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variant
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// <variant> -*- C++ -*-
// Copyright (C) 2016-2025 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
// <http://www.gnu.org/licenses/>.
/** @file variant
* This is the `<variant>` C++ Library header.
*/
#ifndef _GLIBCXX_VARIANT
#define _GLIBCXX_VARIANT 1
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
#define __glibcxx_want_freestanding_variant
#define __glibcxx_want_variant
#define __glibcxx_want_constrained_equality
#include <bits/version.h>
#ifdef __cpp_lib_variant // C++ >= 17
#include <initializer_list>
#include <type_traits>
#include <bits/enable_special_members.h>
#include <bits/exception_defines.h>
#include <bits/functional_hash.h>
#include <bits/invoke.h>
#include <bits/monostate.h>
#include <bits/parse_numbers.h> // _Select_int
#include <bits/stl_iterator_base_funcs.h>
#include <bits/stl_construct.h>
#include <bits/utility.h> // in_place_index_t
#if __cplusplus >= 202002L
# include <concepts>
# include <compare>
#endif
// C++ < 20 || __cpp_concepts < 202002L || __cpp_constexpr < 201811L
#if __cpp_lib_variant < 202106L
# include <ext/aligned_buffer.h> // Use __aligned_membuf for storage.
#endif
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename... _Types> class tuple;
template<typename... _Types> class variant;
template<typename _Variant>
struct variant_size;
template<typename _Variant>
struct variant_size<const _Variant> : variant_size<_Variant> {};
template<typename _Variant>
struct variant_size<volatile _Variant> : variant_size<_Variant> {};
template<typename _Variant>
struct variant_size<const volatile _Variant> : variant_size<_Variant> {};
template<typename... _Types>
struct variant_size<variant<_Types...>>
: std::integral_constant<size_t, sizeof...(_Types)> {};
template<typename _Variant>
inline constexpr size_t variant_size_v = variant_size<_Variant>::value;
template<typename... _Types>
inline constexpr size_t
variant_size_v<variant<_Types...>> = sizeof...(_Types);
template<typename... _Types>
inline constexpr size_t
variant_size_v<const variant<_Types...>> = sizeof...(_Types);
template<size_t _Np, typename _Variant>
struct variant_alternative;
template<size_t _Np, typename... _Types>
struct variant_alternative<_Np, variant<_Types...>>
{
static_assert(_Np < sizeof...(_Types));
using type = typename _Nth_type<_Np, _Types...>::type;
};
template<size_t _Np, typename _Variant>
using variant_alternative_t =
typename variant_alternative<_Np, _Variant>::type;
template<size_t _Np, typename _Variant>
struct variant_alternative<_Np, const _Variant>
{ using type = const variant_alternative_t<_Np, _Variant>; };
template<size_t _Np, typename _Variant>
struct variant_alternative<_Np, volatile _Variant>
{ using type = volatile variant_alternative_t<_Np, _Variant>; };
template<size_t _Np, typename _Variant>
struct variant_alternative<_Np, const volatile _Variant>
{ using type = const volatile variant_alternative_t<_Np, _Variant>; };
inline constexpr size_t variant_npos = -1;
template<size_t _Np, typename... _Types>
constexpr variant_alternative_t<_Np, variant<_Types...>>&
get(variant<_Types...>&);
template<size_t _Np, typename... _Types>
constexpr variant_alternative_t<_Np, variant<_Types...>>&&
get(variant<_Types...>&&);
template<size_t _Np, typename... _Types>
constexpr variant_alternative_t<_Np, variant<_Types...>> const&
get(const variant<_Types...>&);
template<size_t _Np, typename... _Types>
constexpr variant_alternative_t<_Np, variant<_Types...>> const&&
get(const variant<_Types...>&&);
template<typename _Result_type, typename _Visitor, typename... _Variants>
constexpr decltype(auto)
__do_visit(_Visitor&& __visitor, _Variants&&... __variants);
template <typename... _Types, typename _Tp>
_GLIBCXX20_CONSTEXPR
decltype(auto)
__variant_cast(_Tp&& __rhs)
{
if constexpr (is_lvalue_reference_v<_Tp>)
{
if constexpr (is_const_v<remove_reference_t<_Tp>>)
return static_cast<const variant<_Types...>&>(__rhs);
else
return static_cast<variant<_Types...>&>(__rhs);
}
else
return static_cast<variant<_Types...>&&>(__rhs);
}
namespace __detail
{
namespace __variant
{
// used for raw visitation
struct __variant_cookie {};
// used for raw visitation with indices passed in
struct __variant_idx_cookie { using type = __variant_idx_cookie; };
// Used to enable deduction (and same-type checking) for std::visit:
template<typename _Tp> struct __deduce_visit_result { using type = _Tp; };
// Visit variants that might be valueless.
