std::variant<Types...>::variant - cppreference.com
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    std::variant<Types...>::variant

    From cppreference.com
     
    C++
     
    Utilities library
    General utilities
    Relational operators (deprecated in C++20)
     
    std::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 participates in overload resolution only 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_if<other.index()>(std::addressof(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::move(*std::get_if<other.index()>(std::addressof(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 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 each T_i in Types..., except that narrowing conversions aren't considered.

    Formally:

    • 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.
    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 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 participates in overload resolution only 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 participates in overload resolution only 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 participates in overload resolution only 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 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

    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.

    Notes

    MSVC STL initially treated P0608R3 as a change in C++20. As of VS 2022 17.12, MSVC STL also treats P0608R3 as a defect report against C++17.

    Example

    Run this code
    #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';
}

    Output: 

    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.