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

From cppreference.com
< cpp‎ | utility‎ | variant
 
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.
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.
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
  • 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.

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
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/variant/variant&oldid=172821"