template<typename _Visitor, typename... _Variants>
constexpr void
__raw_visit(_Visitor&& __visitor, _Variants&&... __variants)
{
std::__do_visit<__variant_cookie>(std::forward<_Visitor>(__visitor),
std::forward<_Variants>(__variants)...);
}
// Visit variants that might be valueless, passing indices to the visitor.
template<typename _Visitor, typename... _Variants>
constexpr void
__raw_idx_visit(_Visitor&& __visitor, _Variants&&... __variants)
{
std::__do_visit<__variant_idx_cookie>(std::forward<_Visitor>(__visitor),
std::forward<_Variants>(__variants)...);
}
// The __as function templates implement the exposition-only "as-variant"
template<typename... _Types>
constexpr std::variant<_Types...>&
__as(std::variant<_Types...>& __v) noexcept
{ return __v; }
template<typename... _Types>
constexpr const std::variant<_Types...>&
__as(const std::variant<_Types...>& __v) noexcept
{ return __v; }
template<typename... _Types>
constexpr std::variant<_Types...>&&
__as(std::variant<_Types...>&& __v) noexcept
{ return std::move(__v); }
template<typename... _Types>
constexpr const std::variant<_Types...>&&
__as(const std::variant<_Types...>&& __v) noexcept
{ return std::move(__v); }
#if __cpp_lib_variant < 202106L
template<typename _Type, bool = std::is_trivially_destructible_v<_Type>>
struct _Uninitialized;
#else
template<typename _Type, bool /* unused */ = true>
struct _Uninitialized;
#endif
// The primary template is used for trivially destructible types in C++17,
// and for all types in C++20.
template<typename _Type, bool>
struct _Uninitialized
{
template<typename... _Args>
constexpr
_Uninitialized(in_place_index_t<0>, _Args&&... __args)
: _M_storage(std::forward<_Args>(__args)...)
{ }
#if __cpp_lib_variant < 202106L
constexpr const _Type& _M_get() const & noexcept
{ return _M_storage; }
constexpr _Type& _M_get() & noexcept
{ return _M_storage; }
constexpr const _Type&& _M_get() const && noexcept
{ return std::move(_M_storage); }
constexpr _Type&& _M_get() && noexcept
{ return std::move(_M_storage); }
#endif
_Type _M_storage;
};
#if __cpp_lib_variant < 202106L
// This partial specialization is used for non-trivially destructible types
// in C++17, so that _Uninitialized<T> is trivially destructible and can be
// used as a union member in _Variadic_union.
