< cpp‎ | utility
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

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.

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.


[edit] 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).

[edit] Member functions

constructs the variant object
(public member function) [edit]
destroys the variant, along with its contained value
(public member function) [edit]
assigns a variant
(public member function) [edit]
returns the zero-based index of the alternative held by the variant
(public member function) [edit]
checks if the variant is in the invalid state
(public member function) [edit]
constructs a value in the variant, in place
(public member function) [edit]
swaps with another variant
(public member function) [edit]
calls the provided functor with the argument held by the variant
(public member function) [edit]

[edit] Non-member functions

calls the provided functor with the arguments held by one or more variants
(function template) [edit]
checks if a variant currently holds a given type
(function template) [edit]
reads the value of the variant given the index or the type (if the type is unique), throws on error
(function template) [edit]
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) [edit]
compares variant objects as their contained values
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]

[edit] Helper classes

placeholder type for use as the first alternative in a variant of non-default-constructible types
(class) [edit]
exception thrown on invalid accesses to the value of a variant
(class) [edit]
obtains the size of the variant's list of alternatives at compile time
(class template) (variable template)[edit]
obtains the type of the alternative specified by its index, at compile time
(class template) (alias template)[edit]
hash support for std::variant
(class template specialization) [edit]

[edit] Helper objects

index of the variant in the invalid state
(constant) [edit]

[edit] Notes

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

[edit] Example

#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
        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

[edit] 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 std::variant cannot properly support allocators
specialization removed
LWG 3990 C++17 a program could declare an explicit or
partial specialization of std::variant
the program is ill-formed in this
case (no diagnostic required)

[edit] See also

in-place construction tag
a wrapper that may or may not hold an object
(class template) [edit]
objects that hold instances of any CopyConstructible type
(class) [edit]