Re: [PATCH] libstdc++: Format std::float16_t and std::bfloat16_t using respective std::to_chars overloads [PR126731]

On Thu, Aug 20, 2026 at 3:51 PM Tomasz Kaminski <[email protected]> wrote:

>
>
> On Thu, Aug 20, 2026 at 2:36 PM Jonathan Wakely <[email protected]>
> wrote:
>
>> On Thu, 13 Aug 2026 at 10:36 +0200, Tomasz Kamiński wrote:
>> >This patch removes the partial specialization for float16_t and
>> bfloat16_t
>> >that formatted the values by casting to float. In consequence an implicit
>> >specialization of generic floating-point specialization is used, and
>> >to_chars overloads for their types are used.
>> >As explained in r13-3591-g0ae26533b3e268 (that adds corresponding
>> to_chars
>> >overload), the shortest string (produced when no precision is specified)
>> >is shorter for above, than float.
>> >
>> >As formatter specializations were defined under the same condition as
>> >corresponding to_chars overloads, the __formattable_float constrain on
>> >formatter floating-point specialization makes it enable in same cases.
>> >
>> >This reverts parts of r14-3305-g6cf214b4fc97f5 (revereted partially
>>
>> "reverted"
>>
>> >in r14-3329-g27d0cfcb2b33de), leading to (necessary for correctness)
>> >increase of the number template specializations. It also depends on
>> >preserving values of above types (instead of float) in basic_format_arg
>> >introduced r16-616-g9c9a7316adb996.
>>
>> Ah, so I added these specializations in r14-3305-g6cf214b4fc97f5 to
>> support printing extended floating-point types in C++20, but that
>> turned out to not work. So I adjusted the conditions for enabling them
>> in r14-3329-g27d0cfcb2b33de, so that they are only enabled for C++23.
>> But really we should have just removed the specializations again,
>> because the __formattable_float specialization formats them properly
>> since r16-616-g9c9a7316adb996.
>>
>> >The specializations for float32_t and float64_t are left unchanged,
>> >as their to_chars overloads are also implemented in terms of casting
>> >to float/double respectivelly.
>> >
>> >libstdc++-v3/ChangeLog:
>> >
>> >       PR libstdc++/126731
>> >       * include/std/format (std::formatter<_Float16, _CharT>)
>> >       (std::formatter<__format::__bflt16_t, _CharT>): Remove
>> >       partial specializations.
>> >       * testsuite/std/format/formatter/ext_float.cc: Test output
>> >       for closest value to 1/10.
>> >---
>> >As far as I can tell this case will not be affected by P3505R4
>> >"Fix the default floating-point representation in std::format",
>> >as float(0.1f16) is 1.998p-4, while 0.1 f is 1.99999ap-4, so
>> >only later could use 0.1 as shortest string.
>> >
>> >Testing on x86_64-linux. *format* passed in all standard modes,
>> >-m32, debug and assertions. OK for trunk and 16?
>> >(We cannot backport it to eariel standards due dependency described
>> >above).
>>
>> Yes OK for trunk and gcc-16.
>>
>> If I understand correctly, I think we could remove them in gcc-14 and
>> gcc-15, because they're redundant (the __formattable_float
>> specialization can format them). But we would still get incorrect
>> results, because basic_format_arg would convert them to float.
>
> We will get inconsistent results, depending on how you format them:
>  * if (b)float16 is passed as argument to format call, then ye we will
> convert
>    them to float, and get correct result
>
I meant incorrect output in this case.

