std::variant<Types...>::variant
constexpr variant() noexcept(/* see below */); | (1) | (since C++17) |
constexpr variant(const variant& other); | (2) | (since C++17) |
constexpr variant(variant&& other) noexcept(/* see below */); | (3) | (since C++17) |
template< class T > constexpr variant(T&& t) noexcept(/* see below */); | (4) | (since C++17) |
template< class T, class... Args > constexpr explicit variant(std::in_place_type_t<T>, Args&&... args); | (5) | (since C++17) |
template< class T, class U, class... Args > constexpr explicit variant(std::in_place_type_t<T>, std::initializer_list<U> il, Args&&... args); | (6) | (since C++17) |
template< std::size_t I, class... Args > constexpr explicit variant(std::in_place_index_t<I>, Args&&... args); | (7) | (since C++17) |
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); | (8) | (since C++17) |
Constructs a new variant
object.
1) 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 only participates in overload resolution if std::is_default_constructible_v<T_0>
is true.
2) 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 all T_i
in Types...
. It is trivial if std::is_trivially_copy_constructible_v<T_i>
is true for all T_i
in Types...
.
3) 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 only participates in overload resolution if std::is_move_constructible_v<T_i>
is true for all T_i
in Types...
. It is trivial if std::is_trivially_move_constructible_v<T_i>
is true for all T_i
in Types...
.
4) 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
; - If
T_i
is (possibly cv-qualified)bool
,F(T_i)
is only considered ifstd:remove_cvref_t<T>
is alsobool
.
Direct-initializes the contained value as if by direct non-list-initialization from
std::forward<T>(t)
. This overload only participates in overload resolution if sizeof...(Types) > 0
, std::decay_t<U>
(until C++20)std::remove_cvref_t<U>
(since C++20) is neither the same type as variant
, nor a specialization of std::in_place_type_t
, nor a specialization of std::in_place_index_t
, 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. 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
5) 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 only participates in overload resolution if there is exactly one occurrence of T in Types...
and std::is_constructible_v<T, Args...>
is true
.
6) 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 only participates in overload resolution if there is exactly one occurrence of T in Types...
and std::is_constructible_v<T, initializer_list<U>&, Args...>
is true
.
7) 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 only participates in overload resolution if I < sizeof...(Types)
and std::is_constructible_v<T_i, Args...>
is true.
8) 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 only participates in overload resolution 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
1) May throw any exception thrown by the value initialization of the first alternative.
noexcept
specification: noexcept(std::is_nothrow_default_constructible_v<T_0>)
2) May throw any exception thrown by direct-initializing any T_i in
Types...
3) May throw any exception thrown by move-constructing any T_i in
Types...
. noexcept
specification: noexcept( (std::is_nothrow_move_constructible_v<Types> && ...))
4) 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
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 |
Example
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