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std::variant<Types...>::variant

From cppreference.com
< cpp‎ | utility‎ | variant
 
 
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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.
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.
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 only participates in overload resolution if
  • 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] Example

#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 }

[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
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