On 26/11/16 21:38 -0800, Tim Shen wrote:
This 4-patch series contains the following in order:
a.diff: Remove uses-allocator ctors. They are going away, and removing
it reduces the maintenance burden from now on.
Yay! less code.
b.diff: Add constexpr support for get<> and comparisons. This patch
also involves small refactoring of _Variant_storage.
c.diff: Fix some libc++ test failures.
d.diff: Add constexpr support for visit. This patch also removes
__storage, __get_alternative, and __reserved_type_map, since we don't
need to support reference/void types for now.
The underlying design doesn't change - we still use the vtable
approach to achieve O(1) runtime cost even under -O0.
Great stuff.
* include/std/variant: Implement constexpr comparison and get<>.
* testsuite/20_util/variant/compile.cc: Tests.
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 2d9303a..a913074 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -154,31 +154,63 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Alternative>
using __storage = _Alternative;
- template<typename _Type, bool __is_literal = std::is_literal_type_v<_Type>>
+ // _Uninitialized nullify the destructor calls.
This wording confused me slightly. How about:
"_Uninitialized makes destructors trivial"
+ // This is necessary, since we define _Variadic_union as a recursive union,
+ // and we don't want to inspect the union members one by one in its dtor,
+ // it's slow.
Please change "it's slow" to "that's slow".
+ template<typename _Type, bool = std::is_literal_type_v<_Type>>
struct _Uninitialized;
I'm still unsure that is_literal_type is the right trait here. If it's
definitely right then we should probably *not* deprecate it in C++17!
template<typename _Type>
struct _Uninitialized<_Type, false>
{
- constexpr _Uninitialized() = default;
-
template<typename... _Args>
constexpr _Uninitialized(in_place_index_t<0>, _Args&&... __args)
{ ::new (&_M_storage) _Type(std::forward<_Args>(__args)...); }
+ const _Type& _M_get() const &
+ {
+ return *static_cast<const _Type*>(
+ static_cast<const void*>(&_M_storage));
+ }
+
+ _Type& _M_get() &
+ { return *static_cast<_Type*>(static_cast<void*>(&_M_storage)); }
+
+ const _Type&& _M_get() const &&
+ {
+ return std::move(*static_cast<const _Type*>(
+ static_cast<const void*>(&_M_storage)));
+ }
+
+ _Type&& _M_get() &&
+ {
+ return std::move(*static_cast<_Type*>(static_cast<void*>(&_M_storage)));
+ }
+
typename std::aligned_storage<sizeof(_Type), alignof(_Type)>::type
_M_storage;
I think this could use __aligned_membuf, which would reduce the
alignment requirements for some types (e.g. long long on x86-32).
That would also mean you get the _M_ptr() member so don't need all the
casts.
+ ~_Variant_storage()
+ { _M_destroy_impl(std::make_index_sequence<sizeof...(_Types)>{}); }
You can use index_sequence_for<_Types...> here.
@@ -598,9 +645,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_S_apply_all_alts(_Array_type& __vtable, index_sequence<__indices...>)
{ (_S_apply_single_alt<__indices>(__vtable._M_arr[__indices]), ...); }
- template<size_t __index>
+ template<size_t __index, typename T>
This needs to be _Tp not T
+ return __lhs._M_equal_to(__rhs,
+ std::make_index_sequence<sizeof...(_Types)>{});
Another one that could use index_sequence_for<_Types...>
+ return __lhs._M_less_than(__rhs,
+ std::make_index_sequence<sizeof...(_Types)>{});
Same again.
* include/bits/enable_special_members.h: Make
_Enable_default_constructor constexpr.
* include/std/variant (variant::emplace, variant::swap, std::swap,
std::hash): Sfinae on emplace and std::swap; handle __poison_hash
bases
of duplicated types.
diff --git a/libstdc++-v3/include/bits/enable_special_members.h
b/libstdc++-v3/include/bits/enable_special_members.h
index 07c6c99..4f4477b 100644
--- a/libstdc++-v3/include/bits/enable_special_members.h
+++ b/libstdc++-v3/include/bits/enable_special_members.h
@@ -118,7 +118,8 @@ template<typename _Tag>
operator=(_Enable_default_constructor&&) noexcept = default;
// Can be used in other ctors.
- explicit _Enable_default_constructor(_Enable_default_constructor_tag) { }
+ constexpr explicit
+ _Enable_default_constructor(_Enable_default_constructor_tag) { }
};
+ void _M_reset()
+ {
+ _M_reset_impl(std::make_index_sequence<sizeof...(_Types)>{});
+ _M_index = variant_npos;
+ }
+
~_Variant_storage()
- { _M_destroy_impl(std::make_index_sequence<sizeof...(_Types)>{}); }
+ { _M_reset(); }
These can also use index_sequence_for<_Types...>
@@ -1253,14 +1285,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename... _Types>
struct hash<variant<_Types...>>
- : private __poison_hash<remove_const_t<_Types>>...
+ : private __detail::__variant::_Variant_hash_base<
+ variant<_Types...>, std::make_index_sequence<sizeof...(_Types)>>
And again.
{
using result_type = size_t;
using argument_type = variant<_Types...>;
size_t
operator()(const variant<_Types...>& __t) const
- noexcept((... &&
noexcept(hash<decay_t<_Types>>{}(std::declval<_Types>()))))
+ noexcept((noexcept(hash<decay_t<_Types>>{}(std::declval<_Types>()))
+ && ...))
This could be
__and_<is_nothrow_callable<hash<decay_t<_Types>>(_Types)>...>
but I'm not sure it would be an improvement. The is_callable check is
expensive, but maybe we need it anyway to correctly disable this
function if the hash specialization should be posisoned?
@@ -1270,17 +1239,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
template<typename _Visitor, typename... _Variants>
- decltype(auto)
+ constexpr decltype(auto)
visit(_Visitor&& __visitor, _Variants&&... __variants)
{
+ if ((__variants.valueless_by_exception() || ...))
+ __throw_bad_variant_access("Unexpected index");
+
using _Result_type =
decltype(std::forward<_Visitor>(__visitor)(get<0>(__variants)...));
- static constexpr auto _S_vtable =
+ constexpr auto _S_vtable =
If this isn't static now it could be called simply __vtable, the _S_
prefix is misleading. How many of these _S_vtable variables actually
need to be static? If they're all trivial types and constexpr then it
probably doesn't matter either way, there shouldn't be any difference.
__detail::__variant::__gen_vtable<
_Result_type, _Visitor&&, _Variants&&...>::_S_apply();
auto __func_ptr = _S_vtable._M_access(__variants.index()...);
return (*__func_ptr)(std::forward<_Visitor>(__visitor),
- __detail::__variant::__get_storage(__variants)...);
+ std::forward<_Variants>(__variants)...);
}