template<typename _Type>
struct _Uninitialized<_Type, false>
{
template<typename... _Args>
constexpr
_Uninitialized(in_place_index_t<0>, _Args&&... __args)
{
::new ((void*)std::addressof(_M_storage))
_Type(std::forward<_Args>(__args)...);
}
const _Type& _M_get() const & noexcept
{ return *_M_storage._M_ptr(); }
_Type& _M_get() & noexcept
{ return *_M_storage._M_ptr(); }
const _Type&& _M_get() const && noexcept
{ return std::move(*_M_storage._M_ptr()); }
_Type&& _M_get() && noexcept
{ return std::move(*_M_storage._M_ptr()); }
__gnu_cxx::__aligned_membuf<_Type> _M_storage;
};
template<size_t _Np, typename _Union>
constexpr decltype(auto)
__get_n(_Union&& __u) noexcept
{
if constexpr (_Np == 0)
return std::forward<_Union>(__u)._M_first._M_get();
else if constexpr (_Np == 1)
return std::forward<_Union>(__u)._M_rest._M_first._M_get();
else if constexpr (_Np == 2)
return std::forward<_Union>(__u)._M_rest._M_rest._M_first._M_get();
else
return __variant::__get_n<_Np - 3>(
std::forward<_Union>(__u)._M_rest._M_rest._M_rest);
}
#else
template<size_t _Np, typename _Union>
constexpr auto&&
__get_n(_Union&& __u) noexcept
{
if constexpr (_Np == 0)
return std::forward<_Union>(__u)._M_first._M_storage;
else if constexpr (_Np == 1)
return std::forward<_Union>(__u)._M_rest._M_first._M_storage;
else if constexpr (_Np == 2)
return std::forward<_Union>(__u)._M_rest._M_rest._M_first._M_storage;
else
return __variant::__get_n<_Np - 3>(
std::forward<_Union>(__u)._M_rest._M_rest._M_rest);
}
#endif
// Returns the typed storage for __v.
template<size_t _Np, typename _Variant>
constexpr decltype(auto)
__get(_Variant&& __v) noexcept
{ return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
// Gets the _Uninitialized to construct into for __u.
template<size_t _Np, typename _Union>
constexpr decltype(auto)
__construct_n(_Union& __u) noexcept
{
if constexpr (_Np == 0)
return &__u._M_first;
else if constexpr (_Np == 1)
{
std::_Construct(&__u._M_rest);
return &__u._M_rest._M_first;
}
else if constexpr (_Np == 2)
{
std::_Construct(&__u._M_rest);
std::_Construct(&__u._M_rest._M_rest);
return &__u._M_rest._M_rest._M_first;
}
else
{
std::_Construct(&__u._M_rest);
std::_Construct(&__u._M_rest._M_rest);
std::_Construct(&__u._M_rest._M_rest._M_rest);
return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
}
}
template<typename... _Types>
struct _Traits
{
static constexpr bool _S_default_ctor =
is_default_constructible_v<typename _Nth_type<0, _Types...>::type>;
static constexpr bool _S_copy_ctor =
(is_copy_constructible_v<_Types> && ...);
static constexpr bool _S_move_ctor =
(is_move_constructible_v<_Types> && ...);
static constexpr bool _S_copy_assign =
_S_copy_ctor
&& (is_copy_assignable_v<_Types> && ...);
static constexpr bool _S_move_assign =
_S_move_ctor
&& (is_move_assignable_v<_Types> && ...);
static constexpr bool _S_trivial_dtor =
(is_trivially_destructible_v<_Types> && ...);
static constexpr bool _S_trivial_copy_ctor =
(is_trivially_copy_constructible_v<_Types> && ...);
static constexpr bool _S_trivial_move_ctor =
(is_trivially_move_constructible_v<_Types> && ...);
static constexpr bool _S_trivial_copy_assign =
_S_trivial_dtor && _S_trivial_copy_ctor
&& (is_trivially_copy_assignable_v<_Types> && ...);
static constexpr bool _S_trivial_move_assign =
_S_trivial_dtor && _S_trivial_move_ctor
&& (is_trivially_move_assignable_v<_Types> && ...);
// The following nothrow traits are for non-trivial SMFs. Trivial SMFs
// are always nothrow.
static constexpr bool _S_nothrow_default_ctor =
is_nothrow_default_constructible_v<
typename _Nth_type<0, _Types...>::type>;
static constexpr bool _S_nothrow_copy_ctor = false;
static constexpr bool _S_nothrow_move_ctor =
(is_nothrow_move_constructible_v<_Types> && ...);
static constexpr bool _S_nothrow_copy_assign = false;
static constexpr bool _S_nothrow_move_assign =
_S_nothrow_move_ctor
&& (is_nothrow_move_assignable_v<_Types> && ...);
};
// Defines members and ctors.
template<bool __trivially_destructible, typename... _Types>
union _Variadic_union
{
_Variadic_union() = default;
template<size_t _Np, typename... _Args>
_Variadic_union(in_place_index_t<_Np>, _Args&&...) = delete;
};
template<bool __trivially_destructible, typename _First, typename... _Rest>
union _Variadic_union<__trivially_destructible, _First, _Rest...>
{
constexpr _Variadic_union() : _M_rest() { }
template<typename... _Args>
constexpr
_Variadic_union(in_place_index_t<0>, _Args&&... __args)
: _M_first(in_place_index<0>, std::forward<_Args>(__args)...)
