Tested powerpc64le-linux, pushed to trunk.
By defining additional partial specializations of _Nth_type we can
reduce the number of recursive instantiations needed to get from N to 0.
We can also use _Nth_type in variant_alternative, to take advantage of
that new optimization.
By adding a static_assert to variant_alternative we get a nicer error
than 'invalid use of incomplete type'.
By defining partial specializations of std::variant_size_v for the
common case we can avoid instantiating the std::variant_size class
template.
The __tuple_count class template and __tuple_count_v variable template
can be simplified to a single variable template, __count.
By adding a deleted constructor to the _Variant_union primary template
we can (very slightly) improve diagnostics for invalid attempts to
construct a std::variant with an out-of-range index. Instead of a
confusing error about "too many initializers for ..." we get a call to a
deleted function.
By using _Nth_type instead of variant_alternative (for cv-unqualified
variant types) we avoid instantiating variant_alternative.
By adding deleted overloads of variant::emplace we get better
diagnostics for emplace<invalid-index> or emplace<invalid-type>. Instead
of getting errors explaining why each of the four overloads wasn't
valid, we just get one error about calling a deleted function.
libstdc++-v3/ChangeLog:
* include/std/variant (_Nth_type): Define partial
specializations to reduce number of instantiations.
(variant_size_v): Define partial specializations to avoid
instantiations.
(variant_alternative): Use _Nth_type. Add static assert.
(__tuple_count, __tuple_count_v): Replace with ...
(__count): New variable template.
(_Variant_union): Add deleted constructor.
(variant::__to_type): Use _Nth_type.
(variant::emplace): Use _Nth_type. Add deleted overloads for
invalid types and indices.
---
libstdc++-v3/include/std/variant | 119 ++++++++++++++++++++-----------
1 file changed, 78 insertions(+), 41 deletions(-)
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 63468af7012..dc3d032c543 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -61,13 +61,40 @@ namespace __variant
template<size_t _Np, typename... _Types>
struct _Nth_type;
- template<size_t _Np, typename _First, typename... _Rest>
- struct _Nth_type<_Np, _First, _Rest...>
- : _Nth_type<_Np-1, _Rest...> { };
+ template<typename _Tp0, typename... _Rest>
+ struct _Nth_type<0, _Tp0, _Rest...>
+ { using type = _Tp0; };
- template<typename _First, typename... _Rest>
- struct _Nth_type<0, _First, _Rest...>
- { using type = _First; };
+ template<typename _Tp0, typename _Tp1, typename... _Rest>
+ struct _Nth_type<1, _Tp0, _Tp1, _Rest...>
+ { using type = _Tp1; };
+
+ template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
+ struct _Nth_type<2, _Tp0, _Tp1, _Tp2, _Rest...>
+ { using type = _Tp2; };
+
+ template<size_t _Np, typename _Tp0, typename _Tp1, typename _Tp2,
+ typename... _Rest>
+#if __cpp_concepts
+ requires (_Np >= 3)
+#endif
+ struct _Nth_type<_Np, _Tp0, _Tp1, _Tp2, _Rest...>
+ : _Nth_type<_Np - 3, _Rest...>
+ { };
+
+#if ! __cpp_concepts // Need additional specializations to avoid ambiguities.
+ template<typename _Tp0, typename _Tp1, typename... _Rest>
+ struct _Nth_type<0, _Tp0, _Tp1, _Rest...>
+ { using type = _Tp0; };
+
+ template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
+ struct _Nth_type<0, _Tp0, _Tp1, _Tp2, _Rest...>
+ { using type = _Tp0; };
+
+ template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
+ struct _Nth_type<1, _Tp0, _Tp1, _Tp2, _Rest...>
+ { using type = _Tp1; };
+#endif
} // namespace __variant
} // namespace __detail
@@ -102,16 +129,25 @@ namespace __variant
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 _First, typename... _Rest>
- struct variant_alternative<_Np, variant<_First, _Rest...>>
- : variant_alternative<_Np-1, variant<_Rest...>> {};
+ template<size_t _Np, typename... _Types>
+ struct variant_alternative<_Np, variant<_Types...>>
+ {
+ static_assert(_Np < sizeof...(_Types));
- template<typename _First, typename... _Rest>
- struct variant_alternative<0, variant<_First, _Rest...>>
- { using type = _First; };
+ using type
+ = typename __detail::__variant::_Nth_type<_Np, _Types...>::type;
+ };
template<size_t _Np, typename _Variant>
using variant_alternative_t =
@@ -390,7 +426,13 @@ namespace __variant
// Defines members and ctors.
template<typename... _Types>
- union _Variadic_union { };
+ union _Variadic_union
+ {
+ _Variadic_union() = default;
+
+ template<size_t _Np, typename... _Args>
+ _Variadic_union(in_place_index_t<_Np>, _Args&&...) = delete;
+ };
template<typename _First, typename... _Rest>
union _Variadic_union<_First, _Rest...>
@@ -758,28 +800,21 @@ namespace __variant
_Variant_base& operator=(_Variant_base&&) = default;
};
- // For how many times does _Tp appear in _Tuple?
