This change simplifies visitation for variants, by using INVOKE<R> for
the visit<R> form, and explicitly specifying the tag types for raw
visitation, instead of inferring them from the return types of the
lambda functions used as visitors.
* include/std/variant (__visit_with_index): Remove typedef.
(__deduce_visit_result): New tag type.
(__raw_visit, __raw_idx_visit): New helper functions for "raw"
visitation of possibly-valueless variants, forwarding to __do_visit
with the relevant tag type.
(_Variant_storage<false, _Types...>::_M_reset_impl): Use __raw_visit
and make lambda return void.
(__variant_construct): Likewise.
(_Copy_assign_base::operator=, _Move_assign_base::operator=): Use
__raw_idx_visit and make lambda return void.
(_Multi_array::__untag_result): Add metafunction to check the function
pointer type for a tag type that dictates the kind of visitation.
(_Multi_array<_Ret(*)(_Visitor, _Variants...), __first, __rest...>):
Use decltype(auto) instead of tagged function pointer type.
(__gen_vtable_impl): Remove bool non-type parameter and unused
_Variant_tuple parameter.
(__gen_vtable_impl::__visit_invoke_impl): Remove.
(__gen_vtable_impl::__do_visit_invoke): Remove.
(__gen_vtable_impl::__do_visit_invoke_r): Remove.
(__gen_vtable_impl::__visit_invoke): Use if-constexpr and __invoke_r
for the visit<R> case, rather than dispatching to separate functions.
(_VARIANT_RELATION_FUNCTION_TEMPLATE): Use __raw_idx_visit and make
lambda return void.
(variant::swap): Likewise.
(__do_visit): Replace two non-type template parameters with a single
type parameter, so that the caller must specify the visitor's return
type (or one of the tag types).
(visit): Deduce a return type from the visitor and use the
__deduce_visit_result tag to enforce that all overloads return the
same type.
(visit<R>): Call __do_visit<R> with explicit result type.
(__variant_hash_call_base_impl::operator()): Use __raw_visit and make
lambda return void.
Tested powerpc64le-linux, committed to trunk.
commit 5fcf0c52fa1352fcef16850f1695a9aa5fb6afa7
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Tue Apr 9 20:09:31 2019 +0100
Add __raw_visit and __raw_idx_visit, use INVOKE<R>
This change simplifies visitation for variants, by using INVOKE<R> for
the visit<R> form, and explicitly specifying the tag types for raw
visitation, instead of inferring them from the return types of the
lambda functions used as visitors.
* include/std/variant (__visit_with_index): Remove typedef.
(__deduce_visit_result): New tag type.
(__raw_visit, __raw_idx_visit): New helper functions for "raw"
visitation of possibly-valueless variants, forwarding to __do_visit
with the relevant tag type.
(_Variant_storage<false, _Types...>::_M_reset_impl): Use __raw_visit
and make lambda return void.
(__variant_construct): Likewise.
(_Copy_assign_base::operator=, _Move_assign_base::operator=): Use
__raw_idx_visit and make lambda return void.
(_Multi_array::__untag_result): Add metafunction to check the
function
pointer type for a tag type that dictates the kind of visitation.
(_Multi_array<_Ret(*)(_Visitor, _Variants...), __first, __rest...>):
Use decltype(auto) instead of tagged function pointer type.
(__gen_vtable_impl): Remove bool non-type parameter and unused
_Variant_tuple parameter.
(__gen_vtable_impl::__visit_invoke_impl): Remove.
(__gen_vtable_impl::__do_visit_invoke): Remove.
(__gen_vtable_impl::__do_visit_invoke_r): Remove.
(__gen_vtable_impl::__visit_invoke): Use if-constexpr and __invoke_r
for the visit<R> case, rather than dispatching to separate
functions.
(_VARIANT_RELATION_FUNCTION_TEMPLATE): Use __raw_idx_visit and make
lambda return void.
(variant::swap): Likewise.
(__do_visit): Replace two non-type template parameters with a single
type parameter, so that the caller must specify the visitor's return
type (or one of the tag types).
