[PATCH v2] libstdc++: Store basic_format_arg::handle in __format::_Arg_value
Tomasz Kamiński
tkaminsk@redhat.com
Thu Mar 5 06:07:24 GMT 2026
This patch changes the type of _M_handle member of __format::_Arg_value
from __format::_HandleBase union member to basic_format_arg<_Context>::handle.
This allows handle to be stored (using placement new) inside _Arg_value at
compile time, as type _M_handle member now matches stored object.
In addition to above, to make handle usable at compile time, we adjust
the _M_func signature to match the stored function, avoiding the need
for reinterpret cast.
To avoid cycling dependency, where basic_format_arg<_Context> requires
instantiating _Arg_value<_Context> for its _M_val member, that in turn
requires basic_format_arg<_Context>::handle, we define handle as nested
class inside _Arg_value and change basic_format_arg<_Context>::handle
to alias for it.
Finally, the handle(_Tp&) constructor is now constrained with to not accept
handle itself, as otherwise it would be used instead of copy-constructor
when constructing from handle&.
As _Arg_value is already templated on _Context, this change should not lead
to additional template instantiations.
libstdc++-v3/ChangeLog:
* include/std/format (__Arg_value::handle): Define, extracted
with modification from basic_format_arg::handle.
(_Arg_value::_Handle_base): Remove.
(_Arg_value::_M_handle): Change type to handle.
(_Arg_value::_M_get, _Arg_value::_M_set): Check for handle
type directly, and return result unmodified.
(basic_format_arg::__formattable): Remove.
(basic_format_arg::handle): Replace with alias to
_Arg_value::handle.
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
---
v2:
- makes handle a nested class inside _Arg_value
- add new line after defitnion of basic_format_arg::handle alias
The first paragraph already mentioned that storing exactly handle
is required for placement new to work at compile time, so haven't
expanded it.
Testing on x86_64-linux. All *format* test passed.
OK for trunk when test passes?
libstdc++-v3/include/std/format | 110 +++++++++++++++-----------------
1 file changed, 50 insertions(+), 60 deletions(-)
diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 2e4463c6596..b014936a21e 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -4113,10 +4113,52 @@ namespace __format
{
using _CharT = typename _Context::char_type;
- struct _HandleBase
+ class handle
{
+ using _CharT = typename _Context::char_type;
+ using _Func = void(*)(basic_format_parse_context<_CharT>&,
+ _Context&, const void*);
+
+ // Format as const if possible, to reduce instantiations.
+ template<typename _Tp>
+ using __maybe_const_t
+ = __conditional_t<__formattable_with<const _Tp, _Context>,
+ const _Tp, _Tp>;
+
+ template<typename _Tq>
+ static void
+ _S_format(basic_format_parse_context<_CharT>& __parse_ctx,
+ _Context& __format_ctx, const void* __ptr)
+ {
+ using _Td = remove_const_t<_Tq>;
+ typename _Context::template formatter_type<_Td> __f;
+ __parse_ctx.advance_to(__f.parse(__parse_ctx));
+ _Tq& __val = *const_cast<_Tq*>(static_cast<const _Td*>(__ptr));
+ __format_ctx.advance_to(__f.format(__val, __format_ctx));
+ }
+
+ template<typename _Tp>
+ requires (!is_same_v<remove_cv_t<_Tp>, handle>)
+ explicit
+ handle(_Tp& __val) noexcept
+ : _M_ptr(__builtin_addressof(__val))
+ , _M_func(&_S_format<__maybe_const_t<_Tp>>)
+ { }
+
+ friend class basic_format_arg<_Context>;
+
+ public:
+ handle(const handle&) = default;
+ handle& operator=(const handle&) = default;
+
+ [[__gnu__::__always_inline__]]
+ void
+ format(basic_format_parse_context<_CharT>& __pc, _Context& __fc) const
+ { _M_func(__pc, __fc, this->_M_ptr); }
+
+ private:
const void* _M_ptr;
- void (*_M_func)();
+ _Func _M_func;
};
union
@@ -4142,7 +4184,7 @@ namespace __format
const _CharT* _M_str;
basic_string_view<_CharT> _M_sv;
const void* _M_ptr;
- _HandleBase _M_handle;
+ handle _M_handle;
#ifdef __SIZEOF_INT128__
__int128 _M_i128;
unsigned __int128 _M_u128;
@@ -4232,8 +4274,8 @@ namespace __format
else if constexpr (is_same_v<_Tp, _Float64>)
return __u._M_f64;
#endif
- else if constexpr (derived_from<_Tp, _HandleBase>)
- return static_cast<_Tp&>(__u._M_handle);
+ else if constexpr (is_same_v<_Tp, handle>)
+ return __u._M_handle;
// Otherwise, ill-formed.
