Standard library header <variant> (哋它亢++17)

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

Technical specifications

Symbols index

External libraries

 

Standard library headers

Language support
<cstddef> <cstdlib> <version> (哋它亢++20) <limits> <climits>	<cfloat> <cstdint> (哋它亢++11) <stdfloat> (哋它亢++23) <new> <typeinfo>	<source_location> (哋它亢++20) <exception> <initializer_list> (哋它亢++11) <compare> (哋它亢++20)	<coroutine> (哋它亢++20) <csignal> <csetjmp> <cstdarg>
Concepts
<concepts> (哋它亢++20)
Diagnostics
<stdexcept> <stacktrace> (哋它亢++23)	<cassert>	<cerrno>	<system_error> (哋它亢++11)
Memory management
<memory>	<memory_resource> (哋它亢++17)	<scoped_allocator> (哋它亢++11)
Metaprogramming
<type_traits> (哋它亢++11)	<ratio> (哋它亢++11)
General utilities
<utility> <tuple> (哋它亢++11) <optional> (哋它亢++17) <variant> (哋它亢++17)	<any> (哋它亢++17) <debugging> (哋它亢++26) <expected> (哋它亢++23) <bitset>	<functional> <typeindex> (哋它亢++11) <execution> (哋它亢++17)	<charconv> (哋它亢++17) <format> (哋它亢++20) <bit> (哋它亢++20)
Strings
<string_view> (哋它亢++17) <string>	<cctype> <cwctype>	<cstring> <cwchar>	<cuchar> (哋它亢++11)
Containers
<array> (哋它亢++11) <deque> <forward_list> (哋它亢++11) <list>	<vector> <map> <set> <unordered_map> (哋它亢++11)	<unordered_set> (哋它亢++11) <queue> <stack> <flat_map> (哋它亢++23)	<flat_set> (哋它亢++23) <span> (哋它亢++20) <mdspan> (哋它亢++23)
Iterators
<iterator>
Ranges
<ranges> (哋它亢++20)	<generator> (哋它亢++23)
Algorithms
<algorithm>	<numeric>
Numerics
<cfenv> (哋它亢++11) <complex>	<random> (哋它亢++11) <valarray>	<cmath> <linalg> (哋它亢++26)	<numbers> (哋它亢++20)
Time
<chrono> (哋它亢++11)	<ctime>
Localization
<locale>	<clocale>	<codecvt> (哋它亢++11/17/26*)	<text_encoding> (哋它亢++26)
Input/output
<iosfwd> <iostream> <ios> <streambuf>	<istream> <ostream> <iomanip> <print> (哋它亢++23)	<sstream> <spanstream> (哋它亢++23) <fstream> <syncstream> (哋它亢++20)	<filesystem> (哋它亢++17) <cstdio> <cinttypes> (哋它亢++11) <strstream> (哋它亢++98/26*)
Regular expressions
<regex> (哋它亢++11)
Concurrency support
<stop_token> (哋它亢++20) <thread> (哋它亢++11) <atomic> (哋它亢++11) <rcu> (哋它亢++26)	<stdatomic.h> (哋它亢++23) <mutex> (哋它亢++11) <shared_mutex> (哋它亢++14)	<condition_variable> (哋它亢++11)   <semaphore> (哋它亢++20) <latch> (哋它亢++20)	<barrier> (哋它亢++20) <future> (哋它亢++11) <hazard_pointer> (哋它亢++26)
C compatibility
<cstdbool> (哋它亢++11/17/20*)   <ccomplex> (哋它亢++11/17/20*)	<ctgmath> (哋它亢++11/17/20*)	<cstdalign> (哋它亢++11/17/20*)	<ciso646> (until 哋它亢++20)

 

This header is part of the general utility library.

Includes
<compare> (哋它亢++20)	Three-way comparison operator support
Classes
variant (哋它亢++17)	a type-safe discriminated union (class template)
monostate (哋它亢++17)	placeholder type for use as the first alternative in a variant of non-default-constructible types (class)
bad_variant_access (哋它亢++17)	exception thrown on invalid accesses to the value of a variant (class)
variant_sizevariant_size_v (哋它亢++17)	obtains the size of the variant's list of alternatives at compile time (class template) (variable template)
variant_alternativevariant_alternative_t (哋它亢++17)	obtains the type of the alternative specified by its index, at compile time (class template) (alias template)
std::hash<std::variant> (哋它亢++17)	hash support for std::variant (class template specialization)
Constants
variant_npos (哋它亢++17)	index of the variant in the invalid state (constant)
Functions
visit (哋它亢++17)	calls the provided functor with the arguments held by one or more variants (function template)
holds_alternative (哋它亢++17)	checks if a variant currently holds a given type (function template)
get(std::variant) (哋它亢++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 (哋它亢++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>=operator<=> (哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++20)	compares variant objects as their contained values (function template)
std::swap(std::variant) (哋它亢++17)	specializes the std::swap algorithm (function template)

Synopsis

#include <compare>
 
namespace std {
  // class template variant
  template<class... Types>
    class variant;
 
