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

Constructs a new variant object.

Declarations

// 1)
constexpr variant() noexcept(/* see below */);
// 2)
constexpr variant( const variant& other );
// 3)
constexpr variant( variant&& other ) noexcept(/* see below */);
// 4)
template< class T >
constexpr variant( T&& t ) noexcept(/* see below */);
// 5)
template< class T,
          class... Args >
constexpr explicit variant( std::in_place_type_t<T>,
                            Args&&... args );
// 6)
template< class T,
          class U,
          class... Args >
constexpr explicit variant( std::in_place_type_t<T>,
                            std::initializer_list<U> il,
                            Args&&... args );
// 7)
template< std::size_t I,
          class... Args >
constexpr explicit variant( std::in_place_index_t<I>,
                            Args&&... args );
// 8)
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 );

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<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.

Example

#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';
}

Result

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