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libstdc++: Consolidate duplicate metaprogramming utilities

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Currently std::variant uses __index_of<T, Types...> to find the first
occurence of a type in a pack, and __exactly_once<T, Types...> to check
that there is no other occurrence.

We can reuse the __find_uniq_type_in_pack<T, Types...>() function for
both tasks, and remove the recursive templates used to implement
__index_of and __exactly_once.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (__find_uniq_type_in_pack): Move
	definition to here, ...
	* include/std/tuple (__find_uniq_type_in_pack): ... from here.
	* include/std/variant (__detail__variant::__index_of): Remove.
	(__detail::__variant::__exactly_once): Define using
	__find_uniq_type_in_pack instead of __index_of.
	(get<T>, get_if<T>, variant::__index_of): Likewise.
This commit is contained in:
Jonathan Wakely 2021-11-04 11:11:58 +00:00
parent 09aab7e699
commit b57899f30f
3 changed files with 47 additions and 66 deletions
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22
libstdc++-v3/include/bits/utility.h
View file

@ -102,6 +102,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
};
#if __cplusplus >= 201402L
// Return the index of _Tp in _Types, if it occurs exactly once.
// Otherwise, return sizeof...(_Types).
template<typename _Tp, typename... _Types>
constexpr size_t
__find_uniq_type_in_pack()
{
constexpr size_t __sz = sizeof...(_Types);
constexpr bool __found[__sz] = { __is_same(_Tp, _Types) ... };
size_t __n = __sz;
for (size_t __i = 0; __i < __sz; ++__i)
{
if (__found[__i])
{
if (__n < __sz) // more than one _Tp found
return __sz;
__n = __i;
}
}
return __n;
}
// The standard says this macro and alias template should be in <tuple> but we
// we define them here, to be available in <array>, <utility> and <ranges> too.
// _GLIBCXX_RESOLVE_LIB_DEFECTS

22
libstdc++-v3/include/std/tuple
View file

@ -1419,28 +1419,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#define __cpp_lib_tuples_by_type 201304L
// Return the index of _Tp in _Types, if it occurs exactly once.
// Otherwise, return sizeof...(_Types).
// TODO reuse this for __detail::__variant::__exactly_once.
template<typename _Tp, typename... _Types>
constexpr size_t
__find_uniq_type_in_pack()
{
constexpr size_t __sz = sizeof...(_Types);
constexpr bool __found[__sz] = { __is_same(_Tp, _Types) ... };
size_t __n = __sz;
for (size_t __i = 0; __i < __sz; ++__i)
{
if (__found[__i])
{
if (__n < __sz) // more than one _Tp found
return __sz;
__n = __i;
}
}
return __n;
}
/// Return a reference to the unique element of type _Tp of a tuple.
template <typename _Tp, typename... _Types>
constexpr _Tp&

69
libstdc++-v3/include/std/variant
View file

@ -161,19 +161,6 @@ namespace __detail
{
namespace __variant
{
// Returns the first appearance of _Tp in _Types.
// Returns sizeof...(_Types) if _Tp is not in _Types.
template<typename _Tp, typename... _Types>
struct __index_of : std::integral_constant<size_t, 0> {};
template<typename _Tp, typename... _Types>
inline constexpr size_t __index_of_v = __index_of<_Tp, _Types...>::value;
template<typename _Tp, typename _First, typename... _Rest>
struct __index_of<_Tp, _First, _Rest...> :
std::integral_constant<size_t, is_same_v<_Tp, _First>
? 0 : __index_of_v<_Tp, _Rest...> + 1> {};
// used for raw visitation
struct __variant_cookie {};
// used for raw visitation with indices passed in
@ -766,21 +753,9 @@ namespace __variant
_Variant_base& operator=(_Variant_base&&) = default;
};
// How many times does _Tp appear in _Types?
template<typename _Tp, typename... _Types>
inline constexpr size_t __count = 0;
template<typename _Tp, typename _Up, typename... _Types>
inline constexpr size_t __count<_Tp, _Up, _Types...>
= __count<_Tp, _Types...>;
template<typename _Tp, typename... _Types>
inline constexpr size_t __count<_Tp, _Tp, _Types...>
= 1 + __count<_Tp, _Types...>;
// TODO: Reuse this in <tuple> ?
template<typename _Tp, typename... _Types>
inline constexpr bool __exactly_once = __count<_Tp, _Types...> == 1;
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]; };
@ -1139,45 +1114,51 @@ namespace __variant
{
static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
"T must occur exactly once in alternatives");
return __v.index() == __detail::__variant::__index_of_v<_Tp, _Types...>;
return __v.index() == std::__find_uniq_type_in_pack<_Tp, _Types...>();
}
template<typename _Tp, typename... _Types>
constexpr _Tp& get(variant<_Types...>& __v)
constexpr _Tp&
get(variant<_Types...>& __v)
{
static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
"T must occur exactly once in alternatives");
static_assert(!is_void_v<_Tp>, "_Tp must not be void");
return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
return std::get<__n>(__v);
}
template<typename _Tp, typename... _Types>
constexpr _Tp&& get(variant<_Types...>&& __v)
constexpr _Tp&&
get(variant<_Types...>&& __v)
{
static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
"T must occur exactly once in alternatives");
static_assert(!is_void_v<_Tp>, "_Tp must not be void");
return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(
std::move(__v));
constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
return std::get<__n>(std::move(__v));
}
template<typename _Tp, typename... _Types>
constexpr const _Tp& get(const variant<_Types...>& __v)
constexpr const _Tp&
get(const variant<_Types...>& __v)
{
static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
"T must occur exactly once in alternatives");
static_assert(!is_void_v<_Tp>, "_Tp must not be void");
return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
return std::get<__n>(__v);
}
template<typename _Tp, typename... _Types>
constexpr const _Tp&& get(const variant<_Types...>&& __v)
constexpr const _Tp&&
get(const variant<_Types...>&& __v)
{
static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
"T must occur exactly once in alternatives");
static_assert(!is_void_v<_Tp>, "_Tp must not be void");
return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(
std::move(__v));
constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
return std::get<__n>(std::move(__v));
}
template<size_t _Np, typename... _Types>
@ -1214,8 +1195,8 @@ namespace __variant
static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
"T must occur exactly once in alternatives");
static_assert(!is_void_v<_Tp>, "_Tp must not be void");
return std::get_if<__detail::__variant::__index_of_v<_Tp, _Types...>>(
__ptr);
constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
return std::get_if<__n>(__ptr);
}
template<typename _Tp, typename... _Types>
@ -1225,8 +1206,8 @@ namespace __variant
static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
"T must occur exactly once in alternatives");
static_assert(!is_void_v<_Tp>, "_Tp must not be void");
return std::get_if<__detail::__variant::__index_of_v<_Tp, _Types...>>(
__ptr);
constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
return std::get_if<__n>(__ptr);
}
struct monostate { };
@ -1402,8 +1383,8 @@ namespace __variant
using __accepted_type = __to_type<__accepted_index<_Tp>>;
template<typename _Tp>
static constexpr size_t __index_of =
__detail::__variant::__index_of_v<_Tp, _Types...>;
static constexpr size_t __index_of
= std::__find_uniq_type_in_pack<_Tp, _Types...>();
using _Traits = __detail::__variant::_Traits<_Types...>;