[PATCH v2] libstdc++: Implement _Escaping_sink to avoid construction of string [PR119820]
Patrick Palka
ppalka@redhat.com
Tue Oct 14 03:37:53 GMT 2025
On Wed, 10 Sep 2025, Tomasz Kamiński wrote:
> This patch implements _Escaping_sink that stores characters in a local (stack)
> buffer. When the buffer is full, the range of characters is escaped and written
> to the underlying sink.
>
> To support above, the __write_escaped_unicode_part function are defined.
> It takes __str and __prev_esc by reference. The __prev_esc value is updated based
> on the last character written. If the buffer ends with an incomplete code point
> sequence, __str is left non-empty and the characters are not written.
> _Escaping_sink then copies these characters to the front of the buffer to
> reconstruct the full code point.
>
> __formatter__str::_M_format_range now uses _Escaping_sink to escape any
> non-continuous character sequences.
>
> This addresses PR119820 by removing the code constructing string
> completely.
>
> PR libstdc++/PR119820
>
> libstdc++-v3/ChangeLog:
>
> * include/std/format (__format::__write_escape_seqs)
> (__format::_Escaping_sink): Define.
> (__format::__write_escaped_unicode_part): Extract from
> __format::__write_escaped_unicode.
> (__format::__write_escaped_unicode): Forward to
> __write_escaped_unicode_part.
> (__formatter_str::_M_format_range): Use _Escaping sink.
> * testsuite/std/format/ranges/string.cc: New tests for
> character which codepoints will be split in buffer and
> escaping. Invoked test_padding.
Looks good to me!
> ---
> v2 just updates the patch description.
>
> libstdc++-v3/include/std/format | 197 +++++++++++++-----
> .../testsuite/std/format/ranges/string.cc | 89 ++++++++
> 2 files changed, 231 insertions(+), 55 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
> index d6a2170e45d..347f9f0a479 100644
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> @@ -105,6 +105,7 @@ namespace __format
> template<typename _CharT> class _Sink;
> template<typename _CharT> class _Fixedbuf_sink;
> template<typename _Out, typename _CharT> class _Padding_sink;
> + template<typename _Out, typename _CharT> class _Escaping_sink;
>
> // Output iterator that writes to a type-erase character sink.
> template<typename _CharT>
> @@ -1066,6 +1067,17 @@ namespace __format
> return ++__out;
> }
>
> + template<typename _Out, typename _CharT>
> + _Out
> + __write_escape_seqs(_Out __out, basic_string_view<_CharT> __units)
> + {
> + using _UChar = make_unsigned_t<_CharT>;
> + for (_CharT __c : __units)
> + __out = __format::__write_escape_seq(
> + __out, static_cast<_UChar>(__c), _Escapes<_CharT>::_S_x());
> + return __out;
> + }
> +
> template<typename _Out, typename _CharT>
> _Out
> __write_escaped_char(_Out __out, _CharT __c)
> @@ -1124,12 +1136,10 @@ namespace __format
>
> template<typename _CharT, typename _Out>
> _Out
> - __write_escaped_unicode(_Out __out,
> - basic_string_view<_CharT> __str,
> - _Term_char __term)
> + __write_escaped_unicode_part(_Out __out, basic_string_view<_CharT>& __str,
> + bool& __prev_esc, _Term_char __term)
> {
> using _Str_view = basic_string_view<_CharT>;
> - using _UChar = make_unsigned_t<_CharT>;
> using _Esc = _Escapes<_CharT>;
>
> static constexpr char32_t __replace = U'\uFFFD';
> @@ -1143,10 +1153,10 @@ namespace __format
> }();
>
> __unicode::_Utf_view<char32_t, _Str_view> __v(std::move(__str));
> + __str = {};
> +
> auto __first = __v.begin();
> auto const __last = __v.end();
> -
> - bool __prev_esc = true;
> while (__first != __last)
> {
> bool __esc_ascii = false;
> @@ -1185,15 +1195,32 @@ namespace __format
> __out = __format::__write_escaped_char(__out, *__first.base());
> else if (__esc_unicode)
> __out = __format::__write_escape_seq(__out, *__first, _Esc::_S_u());
> - else // __esc_replace
> - for (_CharT __c : _Str_view(__first.base(), __first._M_units()))
> - __out = __format::__write_escape_seq(__out,
> - static_cast<_UChar>(__c),
> - _Esc::_S_x());
> + // __esc_replace
> + else if (_Str_view __units(__first.base(), __first._M_units());
> + __units.end() != __last.base())
> + __out = __format::__write_escape_seqs(__out, __units);
> + else
> + {
> + __str = __units;
> + return __out;
> + }
> +
> __prev_esc = true;
> ++__first;
> -
> }
> +
> + return __out;
> + }
> +