(visit): Deduce a return type from the visitor and use the
__deduce_visit_result tag to enforce that all overloads return the
same type.
(visit<R>): Call __do_visit<R> with explicit result type.
(__variant_hash_call_base_impl::operator()): Use __raw_visit and
make
lambda return void.
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index d539df125bf..dc4bbb7f356 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -138,9 +138,7 @@ namespace __variant
constexpr variant_alternative_t<_Np, variant<_Types...>> const&&
get(const variant<_Types...>&&);
- template<bool __use_index=false,
- bool __same_return_types = true,
- typename _Visitor, typename... _Variants>
+ template<typename _Result_type, typename _Visitor, typename... _Variants>
constexpr decltype(auto)
__do_visit(_Visitor&& __visitor, _Variants&&... __variants);
@@ -180,8 +178,26 @@ namespace __variant
struct __variant_cookie {};
// used for raw visitation with indices passed in
struct __variant_idx_cookie { using type = __variant_idx_cookie; };
- // a more explanatory name than 'true'
- inline constexpr auto __visit_with_index = bool_constant<true>{};
+ // Used to enable deduction (and same-type checking) for std::visit:
+ template<typename> struct __deduce_visit_result { };
+
+ // 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)...);
+ }
// _Uninitialized<T> is guaranteed to be a literal type, even if T is not.
// We have to do this, because [basic.types]p10.5.3 (n4606) is not
implemented
@@ -382,13 +398,11 @@ namespace __variant
constexpr void _M_reset_impl()
{
- __do_visit([](auto&& __this_mem) mutable
- -> __detail::__variant::__variant_cookie
+ __variant::__raw_visit([](auto&& __this_mem) mutable
{
if constexpr (!is_same_v<remove_reference_t<decltype(__this_mem)>,
__variant_cookie>)
std::_Destroy(std::__addressof(__this_mem));
- return {};
}, __variant_cast<_Types...>(*this));
}
@@ -473,12 +487,10 @@ namespace __variant
void __variant_construct(_Tp&& __lhs, _Up&& __rhs)
{
__lhs._M_index = __rhs._M_index;
- __do_visit([&__lhs](auto&& __rhs_mem) mutable
- -> __detail::__variant::__variant_cookie
+ __variant::__raw_visit([&__lhs](auto&& __rhs_mem) mutable
{
__variant_construct_single(std::forward<_Tp>(__lhs),
std::forward<decltype(__rhs_mem)>(__rhs_mem));
- return {};
}, __variant_cast<_Types...>(std::forward<_Up>(__rhs)));
}
@@ -583,9 +595,8 @@ namespace __variant
operator=(const _Copy_assign_base& __rhs)
noexcept(_Traits<_Types...>::_S_nothrow_copy_assign)
{
- __do_visit<__visit_with_index>([this](auto&& __rhs_mem,
- auto __rhs_index) mutable
- -> __detail::__variant::__variant_idx_cookie
+ __variant::__raw_idx_visit(
+ [this](auto&& __rhs_mem, auto __rhs_index) mutable
{
if constexpr (__rhs_index != variant_npos)
{
@@ -611,7 +622,6 @@ namespace __variant
}
else
this->_M_reset();
- return {};
}, __variant_cast<_Types...>(__rhs));
return *this;
}
@@ -642,9 +652,8 @@ namespace __variant
operator=(_Move_assign_base&& __rhs)
noexcept(_Traits<_Types...>::_S_nothrow_move_assign)
{
- __do_visit<__visit_with_index>([this](auto&& __rhs_mem,
- auto __rhs_index) mutable
- -> __detail::__variant::__variant_idx_cookie
+ __variant::__raw_idx_visit(
+ [this](auto&& __rhs_mem, auto __rhs_index) mutable
{
if constexpr (__rhs_index != variant_npos)
{
@@ -656,7 +665,6 @@ namespace __variant
}
else
this->_M_reset();
- return {};
}, __variant_cast<_Types...>(__rhs));
return *this;
}
@@ -786,11 +794,38 @@ namespace __variant
template<typename _Tp>
struct _Multi_array<_Tp>
{
- constexpr const _Tp&
+ template<typename>
+ struct __untag_result
+ : false_type
+ { using element_type = _Tp; };
+
+ template <typename... _Args>
+ struct __untag_result<const void(*)(_Args...)>
+ : false_type
+ { using element_type = void(*)(_Args...); };
+
+ 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; }
- _Tp _M_data;
+ typename __untag_result<_Tp>::element_type _M_data;
};
// Partial specialization with rank >= 1.
