std::variant

Defined in header `<variant>`
template <class... Types> class variant;		(since C++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. Empty variants are also ill-formed (std::variant<std::monostate> can be used instead).

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.

Template parameters

Types

-

the types that may be stored in this variant. All types must be (possibly cv-qualified) non-array object types.

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)

Non-member functions

visit (C++17)	calls the provided functor with the arguments held by one or more variants (function template)
holds_alternative (C++17)	checks if a variant currently holds a given type (function template)
std::get(std::variant) (C++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 (C++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>= (C++17)	compares `variant` objects as their contained values (function template)
std::swap(std::variant) (C++17)	specializes the std::swap algorithm (function)

Helper classes

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

Helper objects

variant_npos

(C++17)

index of the variant in the invalid state
(constant)

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	specialization of std::uses_allocator provided, but `variant` can't properly support allocators	specialization removed

Example

Run this code

#include <variant>
#include <string>
#include <cassert>

using namespace std::literals;

int main()
{
    std::variant<int, float> v, w;
    v = 12; // v contains int
    int i = std::get<int>(v);
    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&) {}

    std::variant<std::string> x("abc"); // converting constructors work when unambiguous
    x = "def"; // converting assignment also works when unambiguous

    std::variant<std::string, bool> y("abc"); // casts to bool when passed a char const *
    assert(std::holds_alternative<bool>(y)); // succeeds
    y = "xyz"s;
    assert(std::holds_alternative<std::string>(y)); //succeeds
}

Language
Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

in_place in_place_type in_place_index in_place_t in_place_type_t in_place_index_t (C++17)	in-place construction tag (class template)
optional (C++17)	a wrapper that may or may not hold an object (class template)
any (C++17)	Objects that hold instances of any CopyConstructible type. (class)