{ }
template<size_t _Np, typename... _Args>
constexpr
_Variadic_union(in_place_index_t<_Np>, _Args&&... __args)
: _M_rest(in_place_index<_Np-1>, std::forward<_Args>(__args)...)
{ }
#if __cpp_lib_variant >= 202106L
_Variadic_union(const _Variadic_union&) = default;
_Variadic_union(_Variadic_union&&) = default;
_Variadic_union& operator=(const _Variadic_union&) = default;
_Variadic_union& operator=(_Variadic_union&&) = default;
~_Variadic_union() = default;
// If any alternative type is not trivially destructible then we need a
// user-provided destructor that does nothing. The active alternative
// will be destroyed by _Variant_storage::_M_reset() instead of here.
constexpr ~_Variadic_union()
requires (!__trivially_destructible)
{ }
#endif
_Uninitialized<_First> _M_first;
_Variadic_union<__trivially_destructible, _Rest...> _M_rest;
};
// _Never_valueless_alt is true for variant alternatives that can
// always be placed in a variant without it becoming valueless.
// For suitably-small, trivially copyable types we can create temporaries
// on the stack and then memcpy them into place.
template<typename _Tp>
struct _Never_valueless_alt
: __and_<bool_constant<sizeof(_Tp) <= 256>, is_trivially_copyable<_Tp>>
{ };
// Specialize _Never_valueless_alt for other types which have a
// non-throwing and cheap move construction and move assignment operator,
// so that emplacing the type will provide the strong exception-safety
// guarantee, by creating and moving a temporary.
// Whether _Never_valueless_alt<T> is true or not affects the ABI of a
// variant using that alternative, so we can't change the value later!
// True if every alternative in _Types... can be emplaced in a variant
// without it becoming valueless. If this is true, variant<_Types...>
// can never be valueless, which enables some minor optimizations.
template <typename... _Types>
constexpr bool __never_valueless()
{
return _Traits<_Types...>::_S_move_assign
&& (_Never_valueless_alt<_Types>::value && ...);
}
// Defines index and the dtor, possibly trivial.
template<bool __trivially_destructible, typename... _Types>
struct _Variant_storage;
template <typename... _Types>
using __select_index =
typename __select_int::_Select_int_base<sizeof...(_Types),
unsigned char,
unsigned short>::type::value_type;
template<typename... _Types>
struct _Variant_storage<false, _Types...>
{
constexpr
_Variant_storage()
: _M_index(static_cast<__index_type>(variant_npos))
{ }
template<size_t _Np, typename... _Args>
constexpr
_Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
: _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
_M_index{_Np}
{ }
constexpr void
_M_reset()
{
if (!_M_valid()) [[__unlikely__]]
return;
std::__do_visit<void>([](auto&& __this_mem) mutable
{
std::_Destroy(std::__addressof(__this_mem));
}, __variant_cast<_Types...>(*this));
_M_index = static_cast<__index_type>(variant_npos);
}
_GLIBCXX20_CONSTEXPR
~_Variant_storage()
{ _M_reset(); }
constexpr bool
_M_valid() const noexcept
{
if constexpr (__variant::__never_valueless<_Types...>())
return true;
return this->_M_index != __index_type(variant_npos);
}
_Variadic_union<false, _Types...> _M_u;
using __index_type = __select_index<_Types...>;
__index_type _M_index;
};
template<typename... _Types>
struct _Variant_storage<true, _Types...>
{
constexpr
_Variant_storage()
: _M_index(static_cast<__index_type>(variant_npos))
{ }
template<size_t _Np, typename... _Args>
constexpr
_Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
: _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
_M_index{_Np}
{ }
constexpr void
_M_reset() noexcept
{ _M_index = static_cast<__index_type>(variant_npos); }
constexpr bool
_M_valid() const noexcept
{
if constexpr (__variant::__never_valueless<_Types...>())
return true;
// It would be nice if we could just return true for -fno-exceptions.
