[PATCH v2 3/4] libstdc++: Specialize _Iter_sink for ostreambuf_iterator

[Anlai Lu]

Hi Tomasz,

Thanks for the patch.  I have reviewed it previously but just now I ran
some additional tests.  During testing, I found two issues that should
be fixed before this is merged.

> On Aug 11, 2026, at 00:11, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
> 
> From: Anlai Lu <agicy@qq.com>
> 
> Add partial specialization of _Iter_sink for ostreambuf_iterator
> that inherits _Streambuf_sink, replacing per-character sputc with
> bulk sputn and zero-copy put-area writes.
> 
> All counting and truncation (_M_max) is handled in this
> specialization so that _Streambuf_sink stays a pure I/O layer.
> _M_overflow counts all characters and only writes up to the limit,
> so format_to_n can compute the total output length. _M_discarding
> returns false for the same reason. Stack writes go through
> _M_out._M_put() which tracks failure on the iterator.  sputn
> exceptions are caught and the iterator is marked failed via
> _M_set_failed().
> 
> The maximum count uses size_t with _S_no_limit sentinel, matching
> the _Ptr_sink convention and avoiding signed comparisons.
> 
> libstdc++-v3/ChangeLog:
> 
> * include/bits/streambuf_iterator.h (__format::_Iter_sink)
> [__glibcxx_format]: Declare with specialization for
> ostreambuf_iterator.
> (std::ostreambuf_iterator) [__glibcxx_format]: Befriend
> __format::_Iter_sink for ostreambuf_iterator.
> * include/std/format (_Iter_sink<_CharT, ostreambuf_iterator<...>>):
> Define new partial specialization.
> 
> Co-authored-by: Tomasz Kamiński <tkaminsk@redhat.com>
> Signed-off-by: Anlai Lu <agicy@qq.com>
> Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
> ---
> Changes in v2:
> - limit unused/reserved buffer to the remaining characters
>   in _M_overflow/_M_reserve and constructor. Mostly via
>   calling _M_trim_buf. Rewrote _M_overflow in process
> - overloads constructor without limit to avoid above
> - befriend _Iter_sink specialization with ostreambuf_iterator
> - remove _M_out member and recosntructor iterator in _M_finish
> 
> Testing on powerprc64. Tested on x86_64, additionally tested
> *format* and *print* in all standard modes, assertions and
> debug. OK for trunk?
> 
> .../include/bits/streambuf_iterator.h         | 12 +++
> libstdc++-v3/include/std/format               | 82 +++++++++++++++++++
> 2 files changed, 94 insertions(+)
> 
> diff --git a/libstdc++-v3/include/bits/streambuf_iterator.h b/libstdc++-v3/include/bits/streambuf_iterator.h
> index 095928ca4d8..d715edc24ae 100644
> --- a/libstdc++-v3/include/bits/streambuf_iterator.h
> +++ b/libstdc++-v3/include/bits/streambuf_iterator.h
> @@ -42,6 +42,14 @@ namespace std _GLIBCXX_VISIBILITY(default)
> {
> _GLIBCXX_BEGIN_NAMESPACE_VERSION
> 
> +#ifdef __glibcxx_format // C++ >= 20 && HOSTED
> +  namespace __format {
> +    template<typename, typename> class _Iter_sink;
> +    template<typename _CharT, typename _Traits>
> +      class _Iter_sink<_CharT, ostreambuf_iterator<_CharT, _Traits>>;
> +  }
> +#endif
> +
>   /**
>    * @addtogroup iterators
>    * @{
> @@ -266,6 +274,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> copy(istreambuf_iterator<_CharT2>, istreambuf_iterator<_CharT2>,
>     ostreambuf_iterator<_CharT2>);
> 
> +#ifdef __glibcxx_format // C++ >= 20 && HOSTED
> +      friend class __format::_Iter_sink<char_type, ostreambuf_iterator>;
> +#endif
> +
>     private:
>       streambuf_type* _M_sbuf;
>       bool _M_failed;
> diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
> index 9223cd2e78b..ad6a56ecc3a 100644
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> @@ -3847,6 +3847,88 @@ namespace __format
>       }
>     };
> 
> +  // Specialization replacing per-character sputc with bulk sputn
> +  // and zero-copy writes into the streambuf's put area.
> +  template<typename _CharT, typename _Traits>
> +    class _Iter_sink<_CharT, ostreambuf_iterator<_CharT, _Traits>>
> +    : public _Streambuf_sink<_CharT, _Traits>
> +    {
> +      using _Base = _Streambuf_sink<_CharT, _Traits>;
> +      using _OutIter = ostreambuf_iterator<_CharT, _Traits>;
> +
> +      static constexpr size_t _S_no_limit = size_t(-1);
> +
> +      void
> +      _M_trim_buf()
> +      {
> + const size_t __avail = _M_max - _M_count;
> + if (this->_M_unused().size() > __avail)
> +  this->_M_reset(this->_M_unused().first(__avail));
> +      }
> +
> +    protected:
> +      size_t _M_max = _S_no_limit;
> +      size_t _M_count = 0;
> +
> +      void
> +      _M_overflow() override
> +      {
> + const size_t __new = this->_M_used().size();
> + if (_M_count >= _M_max)
> +  this->_M_reset(this->_M_buf);
> + else if (size_t __a = _M_max - _M_count; __a > __new)
> +  {
> +    _Base::_M_overflow();
> +    _M_trim_buf();

