[PATCH] libstdc++: Store basic_format_arg::handle in __format::_Arg_value
Jonathan Wakely
jwakely@redhat.com
Wed Mar 4 14:54:43 GMT 2026
On Tue, 24 Feb 2026 at 10:47 +0100, Tomasz Kamiński wrote:
>This patch changes the type of _M_handle member of __format::_Arg_value
>from __format::_HandleBase union member 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 introduce __format::_Handle,
>and change basic_format_arg<_Context>::handle to alias for it. To make
>this unobservable we also define _Handle<_Context>::handle alias, to
>support uses of injected-class name,
>
>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 (__format::_Handle): Define, extracted
> with modification from basic_format_arg::handle.
> (_Arg_value::_Handle_base): Remove.
> (_Arg_value::_M_handle): Change type to _Handle<_Context>.
> (_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 _Handle.
>
>Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
>---
>I do not see there is any drawback of doing the above, and this
>technically would be ABI break, but the actual binary representation
>does not change. So I think it would be coode to merge it to GCC-16.
>
>Testing on x86_64-linux. All *format* test already passed.
>OK for trunk when all test passes.
>
> libstdc++-v3/include/std/format | 122 +++++++++++++++-----------------
> 1 file changed, 57 insertions(+), 65 deletions(-)
>
>diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
>index 2e4463c6596..66bf3c246e5 100644
>--- a/libstdc++-v3/include/std/format
>+++ b/libstdc++-v3/include/std/format
>@@ -4109,15 +4109,60 @@ namespace __format
> using enum _Arg_t;
>
> template<typename _Context>
>- struct _Arg_value
>+ class _Handle
> {
> using _CharT = typename _Context::char_type;
>+ using _Func = void(*)(basic_format_parse_context<_CharT>&,
>+ _Context&, const void*);
>
>- struct _HandleBase
>- {
>- const void* _M_ptr;
>- void (*_M_func)();
We could have just fixed the type of this member, right?
I stored it as void(*)() and used reinterpret_cast, but I think it
could always have been this type instead:
void(*)(basic_format_parse_context<_CharT>&, _Context&, const void*)
But I think it does still makes sense to get rid of the _HandleBase
type and just deal with one type (parameterized on Context) instead of
a base and a derived type.
>- };
>+ // 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:
>+ using handle = _Handle; // emulate injected class name
We could define _Arg_value<Context>::handle instead of a separate
_Handle<Context>, right? Then its injected class name would be
"handle" anyway.
i.e. still remove _Arg_value<C>::_HandleBase, but move
basic_format_arg<C>::handle to be defined as a nested class in
_Arg_value<C> instead. Would that be a little simpler?
>+
>+ _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;
>+ _Func _M_func;
>+ };
>+
>+ template<typename _Context>
>+ struct _Arg_value
>+ {
>+ using _CharT = typename _Context::char_type;
>
> union
> {
>@@ -4142,7 +4187,7 @@ namespace __format
> const _CharT* _M_str;
> basic_string_view<_CharT> _M_sv;
> const void* _M_ptr;
>- _HandleBase _M_handle;
>+ _Handle<_Context> _M_handle;
> #ifdef __SIZEOF_INT128__
> __int128 _M_i128;
> unsigned __int128 _M_u128;
>@@ -4232,8 +4277,8 @@ 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<_Context>>)
>+ return __u._M_handle;
> // Otherwise, ill-formed.
> }
>
>@@ -4254,7 +4299,7 @@ namespace __format
> void
> _M_set(_Tp __v) noexcept
> {
>- if constexpr (derived_from<_Tp, _HandleBase>)
>+ if constexpr (is_same_v<_Tp, _Handle<_Context>>)
> std::construct_at(&_M_handle, __v);
> else
> _S_get<_Tp>(*this) = __v;
>@@ -4279,58 +4324,8 @@ 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::_Handle<_Context>;
Blank line between this using-declaration and the constructor please:
> [[__gnu__::__always_inline__]]
> basic_format_arg() noexcept : _M_type(__format::_Arg_none) { }
>
>@@ -4623,10 +4618,7 @@ 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
>
>
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