// It's possible (but inadvisable) that a std::variant could become
// valueless in a translation unit compiled with -fexceptions and then
// be passed to functions compiled with -fno-exceptions. We would need
// some #ifdef _GLIBCXX_NO_EXCEPTIONS_GLOBALLY property to elide all
// checks for valueless_by_exception().
return this->_M_index != static_cast<__index_type>(variant_npos);
}
_Variadic_union<true, _Types...> _M_u;
using __index_type = __select_index<_Types...>;
__index_type _M_index;
};
// Implementation of v.emplace<N>(args...).
template<size_t _Np, bool _Triv, typename... _Types, typename... _Args>
_GLIBCXX20_CONSTEXPR
inline void
__emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
{
__v._M_reset();
auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
std::_Construct(__addr, in_place_index<0>,
std::forward<_Args>(__args)...);
// Construction didn't throw, so can set the new index now:
__v._M_index = _Np;
}
template<typename... _Types>
using _Variant_storage_alias =
_Variant_storage<_Traits<_Types...>::_S_trivial_dtor, _Types...>;
// The following are (Copy|Move) (ctor|assign) layers for forwarding
// triviality and handling non-trivial SMF behaviors.
template<bool, typename... _Types>
struct _Copy_ctor_base : _Variant_storage_alias<_Types...>
{
using _Base = _Variant_storage_alias<_Types...>;
using _Base::_Base;
_GLIBCXX20_CONSTEXPR
_Copy_ctor_base(const _Copy_ctor_base& __rhs)
noexcept(_Traits<_Types...>::_S_nothrow_copy_ctor)
{
__variant::__raw_idx_visit(
[this](auto&& __rhs_mem, auto __rhs_index) mutable
{
constexpr size_t __j = __rhs_index;
if constexpr (__j != variant_npos)
std::_Construct(std::__addressof(this->_M_u),
in_place_index<__j>, __rhs_mem);
}, __variant_cast<_Types...>(__rhs));
this->_M_index = __rhs._M_index;
}
_Copy_ctor_base(_Copy_ctor_base&&) = default;
_Copy_ctor_base& operator=(const _Copy_ctor_base&) = default;
_Copy_ctor_base& operator=(_Copy_ctor_base&&) = default;
};
template<typename... _Types>
struct _Copy_ctor_base<true, _Types...> : _Variant_storage_alias<_Types...>
{
using _Base = _Variant_storage_alias<_Types...>;
using _Base::_Base;
};
template<typename... _Types>
using _Copy_ctor_alias =
_Copy_ctor_base<_Traits<_Types...>::_S_trivial_copy_ctor, _Types...>;
template<bool, typename... _Types>
struct _Move_ctor_base : _Copy_ctor_alias<_Types...>
{
using _Base = _Copy_ctor_alias<_Types...>;
using _Base::_Base;
_GLIBCXX20_CONSTEXPR
_Move_ctor_base(_Move_ctor_base&& __rhs)
noexcept(_Traits<_Types...>::_S_nothrow_move_ctor)
{
__variant::__raw_idx_visit(
[this](auto&& __rhs_mem, auto __rhs_index) mutable
{
constexpr size_t __j = __rhs_index;
if constexpr (__j != variant_npos)
std::_Construct(std::__addressof(this->_M_u),
in_place_index<__j>,
std::forward<decltype(__rhs_mem)>(__rhs_mem));
}, __variant_cast<_Types...>(std::move(__rhs)));
this->_M_index = __rhs._M_index;
}
_Move_ctor_base(const _Move_ctor_base&) = default;
_Move_ctor_base& operator=(const _Move_ctor_base&) = default;
_Move_ctor_base& operator=(_Move_ctor_base&&) = default;
};
template<typename... _Types>
struct _Move_ctor_base<true, _Types...> : _Copy_ctor_alias<_Types...>
{
using _Base = _Copy_ctor_alias<_Types...>;
using _Base::_Base;
};
template<typename... _Types>
using _Move_ctor_alias =
_Move_ctor_base<_Traits<_Types...>::_S_trivial_move_ctor, _Types...>;
template<bool, typename... _Types>
struct _Copy_assign_base : _Move_ctor_alias<_Types...>
{
using _Base = _Move_ctor_alias<_Types...>;
using _Base::_Base;
_GLIBCXX20_CONSTEXPR
_Copy_assign_base&
operator=(const _Copy_assign_base& __rhs)
noexcept(_Traits<_Types...>::_S_nothrow_copy_assign)
{
__variant::__raw_idx_visit(
[this](auto&& __rhs_mem, auto __rhs_index) mutable
{
constexpr size_t __j = __rhs_index;
if constexpr (__j == variant_npos)
this->_M_reset(); // Make *this valueless.