_M_trim_buf() depends on the value of _M_count.  In this else-if
branch it is called before _M_count is updated, which leads to an
incorrect trim.

> +  }
> + else
> +  {
> +    this->_M_flush();
> +    this->_M_reset(this->_M_buf);
> +  }
> + _M_count += __new;
> +      }
> +
> +      bool
> +      _M_discarding() const override
> +      { return false; }
> +
> +    public:
> +      [[__gnu__::__always_inline__]]
> +      explicit
> +      _Iter_sink(_OutIter __out)
> +      : _Base(__out._M_sbuf)
> +      { }
> +
> +      [[__gnu__::__always_inline__]]
> +      explicit
> +      _Iter_sink(_OutIter __out, iter_difference_t<_OutIter> __max)
> +      : _Base(__out._M_sbuf), _M_max(__max < 0 ? _S_no_limit : size_t(__max))
> +      { _M_trim_buf(); }
> +
> +      typename _Sink<_CharT>::_Reservation
> +      _M_reserve(size_t __n) override
> +      {
> + if (_M_count < _M_max)
> +  if (size_t __a = _M_max - _M_count; __a >= __n)
> +    return _Base::_M_reserve(__n);
> + return { nullptr };
> +      }
> +
> +      format_to_n_result<_OutIter>
> +      _M_finish() &&
> +      {
> + _M_count += this->_M_used().size();
> + if (_M_count < _M_max)
> +  this->_M_flush();

This condition should be _M_count <= _M_max.
When _M_count becomes exactly _M_max, there may still be data
that has not been flushed.

> +
> + _OutIter __out;
> + __out._M_sbuf = this->_M_sbuf;
> + __out._M_failed = this->_M_write_failed;
> + return { std::move(__out), ptrdiff_t(_M_count) };
> +      }
> +    };
> +
>   // Used for contiguous iterators.
>   // No buffer is used, characters are written straight to the iterator.
>   // We do not know the size of the output range, so the span size just grows
> -- 
> 2.55.0

Two issues above can be covered by this test case:

  std::string big(257, 'A');
  std::ostringstream os;
  auto res = std::format_to_n(std::ostreambuf_iterator<char>(os),
                              257, "{}", big);
  VERIFY(res.size == 257);
  VERIFY(os.view() == big);

The existing test suite does not cover this exact-boundary case, which
is why these issues were not caught earlier.  The existing cases in
test_format_to_n have a (len=1000, limit=256) and (len=8, limit=8), but
does not cover the case (len > 256 && len = limit).