On Thu, 05 Mar 2026 at 07:23 +0100, Tomasz Kamiński wrote:
The _Arg_value::_M_set method, initialized the union member, by
assigning to reference to that member produced by _M_get(*this).
However, per language rules, such assignment has undefined behavior,
if alternative was not already active, same as for any object not
within it's lifetime.
"its" without an apostrophe, because English likes to be difficult.
OK with that tweak, thanks.
To address above, we modify _M_set to use placement new for the class
types, and invoke _S_access with two arguments for all other types.
The _S_access (rename of _S_get) is modified to assign the value of
the second parameter (if provided) to the union member. Such direct
assignments are treated specially in the language (see N5032
[class.union.general] p5), and will start lifetime of trivially default
constructible alternative.
libstdc++-v3/ChangeLog:
* include/std/format (_Arg_value::_M_get): Rename to...
(_Arg_value::_M_access): Modified to accept optional
second parameter that is assigned to value.
(_Arg_value::_M_get): Handle rename.
(_Arg_value::_M_set): Use construct_at for basic_string_view,
handle, and two-argument _S_access for other types.
Signed-off-by: Tomasz Kamiński <[email protected]>
Signed-off-by: Ivan Lazaric <[email protected]>
Co-authored-by: Ivan Lazaric <[email protected]>
---
v2:
- fixes typos and add mention to [class.union.general] p5 in commit
description
- add comments to _M_set function
Testing on x86_64-linux. All *format* test passed.
OK for trunk when all test passes?
libstdc++-v3/include/std/format | 62 +++++++++++++++++++--------------
1 file changed, 35 insertions(+), 27 deletions(-)
diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index b014936a21e..9b5d5d499f5 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -4212,67 +4212,70 @@ namespace __format
{ _S_get<_Tp>() = __val; }
#endif
- template<typename _Tp, typename _Self>
+ // Returns reference to the _Arg_value member with the type _Tp.
+ // Value of second argument (if provided), is assigned to that member.
+ template<typename _Tp, typename _Self, typename... _Value>
[[__gnu__::__always_inline__]]
static auto&
- _S_get(_Self& __u) noexcept
+ _S_access(_Self& __u, _Value... __value) noexcept
{
+ static_assert(sizeof...(_Value) <= 1);
if constexpr (is_same_v<_Tp, bool>)
- return __u._M_bool;
+ return (__u._M_bool = ... = __value);
else if constexpr (is_same_v<_Tp, _CharT>)
- return __u._M_c;
+ return (__u._M_c = ... = __value);
else if constexpr (is_same_v<_Tp, int>)
- return __u._M_i;
+ return (__u._M_i = ... = __value);
else if constexpr (is_same_v<_Tp, unsigned>)
- return __u._M_u;
+ return (__u._M_u = ... = __value);
else if constexpr (is_same_v<_Tp, long long>)
- return __u._M_ll;
+ return (__u._M_ll = ... = __value);
else if constexpr (is_same_v<_Tp, unsigned long long>)
- return __u._M_ull;
+ return (__u._M_ull = ... = __value);
else if constexpr (is_same_v<_Tp, float>)
- return __u._M_flt;
+ return (__u._M_flt = ... = __value);
else if constexpr (is_same_v<_Tp, double>)
- return __u._M_dbl;
+ return (__u._M_dbl = ... = __value);
#ifndef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
else if constexpr (is_same_v<_Tp, long double>)
- return __u._M_ldbl;
+ return (__u._M_ldbl = ... = __value);
#else
else if constexpr (is_same_v<_Tp, __ibm128>)
- return __u._M_ibm128;
+ return (__u._M_ibm128 = ... = __value);
else if constexpr (is_same_v<_Tp, __ieee128>)
- return __u._M_ieee128;
+ return (__u._M_ieee128 = ... = __value);
#endif
#ifdef __SIZEOF_FLOAT128__
else if constexpr (is_same_v<_Tp, __float128>)
- return __u._M_float128;
+ return (__u._M_float128 = ... = __value);
#endif
else if constexpr (is_same_v<_Tp, const _CharT*>)
- return __u._M_str;
+ return (__u._M_str = ... = __value);
else if constexpr (is_same_v<_Tp, basic_string_view<_CharT>>)
- return __u._M_sv;
+ return (__u._M_sv = ... = __value);
else if constexpr (is_same_v<_Tp, const void*>)
- return __u._M_ptr;
+ return (__u._M_ptr = ... = __value);
#ifdef __SIZEOF_INT128__
else if constexpr (is_same_v<_Tp, __int128>)
- return __u._M_i128;
+ return (__u._M_i128 = ... = __value);
else if constexpr (is_same_v<_Tp, unsigned __int128>)
- return __u._M_u128;
+ return (__u._M_u128 = ... = __value);
#endif
#ifdef __BFLT16_DIG__
else if constexpr (is_same_v<_Tp, __bflt16_t>)
- return __u._M_bf16;
+ return (__u._M_bf16 = ... = __value);
#endif
#ifdef __FLT16_DIG__
else if constexpr (is_same_v<_Tp, _Float16>)
- return __u._M_f16;
+ return (__u._M_f16 = ... = __value);
#endif
#ifdef __FLT32_DIG__
else if constexpr (is_same_v<_Tp, _Float32>)
- return __u._M_f32;
+ return (__u._M_f32 = ... = __value);
#endif
#ifdef __FLT64_DIG__
else if constexpr (is_same_v<_Tp, _Float64>)
- return __u._M_f64;
+ return (__u._M_f64 = ... = __value);
#endif
else if constexpr (is_same_v<_Tp, handle>)
return __u._M_handle;
@@ -4283,23 +4286,28 @@ namespace __format
[[__gnu__::__always_inline__]]
auto&
_M_get() noexcept
- { return _S_get<_Tp>(*this); }
+ { return _S_access<_Tp>(*this); }
template<typename _Tp>
[[__gnu__::__always_inline__]]
const auto&
_M_get() const noexcept
- { return _S_get<_Tp>(*this); }
+ { return _S_access<_Tp>(*this); }
template<typename _Tp>
[[__gnu__::__always_inline__]]
void
_M_set(_Tp __v) noexcept
{
- if constexpr (is_same_v<_Tp, handle>)
+ // Explicitly construct types without trivial default constructor.
+ if constexpr (is_same_v<_Tp, basic_string_view<_CharT>>)
+ std::construct_at(&_M_sv, __v);
+ else if constexpr (is_same_v<_Tp, handle>)
std::construct_at(&_M_handle, __v);
else
- _S_get<_Tp>(*this) = __v;
+ // Builtin types, has trivial default constructor and assignment
+ // changes active member per N5032 [class.union.general] p5.
+ _S_access<_Tp>(*this, __v);
}
};
--
2.53.0