else if (this->_M_index == __j)
__variant::__get<__j>(*this) = __rhs_mem;
else
{
using _Tj = typename _Nth_type<__j, _Types...>::type;
if constexpr (is_nothrow_copy_constructible_v<_Tj>
|| !is_nothrow_move_constructible_v<_Tj>)
__variant::__emplace<__j>(*this, __rhs_mem);
else
{
using _Variant = variant<_Types...>;
_Variant& __self = __variant_cast<_Types...>(*this);
__self = _Variant(in_place_index<__j>, __rhs_mem);
}
}
}, __variant_cast<_Types...>(__rhs));
return *this;
}
_Copy_assign_base(const _Copy_assign_base&) = default;
_Copy_assign_base(_Copy_assign_base&&) = default;
_Copy_assign_base& operator=(_Copy_assign_base&&) = default;
};
template<typename... _Types>
struct _Copy_assign_base<true, _Types...> : _Move_ctor_alias<_Types...>
{
using _Base = _Move_ctor_alias<_Types...>;
using _Base::_Base;
};
template<typename... _Types>
using _Copy_assign_alias =
_Copy_assign_base<_Traits<_Types...>::_S_trivial_copy_assign, _Types...>;
template<bool, typename... _Types>
struct _Move_assign_base : _Copy_assign_alias<_Types...>
{
using _Base = _Copy_assign_alias<_Types...>;
using _Base::_Base;
_GLIBCXX20_CONSTEXPR
_Move_assign_base&
operator=(_Move_assign_base&& __rhs)
noexcept(_Traits<_Types...>::_S_nothrow_move_assign)
{
__variant::__raw_idx_visit(
[this](auto&& __rhs_mem, auto __rhs_index) mutable
{
constexpr size_t __j = __rhs_index;
if constexpr (__j != variant_npos)
{
if (this->_M_index == __j)
__variant::__get<__j>(*this) = std::move(__rhs_mem);
else
{
using _Tj = typename _Nth_type<__j, _Types...>::type;
if constexpr (is_nothrow_move_constructible_v<_Tj>)
__variant::__emplace<__j>(*this, std::move(__rhs_mem));
else
{
using _Variant = variant<_Types...>;
_Variant& __self = __variant_cast<_Types...>(*this);
__self.template emplace<__j>(std::move(__rhs_mem));
}
}
}
else
this->_M_reset();
}, __variant_cast<_Types...>(__rhs));
return *this;
}
_Move_assign_base(const _Move_assign_base&) = default;
_Move_assign_base(_Move_assign_base&&) = default;
_Move_assign_base& operator=(const _Move_assign_base&) = default;
};
template<typename... _Types>
struct _Move_assign_base<true, _Types...> : _Copy_assign_alias<_Types...>
{
using _Base = _Copy_assign_alias<_Types...>;
using _Base::_Base;
};
template<typename... _Types>
using _Move_assign_alias =
_Move_assign_base<_Traits<_Types...>::_S_trivial_move_assign, _Types...>;
template<typename... _Types>
struct _Variant_base : _Move_assign_alias<_Types...>
{
using _Base = _Move_assign_alias<_Types...>;
constexpr
_Variant_base() noexcept(_Traits<_Types...>::_S_nothrow_default_ctor)
: _Variant_base(in_place_index<0>) { }
template<size_t _Np, typename... _Args>
constexpr explicit
_Variant_base(in_place_index_t<_Np> __i, _Args&&... __args)
: _Base(__i, std::forward<_Args>(__args)...)
