructor is a constexpr constructor, this constructor is also a constexpr constructor. - This overload participates in overload resolution only if I < sizeof...(Types) and
std::is_constructible_v<T_i, std::initializer_list<U>&, Args...>
is true.
Parameters
other
- another variant object whose contained value to copy/move
t
- value to initialize the contained value with
args...
- arguments to initialize the contained value with
il
- initializer list to initialize the contained value with
Exceptions
-
May throw any exception thrown by the value initialization of the first alternative. noexcept specification:
noexcept(std::is_nothrow_default_constructible_v<T_0>)
-
May throw any exception thrown by direct-initializing any
T_i
inTypes...
-
May throw any exception thrown by move-constructing any
T_i
inTypes...
. noexcept specification:
noexcept((std::is_nothrow_move_constructible_v<Types> && ...))
-
May throw any exception thrown by the initialization of the selected alternative
T_j
. noexcept specification:
noexcept(std::is_nothrow_constructible_v<T_j, T>)
5-8) May throw any exception thrown by calling the selected constructor of the selected alternative.
Example
#include <cassert>
#include <iostream>
#include <string>
#include <variant>
#include <vector>
using vector_t = std::vector<int>;
auto& operator<<(auto& out, const vector_t& v)
{
out << "{ ";
for (int e: v)
out << e << ' ';
return out << "}";
}
int main()
{
// value-initializes first alternative
std::variant<int, std::string> var0;
assert(std::holds_alternative<int>(var0) and
var0.index() == 0 and
std::get<int>(var0) == 0);
// initializes first alternative with std::string{"STR"};
std::variant<std::string, int> var1{"STR"};
assert(var1.index() == 0);
std::cout << "1) " << std::get<std::string>(var1) << '\n';
// initializes second alternative with int == 42;
std::variant<std::string, int> var2{42};
assert(std::holds_alternative<int>(var2));
std::cout << "2) " << std::get<int>(var2) << '\n';
// initializes first alternative with std::string{4, 'A'};
std::variant<std::string, vector_t, float> var3
{
std::in_place_type<std::string>, 4, 'A'
};
assert(var3.index() == 0);
std::cout << "3) " << std::get<std::string>(var3) << '\n';
// initializes second alternative with std::vector{1,2,3,4,5};
std::variant<std::string, vector_t, char> var4
{
std::in_place_type<vector_t>, {1,2,3,4,5}
};
assert(var4.index() == 1);
std::cout << "4) " << std::get<vector_t>(var4) << '\n';
// initializes first alternative with std::string{"ABCDE", 3};
std::variant<std::string, vector_t, bool> var5 {std::in_place_index<0>, "ABCDE", 3};
assert(var5.index() == 0);
std::cout << "5) " << std::get<std::string>(var5) << '\n';
// initializes second alternative with std::vector(4, 42);
std::variant<std::string, vector_t, char> var6 {std::in_place_index<1>, 4, 42};
assert(std::holds_alternative<vector_t>(var6));
std::cout << "6) " << std::get<vector_t>(var6) << '\n';
}
1) STR
2) 42
3) AAAA
4) { 1 2 3 4 5 }
5) ABC
6) { 42 42 42 42 }
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 2901 | C++17 | allocator-aware constructors provided but variant can't properly support allocators | constructors removed |
P0739R0 | C++17 | converting constructor template interacts poorly with class template argument deduction | constraint added |
LWG 3024 | C++17 | copy constructor doesn't participate in overload resolution if any member type is not copyable | defined as deleted instead |
P0602R4 | C++17 | copy/move constructors may not be trivial even if underlying constructors are trivial | required to propagate triviality |
P0608R3 | C++17 | converting constructor blindly assembles an overload set, leading to unintended conversions | narrowing and boolean conversions not considered |
P1957R2 | C++17 | converting constructor for bool did not allow implicit conversion | Pointer to bool conversion is narrowing and converting constructor has no exception for bool |