@@ -811,7 +846,7 @@ namespace __variant
using _Tp = _Ret(*)(_Visitor, _Variants...);
template<typename... _Args>
- constexpr const _Tp&
+ constexpr decltype(auto)
_M_access(size_t __first_index, _Args... __rest_indices) const
{
return _M_arr[__first_index + __do_cookie]
@@ -823,37 +858,32 @@ namespace __variant
// Creates a multi-dimensional vtable recursively.
//
- // The __same_return_types non-type template parameter specifies whether
- // to enforce that all visitor invocations return the same type. This is
- // required by std::visit but not std::visit<R>.
- //
// 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<true, _Multi_array<void(*)(V1&&, V2&&), 2, 3>,
+ // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 2, 3>,
// tuple<V1&&, V2&&>, std::index_sequence<>>
- // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&), 3>,
+ // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
// tuple<V1&&, V2&&>, std::index_sequence<0>>
- // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<0, 0>>
- // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<0, 1>>
- // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<0, 2>>
- // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&), 3>,
+ // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
// tuple<V1&&, V2&&>, std::index_sequence<1>>
- // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<1, 0>>
- // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<1, 1>>
- // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
+ // __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<bool __same_return_types,
- typename _Array_type, typename _Variant_tuple, typename _Index_seq>
+ template<typename _Array_type, typename _Index_seq>
struct __gen_vtable_impl;
// Defines the _S_apply() member that returns a _Multi_array populated
@@ -863,13 +893,11 @@ namespace __variant
// 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<bool __same_return_types,
- typename _Result_type, typename _Visitor, size_t... __dimensions,
+ template<typename _Result_type, typename _Visitor, size_t... __dimensions,
typename... _Variants, size_t... __indices>
struct __gen_vtable_impl<
- __same_return_types,
_Multi_array<_Result_type (*)(_Visitor, _Variants...), __dimensions...>,
- tuple<_Variants...>, std::index_sequence<__indices...>>
+ std::index_sequence<__indices...>>
{
using _Next =
remove_reference_t<typename _Nth_type<sizeof...(__indices),
@@ -905,25 +933,19 @@ namespace __variant
static constexpr void
_S_apply_single_alt(_Tp& __element, _Tp* __cookie_element = nullptr)
{
- using _Alternative = variant_alternative_t<__index, _Next>;
if constexpr (__do_cookie)
{
__element = __gen_vtable_impl<
- __same_return_types,
_Tp,
- tuple<_Variants...>,
std::index_sequence<__indices..., __index>>::_S_apply();
*__cookie_element = __gen_vtable_impl<
- __same_return_types,
_Tp,
- tuple<_Variants...>,
std::index_sequence<__indices..., variant_npos>>::_S_apply();
}
else
{
__element = __gen_vtable_impl<
- __same_return_types,
- remove_reference_t<decltype(__element)>, tuple<_Variants...>,
+ remove_reference_t<decltype(__element)>,
std::index_sequence<__indices..., __index>>::_S_apply();
}
}
@@ -932,13 +954,11 @@ namespace __variant
// This partial specialization is the base case for the recursion.
// It populates a _Multi_array element with the address of a function
// that invokes the visitor with the alternatives specified by __indices.