{ }
_Variant_base(const _Variant_base&) = default;
_Variant_base(_Variant_base&&) = default;
_Variant_base& operator=(const _Variant_base&) = default;
_Variant_base& operator=(_Variant_base&&) = default;
};
template<typename _Tp, typename... _Types>
inline constexpr bool __exactly_once
= std::__find_uniq_type_in_pack<_Tp, _Types...>() < sizeof...(_Types);
// Helper used to check for valid conversions that don't involve narrowing.
template<typename _Ti> struct _Arr { _Ti _M_x[1]; };
// "Build an imaginary function FUN(Ti) for each alternative type Ti"
template<size_t _Ind, typename _Tp, typename _Ti, typename = void>
struct _Build_FUN
{
// This function means 'using _Build_FUN<I, T, Ti>::_S_fun;' is valid,
// but only static functions will be considered in the call below.
void _S_fun() = delete;
};
// "... for which Ti x[] = {std::forward<T>(t)}; is well-formed."
template<size_t _Ind, typename _Tp, typename _Ti>
struct _Build_FUN<_Ind, _Tp, _Ti,
void_t<decltype(_Arr<_Ti>{{std::declval<_Tp>()}})>>
{
// This is the FUN function for type _Ti, with index _Ind
static integral_constant<size_t, _Ind> _S_fun(_Ti);
};
template<typename _Tp, typename _Variant,
typename = make_index_sequence<variant_size_v<_Variant>>>
struct _Build_FUNs;
template<typename _Tp, typename... _Ti, size_t... _Ind>
struct _Build_FUNs<_Tp, variant<_Ti...>, index_sequence<_Ind...>>
: _Build_FUN<_Ind, _Tp, _Ti>...
{
using _Build_FUN<_Ind, _Tp, _Ti>::_S_fun...;
};
// The index j of the overload FUN(Tj) selected by overload resolution
// for FUN(std::forward<_Tp>(t))
template<typename _Tp, typename _Variant>
using _FUN_type
= decltype(_Build_FUNs<_Tp, _Variant>::_S_fun(std::declval<_Tp>()));
// The index selected for FUN(std::forward<T>(t)), or variant_npos if none.
template<typename _Tp, typename _Variant, typename = void>
inline constexpr size_t
__accepted_index = variant_npos;
template<typename _Tp, typename _Variant>
inline constexpr size_t
__accepted_index<_Tp, _Variant, void_t<_FUN_type<_Tp, _Variant>>>
= _FUN_type<_Tp, _Variant>::value;
template<typename _Maybe_variant_cookie, typename _Variant,
typename = __remove_cvref_t<_Variant>>
inline constexpr bool
__extra_visit_slot_needed = false;
template<typename _Var, typename... _Types>
inline constexpr bool
__extra_visit_slot_needed<__variant_cookie, _Var, variant<_Types...>>
= !__variant::__never_valueless<_Types...>();
template<typename _Var, typename... _Types>
inline constexpr bool
__extra_visit_slot_needed<__variant_idx_cookie, _Var, variant<_Types...>>
= !__variant::__never_valueless<_Types...>();
// Used for storing a multi-dimensional vtable.
template<typename _Tp, size_t... _Dimensions>
struct _Multi_array;
// Partial specialization with rank zero, stores a single _Tp element.
template<typename _Tp>
struct _Multi_array<_Tp>
{
template<typename>
struct __untag_result
: false_type
{ using element_type = _Tp; };
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wignored-qualifiers"
template <typename... _Args>
struct __untag_result<const void(*)(_Args...)>
: false_type
{ using element_type = void(*)(_Args...); };
#pragma GCC diagnostic pop
template <typename... _Args>
struct __untag_result<__variant_cookie(*)(_Args...)>
: false_type
{ using element_type = void(*)(_Args...); };
template <typename... _Args>
struct __untag_result<__variant_idx_cookie(*)(_Args...)>
: false_type
{ using element_type = void(*)(_Args...); };
template <typename _Res, typename... _Args>
struct __untag_result<__deduce_visit_result<_Res>(*)(_Args...)>
: true_type
{ using element_type = _Res(*)(_Args...); };
using __result_is_deduced = __untag_result<_Tp>;
constexpr const typename __untag_result<_Tp>::element_type&
_M_access() const
{ return _M_data; }
typename __untag_result<_Tp>::element_type _M_data;
};
// Partial specialization with rank >= 1.
