std::variant

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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 meet the Destructible requirements (in particular, array types and non-object types are not allowed).

Member functions

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

Non-member functions

(C++17)
 calls the provided functor with the arguments held by one or more variants
(function template)
(C++17)
 checks if a variant currently holds a given type
(function template)
(C++17)
 reads the value of the variant given the index or the type (if the type is unique), throws on error
(function template)
(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)
(C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)
 compares variant objects as their contained values
(function template)
(C++17)
 specializes the std::swap algorithm
(function template)

Helper classes

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

Helper objects

(C++17)
 index of the variant in the invalid state
(constant)

Notes

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

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 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

See also

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