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std::variant - cppreference.com

template< class... Types > class variant;

(since C++17)

The class template std::variant represents a type-safe union.

An instance of 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 T object is nested within the variant object.

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 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] Observers 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] Modifiers constructs a value in the variant, in place
(public member function) [edit] swaps with another variant
(public member function) [edit] Visitation calls the provided functor with the argument held by the variant
(public member function) [edit] Non-member functions Helper classes Helper objects Notes 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
 
    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 C++ standards.

DR Applied to Behavior as published Correct behavior LWG 2901 C++17 specialization of std::uses_allocator provided,
but 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) LWG 4141 C++17 the requirement for storage
allocation was confusing the contained object must be
nested within the variant object See also

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