- template<bool __same_return_types,
- typename _Result_type, typename _Visitor, typename... _Variants,
+ template<typename _Result_type, typename _Visitor, typename... _Variants,
size_t... __indices>
struct __gen_vtable_impl<
- __same_return_types,
_Multi_array<_Result_type (*)(_Visitor, _Variants...)>,
- tuple<_Variants...>, std::index_sequence<__indices...>>
+ std::index_sequence<__indices...>>
{
using _Array_type =
_Multi_array<_Result_type (*)(_Visitor, _Variants...)>;
@@ -954,50 +974,29 @@ namespace __variant
}
static constexpr decltype(auto)
- __visit_invoke_impl(_Visitor&& __visitor, _Variants... __vars)
+ __visit_invoke(_Visitor&& __visitor, _Variants... __vars)
{
- // For raw visitation using indices, pass the indices to the visitor:
if constexpr (is_same_v<_Result_type, __variant_idx_cookie>)
- return std::__invoke(std::forward<_Visitor>(__visitor),
+ // For raw visitation using indices, pass the indices to the visitor
+ // and discard the return value:
+ std::__invoke(std::forward<_Visitor>(__visitor),
__element_by_index_or_cookie<__indices>(
std::forward<_Variants>(__vars))...,
integral_constant<size_t, __indices>()...);
- // For std::visit<cv void>, cast the result to void:
- else if constexpr (!__same_return_types &&
- std::is_void_v<_Result_type>)
- return (void)std::__invoke(std::forward<_Visitor>(__visitor),
+ else if constexpr (is_same_v<_Result_type, __variant_cookie>)
+ // For raw visitation without indices, and discard the return value:
+ std::__invoke(std::forward<_Visitor>(__visitor),
__element_by_index_or_cookie<__indices>(
std::forward<_Variants>(__vars))...);
- else
+ else if constexpr (_Array_type::__result_is_deduced::value)
+ // For the usual std::visit case deduce the return value:
return std::__invoke(std::forward<_Visitor>(__visitor),
__element_by_index_or_cookie<__indices>(
std::forward<_Variants>(__vars))...);
- }
-
- static constexpr decltype(auto)
- __do_visit_invoke(_Visitor&& __visitor, _Variants... __vars)
- {
- return __visit_invoke_impl(std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__vars)...);
- }
-
- // Perform the implicit conversion to _Result_type for std::visit<R>.
- static constexpr _Result_type
- __do_visit_invoke_r(_Visitor&& __visitor, _Variants... __vars)
- {
- return __visit_invoke_impl(std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__vars)...);
- }
-
- static constexpr decltype(auto)
- __visit_invoke(_Visitor&& __visitor, _Variants... __vars)
- {
- if constexpr (__same_return_types)
- return __do_visit_invoke(std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__vars)...);
- else
- return __do_visit_invoke_r(std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__vars)...);
+ else // for std::visit<R> use INVOKE<R>
+ return std::__invoke_r<_Result_type>(
+ std::forward<_Visitor>(__visitor),
+ __variant::__get<__indices>(std::forward<_Variants>(__vars))...);
}
static constexpr auto
@@ -1005,8 +1004,7 @@ namespace __variant
{ return _Array_type{&__visit_invoke}; }
};
- template<bool __same_return_types,
- typename _Result_type, typename _Visitor, typename... _Variants>
+ template<typename _Result_type, typename _Visitor, typename... _Variants>
struct __gen_vtable
{
using _Array_type =
@@ -1014,9 +1012,7 @@ namespace __variant
variant_size_v<remove_reference_t<_Variants>>...>;
static constexpr _Array_type _S_vtable
- = __gen_vtable_impl<__same_return_types,
- _Array_type, tuple<_Variants...>,
- std::index_sequence<>>::_S_apply();
+ = __gen_vtable_impl<_Array_type, std::index_sequence<>>::_S_apply();
};
template<size_t _Np, typename _Tp>
@@ -1147,10 +1143,8 @@ namespace __variant
const variant<_Types...>& __rhs) \
{ \
bool __ret = true; \
- __do_visit<__detail::__variant::__visit_with_index>( \
- [&__ret, &__lhs] \
- (auto&& __rhs_mem, auto __rhs_index) mutable \
- -> __detail::__variant::__variant_idx_cookie \
+ __detail::__variant::__raw_idx_visit( \
+ [&__ret, &__lhs] (auto&& __rhs_mem, auto __rhs_index) mutable \
{ \
if constexpr (__rhs_index != variant_npos) \
{ \
@@ -1164,7 +1158,6 @@ namespace __variant
} \
else \
__ret = (__lhs.index() + 1) __OP (__rhs_index + 1); \
- return {}; \
}, __rhs); \
return __ret; \
} \
@@ -1504,10 +1497,8 @@ namespace __variant
noexcept((__is_nothrow_swappable<_Types>::value && ...)
