public inbox for libstdc++-cvs@sourceware.org
help / color / mirror / Atom feed
* [gcc r12-4890] libstdc++: Refactor emplace-like functions in std::variant
@ 2021-11-04 9:39 Jonathan Wakely
0 siblings, 0 replies; only message in thread
From: Jonathan Wakely @ 2021-11-04 9:39 UTC (permalink / raw)
To: gcc-cvs, libstdc++-cvs
https://gcc.gnu.org/g:a45d577b2b05e231e7a9abd43165f1e3120af013
commit r12-4890-ga45d577b2b05e231e7a9abd43165f1e3120af013
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Tue Nov 2 21:07:37 2021 +0000
libstdc++: Refactor emplace-like functions in std::variant
libstdc++-v3/ChangeLog:
* include/std/variant (__detail::__variant::__emplace): New
function template.
(_Copy_assign_base::operator=): Reorder conditions to match
bulleted list of effects in the standard. Use __emplace instead
of _M_reset followed by _Construct.
(_Move_assign_base::operator=): Likewise.
(__construct_by_index): Remove.
(variant::emplace): Use __emplace instead of _M_reset followed
by __construct_by_index.
(variant::swap): Hoist valueless cases out of visitor. Use
__emplace to replace _M_reset followed by _Construct.
Diff:
---
libstdc++-v3/include/std/variant | 179 ++++++++++++++++++---------------------
1 file changed, 82 insertions(+), 97 deletions(-)
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index dc3d032c543..c4c307b7bb2 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -590,6 +590,19 @@ namespace __variant
__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 = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
+ std::_Construct(__addr, 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...>;
@@ -655,6 +668,7 @@ namespace __variant
}, __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;
@@ -685,36 +699,24 @@ namespace __variant
__variant::__raw_idx_visit(
[this](auto&& __rhs_mem, auto __rhs_index) mutable
{
- if constexpr (__rhs_index != variant_npos)
+ 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
{
- if (this->_M_index == __rhs_index)
- __variant::__get<__rhs_index>(*this) = __rhs_mem;
+ 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 __rhs_type = __remove_cvref_t<decltype(__rhs_mem)>;
- if constexpr (is_nothrow_copy_constructible_v<__rhs_type>
- || !is_nothrow_move_constructible_v<__rhs_type>)
- {
- // The standard says emplace<__rhs_index>(__rhs_mem)
- // should be used here, but this is equivalent. Either
- // copy construction doesn't throw, so we have strong
- // exception safety guarantee, or both copy construction
- // and move construction can throw, so emplace only
- // gives basic exception safety anyway.
- this->_M_reset();
- std::_Construct(std::__addressof(this->_M_u),
- in_place_index<__rhs_index>,
- __rhs_mem);
- this->_M_index = __rhs_index;
- }
- else
- __variant_cast<_Types...>(*this)
- = variant<_Types...>(std::in_place_index<__rhs_index>,
- __rhs_mem);
+ using _Variant = variant<_Types...>;
+ _Variant& __self = __variant_cast<_Types...>(*this);
+ __self = _Variant(in_place_index<__j>, __rhs_mem);
}
}
- else
- this->_M_reset();
}, __variant_cast<_Types...>(__rhs));
return *this;
}
@@ -749,13 +751,23 @@ namespace __variant
__variant::__raw_idx_visit(
[this](auto&& __rhs_mem, auto __rhs_index) mutable
{
- if constexpr (__rhs_index != variant_npos)
+ constexpr size_t __j = __rhs_index;
+ if constexpr (__j != variant_npos)
{
- if (this->_M_index == __rhs_index)
- __variant::__get<__rhs_index>(*this) = std::move(__rhs_mem);
+ if (this->_M_index == __j)
+ __variant::__get<__j>(*this) = std::move(__rhs_mem);
else
- __variant_cast<_Types...>(*this)
- .template emplace<__rhs_index>(std::move(__rhs_mem));
+ {
+ 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();
@@ -1164,17 +1176,6 @@ namespace __variant
>;
}
- template<size_t _Np, typename _Variant, typename... _Args>
- _GLIBCXX20_CONSTEXPR
- inline void
- __construct_by_index(_Variant& __v, _Args&&... __args)
- {
- std::_Construct(std::__addressof(__variant::__get<_Np>(__v)),
- std::forward<_Args>(__args)...);
- // Construction didn't throw, so can set the new index now:
- __v._M_index = _Np;
- }
-
} // namespace __variant
} // namespace __detail
@@ -1415,11 +1416,6 @@ namespace __variant
friend _GLIBCXX20_CONSTEXPR decltype(auto)
__variant_cast(_Tp&&);
- template<size_t _Np, typename _Variant, typename... _Args>
- friend _GLIBCXX20_CONSTEXPR void
- __detail::__variant::__construct_by_index(_Variant& __v,
- _Args&&... __args);
-
static_assert(sizeof...(_Types) > 0,
"variant must have at least one alternative");
static_assert(!(std::is_reference_v<_Types> || ...),
@@ -1589,17 +1585,14 @@ namespace __variant
// to avoid becoming valueless.