}
@@ -4254,7 +4296,7 @@ namespace __format
void
_M_set(_Tp __v) noexcept
{
- if constexpr (derived_from<_Tp, _HandleBase>)
+ if constexpr (is_same_v<_Tp, handle>)
std::construct_at(&_M_handle, __v);
else
_S_get<_Tp>(*this) = __v;
@@ -4279,57 +4321,8 @@ namespace __format
{
using _CharT = typename _Context::char_type;
- template<typename _Tp>
- static constexpr bool __formattable
- = __format::__formattable_with<_Tp, _Context>;
-
public:
- class handle : public __format::_Arg_value<_Context>::_HandleBase
- {
- using _Base = typename __format::_Arg_value<_Context>::_HandleBase;
-
- // Format as const if possible, to reduce instantiations.
- template<typename _Tp>
- using __maybe_const_t
- = __conditional_t<__formattable<const _Tp>, const _Tp, _Tp>;
-
- template<typename _Tq>
- static void
- _S_format(basic_format_parse_context<_CharT>& __parse_ctx,
- _Context& __format_ctx, const void* __ptr)
- {
- using _Td = remove_const_t<_Tq>;
- typename _Context::template formatter_type<_Td> __f;
- __parse_ctx.advance_to(__f.parse(__parse_ctx));
- _Tq& __val = *const_cast<_Tq*>(static_cast<const _Td*>(__ptr));
- __format_ctx.advance_to(__f.format(__val, __format_ctx));
- }
-
- template<typename _Tp>
- explicit
- handle(_Tp& __val) noexcept
- {
- this->_M_ptr = __builtin_addressof(__val);
- auto __func = _S_format<__maybe_const_t<_Tp>>;
- this->_M_func = reinterpret_cast<void(*)()>(__func);
- }
-
- friend class basic_format_arg<_Context>;
-
- public:
- handle(const handle&) = default;
- handle& operator=(const handle&) = default;
-
- [[__gnu__::__always_inline__]]
- void
- format(basic_format_parse_context<_CharT>& __pc, _Context& __fc) const
- {
- using _Func = void(*)(basic_format_parse_context<_CharT>&,
- _Context&, const void*);
- auto __f = reinterpret_cast<_Func>(this->_M_func);
- __f(__pc, __fc, this->_M_ptr);
- }
- };
+ using handle = __format::_Arg_value<_Context>::handle;
[[__gnu__::__always_inline__]]
basic_format_arg() noexcept : _M_type(__format::_Arg_none) { }
@@ -4623,10 +4616,7 @@ namespace __format
case _Arg_ptr:
return std::forward<_Visitor>(__vis)(_M_val._M_ptr);
case _Arg_handle:
- {
- auto& __h = static_cast<handle&>(_M_val._M_handle);
- return std::forward<_Visitor>(__vis)(__h);
- }
+ return std::forward<_Visitor>(__vis)(_M_val._M_handle);
#ifdef __SIZEOF_INT128__
case _Arg_i128:
return std::forward<_Visitor>(__vis)(_M_val._M_i128);
--
2.53.0
More information about the Libstdc++
mailing list