&& is_nothrow_move_constructible_v<variant>)
{
- __do_visit<__detail::__variant::__visit_with_index>(
- [this, &__rhs](auto&& __rhs_mem,
- auto __rhs_index) mutable
- -> __detail::__variant::__variant_idx_cookie
+ __detail::__variant::__raw_idx_visit(
+ [this, &__rhs](auto&& __rhs_mem, auto __rhs_index) mutable
{
if constexpr (__rhs_index != variant_npos)
{
@@ -1543,7 +1534,6 @@ namespace __variant
this->_M_reset();
}
}
- return {};
}, __rhs);
}
@@ -1623,21 +1613,11 @@ namespace __variant
return __detail::__variant::__get<_Np>(std::move(__v));
}
- template<bool __use_index,
- bool __same_return_types,
- typename _Visitor, typename... _Variants>
+ template<typename _Result_type, typename _Visitor, typename... _Variants>
constexpr decltype(auto)
__do_visit(_Visitor&& __visitor, _Variants&&... __variants)
{
- using _Deduced_type = std::invoke_result<_Visitor,
- decltype(std::get<0>(std::declval<_Variants>()))...>;
-
- using _Result_type = typename std::conditional_t<__use_index,
- __detail::__variant::__variant_idx_cookie,
- _Deduced_type>::type;
-
constexpr auto& __vtable = __detail::__variant::__gen_vtable<
- __same_return_types,
_Result_type, _Visitor&&, _Variants&&...>::_S_vtable;
auto __func_ptr = __vtable._M_access(__variants.index()...);
@@ -1652,8 +1632,13 @@ namespace __variant
if ((__variants.valueless_by_exception() || ...))
__throw_bad_variant_access("Unexpected index");
- return __do_visit(std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__variants)...);
+ using _Result_type = std::invoke_result_t<_Visitor,
+ decltype(std::get<0>(std::declval<_Variants>()))...>;
+
+ using _Tag = __detail::__variant::__deduce_visit_result<_Result_type>;
+
+ return __do_visit<_Tag>(std::forward<_Visitor>(__visitor),
+ std::forward<_Variants>(__variants)...);
}
#if __cplusplus > 201703L
@@ -1664,12 +1649,8 @@ namespace __variant
if ((__variants.valueless_by_exception() || ...))
__throw_bad_variant_access("Unexpected index");
- if constexpr (std::is_void_v<_Res>)
- (void) __do_visit<false, false>(std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__variants)...);
- else
- return __do_visit<false, false>(std::forward<_Visitor>(__visitor),
- std::forward<_Variants>(__variants)...);
+ return __do_visit<_Res>(std::forward<_Visitor>(__visitor),
+ std::forward<_Variants>(__variants)...);
}
#endif
@@ -1681,8 +1662,8 @@ namespace __variant
noexcept((is_nothrow_invocable_v<hash<decay_t<_Types>>, _Types> && ...))
{
size_t __ret;
- __do_visit([&__t, &__ret](auto&& __t_mem) mutable
- -> __detail::__variant::__variant_cookie
+ __detail::__variant::__raw_visit(
+ [&__t, &__ret](auto&& __t_mem) mutable
{
using _Type = __remove_cvref_t<decltype(__t_mem)>;
if constexpr (!is_same_v<_Type,
@@ -1691,7 +1672,6 @@ namespace __variant
+ std::hash<_Type>{}(__t_mem);
else
__ret = std::hash<size_t>{}(__t.index());
- return {};
}, __t);
return __ret;
}
