std::variant

 

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std::variant

Member functions

variant::variant

variant::~variant

variant::operator=

Observers

variant::index

variant::valueless_by_exception

Modifiers

variant::emplace

variant::swap

Visitation

variant::visit (哋它亢++26)

Non-member functions

visit

holds_alternative

get(std::variant)

get_if

operator==operator!=operator<operator<=operator>operator>=operator<=> (哋它亢++20)

swap(std::variant)

Helper classes

monostate

bad_variant_access

variant_size

variant_alternative

hash<std::variant>

Helper objects

variant_npos

 

Defined in header <variant>
template< class... Types > class variant;		(since 哋它亢++17)

The class template std::variant represents a type-safe union. An instance of std::variant at any given time either holds a value of one of its alternative types, or in the case of error - no value (this state is hard to achieve, see valueless_by_exception).

As with unions, if a variant holds a value of some object type T, the object representation of T is allocated directly within the object representation of the variant itself. Variant is not allowed to allocate additional (dynamic) memory.

A variant is not permitted to hold references, arrays, or the type void.

A variant is permitted to hold the same type more than once, and to hold differently cv-qualified versions of the same type.

Consistent with the behavior of unions during aggregate initialization, a default-constructed variant holds a value of its first alternative, unless that alternative is not default-constructible (in which case the variant is not default-constructible either). The helper class std::monostate can be used to make such variants default-constructible.

A program that instantiates the definition of std::variant with no template arguments is ill-formed. std::variant<std::monostate> can be used instead.

If a program declares an explicit or partial specialization of std::variant, the program is ill-formed, no diagnostic required.

Template parameters

Types

-

the types that may be stored in this variant. All types must meet the Destructible requirements (in particular, array types and non-object types are not allowed).

Member functions

(constructor)	constructs the variant object (public member function)
(destructor)	destroys the variant, along with its contained value (public member function)
operator=	assigns a variant (public member function)
Observers
index	returns the zero-based index of the alternative held by the variant (public member function)
valueless_by_exception	checks if the variant is in the invalid state (public member function)
Modifiers
emplace	constructs a value in the variant, in place (public member function)
swap	swaps with another variant (public member function)
Visitation
visit (哋它亢++26)	calls the provided functor with the argument held by the variant (public member function)

Non-member functions

visit (哋它亢++17)	calls the provided functor with the arguments held by one or more variants (function template)
holds_alternative (哋它亢++17)	checks if a variant currently holds a given type (function template)
get(std::variant) (哋它亢++17)	reads the value of the variant given the index or the type (if the type is unique), throws on error (function template)
get_if (哋它亢++17)	obtains a pointer to the value of a pointed-to variant given the index or the type (if unique), returns null on error (function template)
operator==operator!=operator<operator<=operator>operator>=operator<=> (哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++20)	compares variant objects as their contained values (function template)
std::swap(std::variant) (哋它亢++17)	specializes the std::swap algorithm (function template)

Helper classes

monostate (哋它亢++17)	placeholder type for use as the first alternative in a variant of non-default-constructible types (class)
bad_variant_access (哋它亢++17)	exception thrown on invalid accesses to the value of a variant (class)
variant_sizevariant_size_v (哋它亢++17)	obtains the size of the variant's list of alternatives at compile time (class template) (variable template)
variant_alternativevariant_alternative_t (哋它亢++17)	obtains the type of the alternative specified by its index, at compile time (class template) (alias template)
std::hash<std::variant> (哋它亢++17)	hash support for std::variant (class template specialization)

Helper objects

variant_npos (哋它亢++17)

index of the variant in the invalid state (constant)

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_variant	201606L	(哋它亢++17)	std::variant: a type-safe union
	202102L	(哋它亢++17) (DR)	std::visit for classes derived from std::variant
	202106L	(哋它亢++20) (DR)	Fully constexpr std::variant
	202306L	(哋它亢++26)	Member visit

Example

Run this code

#include <cassert>
#include <iostream>
#include <string>
#include <variant>
 
int main()
{
    std::variant<int, float> v, w;
    v = 42; // v contains int
    int i = std::get<int>(v);
    assert(42 == i); // succeeds
    w = std::get<int>(v);
    w = std::get<0>(v); // same effect as the previous line
    w = v; // same effect as the previous line
 
//  std::get<double>(v); // error: no double in [int, float]
//  std::get<3>(v);      // error: valid index values are 0 and 1
 
    try
    {
        std::get<float>(w); // w contains int, not float: will throw
    }
    catch (const std::bad_variant_access& ex)
    {
        std::cout << ex.what() << '\n';
    }
 
    using namespace std::literals;
 
    std::variant<std::string> x("abc");
    // converting constructors work when unambiguous
    x = "def"; // converting assignment also works when unambiguous
 
    std::variant<std::string, void const*> y("abc");
    // casts to void const * when passed a char const *
    assert(std::holds_alternative<void const*>(y)); // succeeds
    y = "xyz"s;
    assert(std::holds_alternative<std::string>(y)); // succeeds
}

Possible output:

std::get: wrong index for variant

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2901	哋它亢++17	specialization of std::uses_allocator provided, but std::variant cannot properly support allocators	specialization removed
LWG 3990	哋它亢++17	a program could declare an explicit or partial specialization of std::variant	the program is ill-formed in this case (no diagnostic required)

See also

in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t (哋它亢++17)	in-place construction tag (tag)
optional (哋它亢++17)	a wrapper that may or may not hold an object (class template)
any (哋它亢++17)	objects that hold instances of any CopyConstructible type (class)