if constexpr (is_nothrow_constructible_v<type, _Args...>)
{
- this->_M_reset();
- __variant::__construct_by_index<_Np>(*this,
- std::forward<_Args>(__args)...);
+ __variant::__emplace<_Np>(*this, std::forward<_Args>(__args)...);
}
else if constexpr (is_scalar_v<type>)
{
// This might invoke a potentially-throwing conversion operator:
const type __tmp(std::forward<_Args>(__args)...);
- // But these steps won't throw:
- this->_M_reset();
- __variant::__construct_by_index<_Np>(*this, __tmp);
+ // But this won't throw:
+ __variant::__emplace<_Np>(*this, __tmp);
}
else if constexpr (__variant::_Never_valueless_alt<type>()
&& _Traits::_S_move_assign)
@@ -1614,9 +1607,7 @@ namespace __variant
{
// This case only provides the basic exception-safety guarantee,
// i.e. the variant can become valueless.
- this->_M_reset();
- __variant::__construct_by_index<_Np>(*this,
- std::forward<_Args>(__args)...);
+ __variant::__emplace<_Np>(*this, std::forward<_Args>(__args)...);
}
return std::get<_Np>(*this);
}
@@ -1636,9 +1627,8 @@ namespace __variant
initializer_list<_Up>&,
_Args...>)
{
- this->_M_reset();
- __variant::__construct_by_index<_Np>(*this, __il,
- std::forward<_Args>(__args)...);
+ __variant::__emplace<_Np>(*this, __il,
+ std::forward<_Args>(__args)...);
}
else if constexpr (__variant::_Never_valueless_alt<type>()
&& _Traits::_S_move_assign)
@@ -1653,9 +1643,8 @@ namespace __variant
{
// This case only provides the basic exception-safety guarantee,
// i.e. the variant can become valueless.
- this->_M_reset();
- __variant::__construct_by_index<_Np>(*this, __il,
- std::forward<_Args>(__args)...);
+ __variant::__emplace<_Np>(*this, __il,
+ std::forward<_Args>(__args)...);
}
return std::get<_Np>(*this);
}
@@ -1686,61 +1675,57 @@ namespace __variant
noexcept((__is_nothrow_swappable<_Types>::value && ...)
&& is_nothrow_move_constructible_v<variant>)
{
- __detail::__variant::__raw_idx_visit(
+ static_assert((is_move_constructible_v<_Types> && ...));
+
+ // Handle this here to simplify the visitation.
+ if (__rhs.valueless_by_exception()) [[__unlikely__]]
+ {
+ if (!this->valueless_by_exception()) [[__likely__]]
+ __rhs.swap(*this);
+ return;
+ }
+
+ namespace __variant = __detail::__variant;
+
+ __variant::__raw_idx_visit(
[this, &__rhs](auto&& __rhs_mem, auto __rhs_index) mutable
{
- if constexpr (__rhs_index != variant_npos)
+ constexpr size_t __j = __rhs_index;
+ if constexpr (__j != variant_npos)
{
- if (this->index() == __rhs_index)
+ if (this->index() == __j)
{
- auto& __this_mem =
- std::get<__rhs_index>(*this);
using std::swap;
- swap(__this_mem, __rhs_mem);
+ swap(std::get<__j>(*this), __rhs_mem);
}
else
{
- constexpr size_t __j = __rhs_index;
- if (!this->valueless_by_exception()) [[__likely__]]
- {
- auto __tmp(std::move(__rhs_mem));
- __rhs = std::move(*this);
- this->_M_reset();
- std::_Construct(std::__addressof(this->_M_u),
- in_place_index<__j>,
- std::move(__tmp));
- this->_M_index = __j;
- }
+ auto __tmp(std::move(__rhs_mem));
+
+ if constexpr (_Traits::_S_trivial_move_assign)
+ __rhs = std::move(*this);
else
- {
- this->_M_reset();
- std::_Construct(std::__addressof(this->_M_u),
- in_place_index<__j>,
- std::move(__rhs_mem));
- this->_M_index = __j;
- __rhs._M_reset();
- }
- }
- }
- else
- {
- if (!this->valueless_by_exception()) [[__likely__]]
- {
- __rhs = std::move(*this);
- this->_M_reset();
+ __variant::__raw_idx_visit(
+ [&__rhs](auto&& __this_mem, auto __this_index) mutable
+ {
+ constexpr size_t __k = __this_index;
+ if constexpr (__k != variant_npos)
+ __variant::__emplace<__k>(__rhs,
+ std::move(__this_mem));
+ }, *this);
+
+ __variant::__emplace<__j>(*this, std::move(__tmp));
}
}
}, __rhs);
}
- private:
-
#if defined(__clang__) && __clang_major__ <= 7
public:
using _Base::_M_u; // See https://bugs.llvm.org/show_bug.cgi?id=31852
- private:
#endif
+ private:
template<size_t _Np, typename _Vp>
friend constexpr decltype(auto)
__detail::__variant::__get(_Vp&& __v) noexcept;
^ permalink raw reply [flat|nested] only message in thread
only message in thread, other threads:[~2021-11-04 9:39 UTC | newest]
Thread overview: (only message) (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-11-04 9:39 [gcc r12-4890] libstdc++: Refactor emplace-like functions in std::variant Jonathan Wakely
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).