>  * if you call formatter<(b)float16> direclty (for example becaue you are
> writting
>   formatter for a class with a member), you will get correct output
>
>> So the extra specializations in gcc-14 and gcc-15 are not doing any harm,
>> and
>> it doesn't seem worth backporting it.
>>
> They are keeping us consistently wrong; if we remove them, we will be
> inconsistent.
> and wrong in the usual case.
>
>>
>>
>> > libstdc++-v3/include/std/format               | 44 -------------------
>> > .../std/format/formatter/ext_float.cc         | 22 ++++++----
>> > 2 files changed, 13 insertions(+), 53 deletions(-)
>> >
>> >diff --git a/libstdc++-v3/include/std/format
>> b/libstdc++-v3/include/std/format
>> >index 729cb89ec60..8d530e883c3 100644
>> >--- a/libstdc++-v3/include/std/format
>> >+++ b/libstdc++-v3/include/std/format
>> >@@ -3003,28 +3003,6 @@ namespace __format
>> >     };
>> > #endif
>> >
>> >-#if defined(__STDCPP_FLOAT16_T__) &&
>> defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
>> >-  // Reuse __formatter_fp<C>::format<float, Out> for _Float16.
>> >-  template<__format::__char _CharT>
>> >-    struct formatter<_Float16, _CharT>
>> >-    {
>> >-      formatter() = default;
>> >-
>> >-      [[__gnu__::__always_inline__]]
>> >-      constexpr typename basic_format_parse_context<_CharT>::iterator
>> >-      parse(basic_format_parse_context<_CharT>& __pc)
>> >-      { return _M_f.parse(__pc); }
>> >-
>> >-      template<typename _Out>
>> >-      typename basic_format_context<_Out, _CharT>::iterator
>> >-      format(_Float16 __u, basic_format_context<_Out, _CharT>& __fc)
>> const
>> >-      { return _M_f.format((float)__u, __fc); }
>> >-
>> >-    private:
>> >-      __format::__formatter_fp<_CharT> _M_f;
>> >-    };
>> >-#endif
>> >-
>> > #if defined(__FLT32_DIG__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
>> >   // Reuse __formatter_fp<C>::format<float, Out> for _Float32.
>> >   template<__format::__char _CharT>
>> >@@ -3114,28 +3092,6 @@ namespace __format
>> >     };
>> > #endif
>> >
>> >-#if defined(__STDCPP_BFLOAT16_T__) &&
>> defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
>> >-  // Reuse __formatter_fp<C>::format<float, Out> for bfloat16_t.
>> >-  template<__format::__char _CharT>
>> >-    struct formatter<__format::__bflt16_t, _CharT>
>> >-    {
>> >-      formatter() = default;
>> >-
>> >-      [[__gnu__::__always_inline__]]
>> >-      constexpr typename basic_format_parse_context<_CharT>::iterator
>> >-      parse(basic_format_parse_context<_CharT>& __pc)
>> >-      { return _M_f.parse(__pc); }
>> >-
>> >-      template<typename _Out>
>> >-      typename basic_format_context<_Out, _CharT>::iterator
>> >-      format(__gnu_cxx::__bfloat16_t __u,
>> >-             basic_format_context<_Out, _CharT>& __fc) const
>> >-      { return _M_f.format((float)__u, __fc); }
>> >-
>> >-    private:
>> >-      __format::__formatter_fp<_CharT> _M_f;
>> >-    };
>> >-#endif
>> > #endif // __cpp_lib_to_chars
>> >
>> >   /** Format a pointer.
>> >diff --git a/libstdc++-v3/testsuite/std/format/formatter/ext_float.cc
>> b/libstdc++-v3/testsuite/std/format/formatter/ext_float.cc
>> >index 82e696fa797..7b231b07006 100644
>> >--- a/libstdc++-v3/testsuite/std/format/formatter/ext_float.cc
>> >+++ b/libstdc++-v3/testsuite/std/format/formatter/ext_float.cc
>> >@@ -4,10 +4,14 @@
>> > #include <testsuite_hooks.h>
>> >
>> > template<typename T>
>> >-bool format_float()
>> >+void
>> >+verify_output()
>> > {
>> >-    auto s = std::format("{:#} != {:<+7.3f}", (T)-0.0, (T)0.5);
>> >-    return s == "-0. != +0.500 ";
>> >+  auto s = std::format("{:#} != {:<+7.3f}", T(-0.0), T(0.5));
>> >+  VERIFY( s == "-0. != +0.500 ");
>> >+
>> >+  s = std::format("{}", T(1)/T(10));
>> >+  VERIFY( s == "0.1");
>> > }
>> >
>> > #if __cplusplus > 202002L
>> >@@ -23,7 +27,7 @@ test_float16()
>> > {
>> > #if __FLT16_DIG__
>> >   if constexpr (formattable<_Float16>)
>> >-    VERIFY( format_float<_Float16>() );
>> >+    verify_output<_Float16>();
>> >   else
>> >     std::puts("Cannot format _Float16 on this target");
>> > #endif
>> >@@ -34,7 +38,7 @@ test_float32()
>> > {
>> > #if __FLT32_DIG__
>> >   if constexpr (formattable<_Float32>)
>> >-    VERIFY( format_float<_Float32>() );
>> >+    verify_output<_Float32>();
>> >   else
>> >     std::puts("Cannot format _Float32 on this target");
>> > #endif
>> >@@ -45,7 +49,7 @@ test_float64()
>> > {
>> > #if __FLT64_DIG__
>> >   if constexpr (formattable<_Float64>)
>> >-    VERIFY( format_float<_Float64>() );
>> >+    verify_output<_Float64>();
>> >   else
>> >     std::puts("Cannot format _Float64 on this target");
>> > #endif
>> >@@ -56,13 +60,13 @@ test_float128()
>> > {
>> > #ifdef __SIZEOF_FLOAT128__
>> >   if constexpr (formattable<__float128>)
>> >-    VERIFY( format_float<__float128>() );
>> >+    verify_output<__float128>();
>> >   else
>> >     std::puts("Cannot format __float128 on this target");
>> > #endif
>> > #if __FLT128_DIG__
>> >   if constexpr (formattable<_Float128>)
>> >-    VERIFY( format_float<_Float128>() );
>> >+    verify_output<_Float128>();
>> >   else
>> >     std::puts("Cannot format _Float128 on this target");
>> > #endif
>> >@@ -75,7 +79,7 @@ test_bfloat16()
>> >   using bfloat16_t = decltype(0.0bf16);
>> >
>> >   if constexpr (formattable<bfloat16_t>)
>> >-    VERIFY( format_float<bfloat16_t>() );
>> >+    verify_output<bfloat16_t>();
>> >   else
>> >     std::puts("Cannot format bfloat16_t on this target");
>> > #endif
>> >--
>> >2.55.0
>> >
>> >
>>
>>