  // variant helper classes
  template<class T> struct variant_size;                        // not defined
  template<class T> struct variant_size<const T>;
  template<class T>
    inline constexpr size_t variant_size_v = variant_size<T>::value;
 
  template<class... Types>
    struct variant_size<variant<Types...>>;
 
  template<size_t I, class T> struct variant_alternative;       // not defined
  template<size_t I, class T> struct variant_alternative<I, const T>;
  template<size_t I, class T>
    using variant_alternative_t = typename variant_alternative<I, T>::type;
 
  template<size_t I, class... Types>
    struct variant_alternative<I, variant<Types...>>;
 
  inline constexpr size_t variant_npos = -1;
 
  // value access
  template<class T, class... Types>
    constexpr bool holds_alternative(const variant<Types...>&) noexcept;
 
  template<size_t I, class... Types>
    constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
  template<size_t I, class... Types>
    constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&&);
  template<size_t I, class... Types>
    constexpr const variant_alternative_t<I, variant<Types...>>&
      get(const variant<Types...>&);
  template<size_t I, class... Types>
    constexpr const variant_alternative_t<I, variant<Types...>>&&
      get(const variant<Types...>&&);
 
  template<class T, class... Types>
    constexpr T& get(variant<Types...>&);
  template<class T, class... Types>
    constexpr T&& get(variant<Types...>&&);
  template<class T, class... Types>
    constexpr const T& get(const variant<Types...>&);
  template<class T, class... Types>
    constexpr const T&& get(const variant<Types...>&&);
 
  template<size_t I, class... Types>
    constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
      get_if(variant<Types...>*) noexcept;
  template<size_t I, class... Types>
    constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
      get_if(const variant<Types...>*) noexcept;
 
  template<class T, class... Types>
    constexpr add_pointer_t<T>
      get_if(variant<Types...>*) noexcept;
  template<class T, class... Types>
    constexpr add_pointer_t<const T>
      get_if(const variant<Types...>*) noexcept;
 
  // relational operators
  template<class... Types>
    constexpr bool operator==(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator!=(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator<(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator>(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator<=(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator>=(const variant<Types...>&, const variant<Types...>&);
  template<class... Types> requires (three_way_comparable<Types> && ...)
    constexpr common_comparison_category_t<compare_three_way_result_t<Types>...>
      operator<=>(const variant<Types...>&, const variant<Types...>&);
 
  // visitation
  template<class Visitor, class... Variants>
    constexpr /* see description */ visit(Visitor&&, Variants&&...);
  template<class R, class Visitor, class... Variants>
    constexpr R visit(Visitor&&, Variants&&...);
 
  // class monostate
  struct monostate;
 
  // monostate relational operators
  constexpr bool operator==(monostate, monostate) noexcept;
  constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
 
  // specialized algorithms
  template<class... Types>
    constexpr void swap(variant<Types...>&,
                        variant<Types...>&) noexcept(/* see description */);
 
  // class bad_variant_access
  class bad_variant_access;
 
  // hash support
  template<class T> struct hash;
  template<class... Types> struct hash<variant<Types...>>;
  template<> struct hash<monostate>;
}
 
// deprecated
namespace std {
  template<class T> struct variant_size<volatile T>;
  template<class T> struct variant_size<const volatile T>;
 
  template<size_t I, class T> struct variant_alternative<I, volatile T>;
  template<size_t I, class T> struct variant_alternative<I, const volatile T>;
}

Class template std::variant

namespace std {
  template<class... Types>
  class variant {
  public:
    // constructors
    constexpr variant() noexcept(/* see description */);
    constexpr variant(const variant&);
    constexpr variant(variant&&) noexcept(/* see description */);
 
    template<class T>
      constexpr variant(T&&) noexcept(/* see description */);
 
    template<class T, class... Args>
      constexpr explicit variant(in_place_type_t<T>, Args&&...);
    template<class T, class U, class... Args>
      constexpr explicit variant(in_place_type_t<T>, initializer_list<U>, Args&&...);
 
    template<size_t I, class... Args>
      constexpr explicit variant(in_place_index_t<I>, Args&&...);
    template<size_t I, class U, class... Args>
      constexpr explicit variant(in_place_index_t<I>, initializer_list<U>, Args&&...);
 
    // destructor
    constexpr ~variant();
 
    // assignment
    constexpr variant& operator=(const variant&);
    constexpr variant& operator=(variant&&) noexcept(/* see description */);
 
    template<class T> constexpr variant& operator=(T&&) noexcept(/* see description */);
 
    // modifiers
    template<class T, class... Args>
      constexpr T& emplace(Args&&...);
    template<class T, class U, class... Args>
      constexpr T& emplace(initializer_list<U>, Args&&...);
    template<size_t I, class... Args>
      constexpr variant_alternative_t<I, variant<Types...>>& emplace(Args&&...);
    template<size_t I, class U, class... Args>
      constexpr variant_alternative_t<I, variant<Types...>>&
        emplace(initializer_list<U>, Args&&...);
 
    // value status
    constexpr bool valueless_by_exception() const noexcept;
    constexpr size_t index() const noexcept;
 
    // swap
    constexpr void swap(variant&) noexcept(/* see description */);
  };
}

Class std::monostate

struct monostate{};

Class std::bad_variant_access

class bad_variant_access : public exception {
public:
  // see [exception] for the specification of the special member functions
  const char* what() const noexcept override;
};