std::variant::variant(3)

NAME
std::variant::variant - std::variant::variant

Synopsis
   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 > (4) (since C++17)
   constexpr variant( T&& t ) noexcept(/* see below */);
   template< class T,
   class... Args > (5) (since C++17)
   constexpr explicit variant( std::in_place_type_t<T>,
   Args&&... args );
   template< class T,
   class U,
   class... Args > (6) (since C++17)
   constexpr explicit variant( std::in_place_type_t<T>,
   std::initializer_list<U> il,
   Args&&... args );
   template< std::size_t I,
   class... Args > (7) (since C++17)
   constexpr explicit variant( std::in_place_index_t<I>,
   Args&&... args );
   template< std::size_t I,
   class U,
   class... Args > (8) (since C++17)
   constexpr explicit variant( std::in_place_index_t<I>,
   std::initializer_list<U> il,
   Args&&... args );
   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<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 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
   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;
   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<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 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

Example
// Run this code
 #include <cassert>
 #include <iostream>
 #include <string>
 #include <variant>
 #include <vector>
 template <class Os> Os& operator<< (Os& os, const std::vector<int>& v) {
     os << "{ ";
     for (int e: v) { std::cout << e << ' '; }
     return os << "}";
 }
 int main()
 {
     {
         std::variant<int, std::string> var; // value-initializes first alternative
         assert(std::holds_alternative<int>(var) && var.index() == 0 &&
                std::get<int>(var) == 0);
     }
     {
         std::variant<std::string, int> var{"STR"};
             // initializes first alternative with std::string{"STR"};
         assert(var.index() == 0);
         std::cout << "1) " << std::get<std::string>(var) << '\n';
     }
     {
         std::variant<std::string, int> var{42};
             // initializes second alternative with int = 42;
         assert(std::holds_alternative<int>(var));
         std::cout << "2) " << std::get<int>(var) << '\n';
     }
     {
         std::variant<std::string, std::vector<int>, float> var{
             std::in_place_type<std::string>, 4, 'A'};
             // initializes first alternative with std::string{4, 'A'};
         assert(var.index() == 0);
         std::cout << "3) " << std::get<std::string>(var) << '\n';
     }
     {
         std::variant<std::string, std::vector<int>, char> var{
             std::in_place_type<std::vector<int>>, {1,2,3,4,5} };
             // initializes second alternative with std::vector{1,2,3,4,5};
         assert(var.index() == 1);
         std::cout << "4) " << std::get<std::vector<int>>(var) << '\n';
     }
     {
         std::variant<std::string, std::vector<int>, bool> var{
             std::in_place_index<0>, "ABCDE", 3};
             // initializes first alternative with std::string{"ABCDE", 3};
         assert(var.index() == 0);
         std::cout << "5) " << std::get<std::string>(var) << '\n';
     }
     {
         std::variant<std::string, std::vector<int>, char> var{
             std::in_place_index<1>, 4, 42};
             // initializes second alternative with std::vector(4, 42);
         assert(std::holds_alternative<std::vector<int>>(var));
         std::cout << "6) " << std::get<std::vector<int>>(var) << '\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.
      DR Applied to Behavior as published Correct behavior
                       allocator-aware constructors
   LWG 2901 C++17 provided but constructors removed
                       variant can't properly support
                       allocators
                       converting constructor template
   P0739R0 C++17 interacts constraint added
                       poorly with class template argument
                       deduction
                       copy constructor doesn't participate
   LWG 3024 C++17 in defined as deleted instead
                       overload resolution if any member
                       type is not copyable
                       copy/move constructors may not be required to propagate
   P0602R4 C++17 trivial even if underlying triviality
                       constructors are trivial
                       converting constructor blindly
   P0608R3 C++17 assembles narrowing and boolean
                       an overload set, leading to conversions not considered
                       unintended conversions
                        Pointer to bool conversion
                       converting constructor for bool did is narrowing and
   P1957R2 C++17 not allow converting constructor has
                       implicit conversion no
                        exception for bool