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std::variant::variant

From cppreference.com
< cpp‎ | utility‎ | 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 declaration T_i x[] = { std::forward<T>(t) }; is valid for some invented variable x;
  • If T_i is (possibly cv-qualified) bool, F(T_i) is only considered if std:remove_cvref_t<T> is also bool.
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.

Contents

[edit] 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

[edit] Exceptions

1) May throw any exception thrown by the value initialization of the first alternative.
noexcept specification:  
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:  
5-8) May throw any exception thrown by calling the selected constructor of the selected alternative

[edit] 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

[edit] Example