- template<typename _Tp, typename _Tuple>
- struct __tuple_count;
+ // How many times does _Tp appear in _Types?
+ template<typename _Tp, typename... _Types>
+ inline constexpr size_t __count = 0;
- template<typename _Tp, typename _Tuple>
- inline constexpr size_t __tuple_count_v =
- __tuple_count<_Tp, _Tuple>::value;
+ template<typename _Tp, typename _Up, typename... _Types>
+ inline constexpr size_t __count<_Tp, _Up, _Types...>
+ = __count<_Tp, _Types...>;
template<typename _Tp, typename... _Types>
- struct __tuple_count<_Tp, tuple<_Types...>>
- : integral_constant<size_t, 0> { };
-
- template<typename _Tp, typename _First, typename... _Rest>
- struct __tuple_count<_Tp, tuple<_First, _Rest...>>
- : integral_constant<
- size_t,
- __tuple_count_v<_Tp, tuple<_Rest...>> + is_same_v<_Tp, _First>> { };
+ 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 =
- __tuple_count_v<_Tp, tuple<_Types...>> == 1;
+ inline constexpr bool __exactly_once = __count<_Tp, _Types...> == 1;
// Helper used to check for valid conversions that don't involve narrowing.
template<typename _Ti> struct _Arr { _Ti _M_x[1]; };
@@ -1411,7 +1446,8 @@ namespace __variant
= __detail::__variant::__accepted_index<_Tp, variant>::value;
template<size_t _Np, typename = enable_if_t<(_Np < sizeof...(_Types))>>
- using __to_type = variant_alternative_t<_Np, variant>;
+ using __to_type
+ = typename __detail::__variant::_Nth_type<_Np, _Types...>::type;
template<typename _Tp, typename = enable_if_t<__not_self<_Tp>>>
using __accepted_type = __to_type<__accepted_index<_Tp>>;
@@ -1543,15 +1579,12 @@ namespace __variant
template<size_t _Np, typename... _Args>
_GLIBCXX20_CONSTEXPR
- enable_if_t<is_constructible_v<variant_alternative_t<_Np, variant>,
- _Args...>,
- variant_alternative_t<_Np, variant>&>
+ enable_if_t<is_constructible_v<__to_type<_Np>, _Args...>,
+ __to_type<_Np>&>
emplace(_Args&&... __args)
{
- static_assert(_Np < sizeof...(_Types),
- "The index must be in [0, number of alternatives)");
- using type = variant_alternative_t<_Np, variant>;
namespace __variant = std::__detail::__variant;
+ using type = typename __variant::_Nth_type<_Np, _Types...>::type;
// Provide the strong exception-safety guarantee when possible,
// to avoid becoming valueless.
if constexpr (is_nothrow_constructible_v<type, _Args...>)
@@ -1590,15 +1623,13 @@ namespace __variant
template<size_t _Np, typename _Up, typename... _Args>
_GLIBCXX20_CONSTEXPR
- enable_if_t<is_constructible_v<variant_alternative_t<_Np, variant>,
+ enable_if_t<is_constructible_v<__to_type<_Np>,
initializer_list<_Up>&, _Args...>,
- variant_alternative_t<_Np, variant>&>
+ __to_type<_Np>&>
emplace(initializer_list<_Up> __il, _Args&&... __args)
{
- static_assert(_Np < sizeof...(_Types),
- "The index must be in [0, number of alternatives)");
- using type = variant_alternative_t<_Np, variant>;
namespace __variant = std::__detail::__variant;
+ using type = typename __variant::_Nth_type<_Np, _Types...>::type;
// Provide the strong exception-safety guarantee when possible,
// to avoid becoming valueless.
if constexpr (is_nothrow_constructible_v<type,
@@ -1629,6 +1660,12 @@ namespace __variant
return std::get<_Np>(*this);
}
+ template<size_t _Np, typename... _Args>
+ enable_if_t<!(_Np < sizeof...(_Types))> emplace(_Args&&...) = delete;
+
+ template<typename _Tp, typename... _Args>
+ enable_if_t<!__exactly_once<_Tp>> emplace(_Args&&...) = delete;
+
constexpr bool valueless_by_exception() const noexcept
{ return !this->_M_valid(); }
--
2.31.1