template<typename _Ret,
typename _Visitor,
typename... _Variants,
size_t __first, size_t... __rest>
struct _Multi_array<_Ret(*)(_Visitor, _Variants...), __first, __rest...>
{
static constexpr size_t __index =
sizeof...(_Variants) - sizeof...(__rest) - 1;
using _Variant = typename _Nth_type<__index, _Variants...>::type;
static constexpr int __do_cookie =
__extra_visit_slot_needed<_Ret, _Variant> ? 1 : 0;
using _Tp = _Ret(*)(_Visitor, _Variants...);
template<typename... _Args>
constexpr decltype(auto)
_M_access(size_t __first_index, _Args... __rest_indices) const
{
return _M_arr[__first_index + __do_cookie]
._M_access(__rest_indices...);
}
_Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
};
// Creates a multi-dimensional vtable recursively.
//
// For example,
// visit([](auto, auto){},
// variant<int, char>(), // typedef'ed as V1
// variant<float, double, long double>()) // typedef'ed as V2
// will trigger instantiations of:
// __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 2, 3>,
// tuple<V1&&, V2&&>, std::index_sequence<>>
// __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
// tuple<V1&&, V2&&>, std::index_sequence<0>>
// __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<0, 0>>
// __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<0, 1>>
// __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<0, 2>>
// __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
// tuple<V1&&, V2&&>, std::index_sequence<1>>
// __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<1, 0>>
// __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<1, 1>>
// __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<1, 2>>
// The returned multi-dimensional vtable can be fast accessed by the visitor
// using index calculation.
template<typename _Array_type, typename _Index_seq>
struct __gen_vtable_impl;
// Defines the _S_apply() member that returns a _Multi_array populated
// with function pointers that perform the visitation expressions e(m)
// for each valid pack of indexes into the variant types _Variants.
//
// This partial specialization builds up the index sequences by recursively
// calling _S_apply() on the next specialization of __gen_vtable_impl.
// The base case of the recursion defines the actual function pointers.
template<typename _Result_type, typename _Visitor, size_t... __dimensions,
typename... _Variants, size_t... __indices>
struct __gen_vtable_impl<
_Multi_array<_Result_type (*)(_Visitor, _Variants...), __dimensions...>,
std::index_sequence<__indices...>>
{
using _Next =
remove_reference_t<typename _Nth_type<sizeof...(__indices),
_Variants...>::type>;
using _Array_type =
_Multi_array<_Result_type (*)(_Visitor, _Variants...),
__dimensions...>;
static constexpr _Array_type
_S_apply()
{
_Array_type __vtable{};
_S_apply_all_alts(
__vtable, make_index_sequence<variant_size_v<_Next>>());
return __vtable;
}
template<size_t... __var_indices>
static constexpr void
_S_apply_all_alts(_Array_type& __vtable,
std::index_sequence<__var_indices...>)
{
if constexpr (__extra_visit_slot_needed<_Result_type, _Next>)
(_S_apply_single_alt<true, __var_indices>(
__vtable._M_arr[__var_indices + 1],
&(__vtable._M_arr[0])), ...);
else
(_S_apply_single_alt<false, __var_indices>(
__vtable._M_arr[__var_indices]), ...);
}
template<bool __do_cookie, size_t __index, typename _Tp>
static constexpr void
_S_apply_single_alt(_Tp& __element, _Tp* __cookie_element = nullptr)
{
if constexpr (__do_cookie)
{
__element = __gen_vtable_impl<
_Tp,
std::index_sequence<__indices..., __index>>::_S_apply();
*__cookie_element = __gen_vtable_impl<
_Tp,