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_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.

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
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

Category:¶

* conditionally noexcept