std::variant (3) - Linux Manuals

std::variant: std::variant

NAME

std::variant - std::variant

Synopsis

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

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

                       constructs the variant object
constructor (public member function)
                       destroys the variant, along with its contained value
destructor (public member function)
                       assigns a variant
operator= (public member function)

Observers

                       returns the zero-based index of the alternative held by the variant
index (public member function)
                       checks if the variant is in the invalid state
valueless_by_exception (public member function)

Modifiers

                       constructs a value in the variant, in place
emplace (public member function)
                       swaps with another variant
swap (public member function)

Non-member functions

visit calls the provided functor with the arguments held by one or more variants
                        (function template)
(C++17)

holds_alternative checks if a variant currently holds a given type
                        (function template)
(C++17)

std::get(std::variant) reads the value of the variant given the index or the type (if the type is unique), throws on error
                        (function template)
(C++17)

get_if 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)

operator==
operator!=
operator< compares variant objects as their contained values
operator<= (function template)
operator>
operator>=

(C++17)

std::swap(std::variant) specializes the std::swap algorithm
                        (function)
(C++17)

Helper classes

monostate placeholder type for use as the first alternative in a variant of non-default-constructible types
                        (class)
(C++17)

bad_variant_access exception thrown on invalid accesses to the value of a variant
                        (class)
(C++17)

variant_size obtains the size of the variant's list of alternatives at compile time
variant_size_v (class template) (variable template)

(C++17)

variant_alternative obtains the type of the alternative specified by its index, at compile time
variant_alternative_t (class template) (alias template)

(C++17)

std::hash<std::variant> specializes the std::hash algorithm
                        (class template specialization)
(C++17)

Helper objects

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

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>

  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&) {}

      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
  }