> + template<typename _CharT, typename _Out>
> + _Out
> + __write_escaped_unicode(_Out __out, basic_string_view<_CharT> __str,
> + _Term_char __term)
> + {
> + bool __prev_escape = true;
> + __out = __format::__write_escaped_unicode_part(__out, __str,
> + __prev_escape, __term);
> + __out = __format::__write_escape_seqs(__out, __str);
> return __out;
> }
>
> @@ -1412,55 +1439,28 @@ namespace __format
> size_t(ranges::distance(__rg)));
> return format(__str, __fc);
> }
> - else if (!_M_spec._M_debug)
> + else
> {
> + auto __handle_debug = [this, &__rg]<typename _NOut>(_NOut __nout)
> + {
> + if (!_M_spec._M_debug)
> + return ranges::copy(__rg, std::move(__nout)).out;
> +
> + _Escaping_sink<_NOut, _CharT>
> + __sink(std::move(__nout), _Term_quote);
> + ranges::copy(__rg, __sink.out());
> + return __sink._M_finish();
> + };
> +
> const size_t __padwidth = _M_spec._M_get_width(__fc);
> if (__padwidth == 0 && _M_spec._M_prec_kind == _WP_none)
> - return ranges::copy(__rg, __fc.out()).out;
> + return __handle_debug(__fc.out());
>
> - _Padding_sink<_Out, _CharT> __sink(__fc.out(), __padwidth,
> - _M_spec._M_get_precision(__fc));
> - ranges::copy(__rg, __sink.out());
> + _Padding_sink<_Out, _CharT>
> + __sink(__fc.out(), __padwidth, _M_spec._M_get_precision(__fc));
> + __handle_debug(__sink.out());
> return __sink._M_finish(_M_spec._M_align, _M_spec._M_fill);
> }
> - else if constexpr (ranges::forward_range<_Rg> || ranges::sized_range<_Rg>)
> - {
> - const size_t __n(ranges::distance(__rg));
> - size_t __w = __n;
> - if constexpr (!__unicode::__literal_encoding_is_unicode<_CharT>())
> - if (size_t __max = _M_spec._M_get_precision(__fc); __n > __max)
> - __w == __max;
> -
> - if (__w <= __format::__stackbuf_size<_CharT>)
> - {
> - _CharT __buf[__format::__stackbuf_size<_CharT>];
> - ranges::copy_n(ranges::begin(__rg), __w, __buf);
> - return _M_format_escaped(_String_view(__buf, __n), __fc);
> - }
> - else if constexpr (ranges::random_access_range<_Rg>)
> - {
> - ranges::iterator_t<_Rg> __first = ranges::begin(__rg);
> - ranges::subrange __sub(__first, __first + __w);
> - return _M_format_escaped(_String(from_range, __sub), __fc);
> - }
> - else if (__w <= __n)
> - {
> - ranges::subrange __sub(
> - counted_iterator(ranges::begin(__rg), __w),
> - default_sentinel);
> - return _M_format_escaped(_String(from_range, __sub), __fc);
> - }
> - else if constexpr (ranges::sized_range<_Rg>)
> - return _M_format_escaped(_String(from_range, __rg), __fc);
> - else
> - {
> - // N.B. preserve the computed size
> - ranges::subrange __sub(__rg, __n);
> - return _M_format_escaped(_String(from_range, __sub), __fc);
> - }
> - }
> - else
> - return _M_format_escaped(_String(from_range, __rg), __fc);
> }
>
> constexpr void
> @@ -3915,6 +3915,93 @@ namespace __format
> }
> };
>
> + template<typename _Out, typename _CharT>
> + class _Escaping_sink : public _Buf_sink<_CharT>
> + {
> + using _Esc = _Escapes<_CharT>;
> +
> + _Out _M_out;
> + _Term_char _M_term : 2;
> + unsigned _M_prev_escape : 1;
> + unsigned _M_out_discards : 1;
> +
> + void
> + _M_sync_discarding()
> + {
> + if constexpr (is_same_v<_Out, _Sink_iter<_CharT>>)
> + _M_out_discards = _M_out._M_discarding();
> + }
> +
> + void
> + _M_write()
> + {
> + span<_CharT> __bytes = this->_M_used();
> + basic_string_view<_CharT> __str(__bytes.data(), __bytes.size());
> +
> + size_t __rem = 0;
> + if constexpr (__unicode::__literal_encoding_is_unicode<_CharT>())
> + {
> + bool __prev_escape = _M_prev_escape;
> + _M_out = __format::__write_escaped_unicode_part(
> + std::move(_M_out), __str, __prev_escape, _M_term);
> + _M_prev_escape = __prev_escape;
> +
> + __rem = __str.size();
> + if (__rem > 0 && __str.data() != this->_M_buf) [[unlikely]]
> + ranges::move(__str, this->_M_buf);
> + }
> + else
> + _M_out = __format::__write_escaped_ascii(
> + std::move(_M_out), __str, _M_term);
> +
> + this->_M_reset(this->_M_buf, __rem);
> + _M_sync_discarding();
> + }
> +
> + void
> + _M_overflow() override
> + {
> + if (_M_out_discards)
> + this->_M_rewind();
> + else
> + _M_write();
> + }
> +
> + bool
> + _M_discarding() const override
> + { return _M_out_discards; }
> +
> + public:
> + [[__gnu__::__always_inline__]]
> + explicit
> + _Escaping_sink(_Out __out, _Term_char __term)
> + : _M_out(std::move(__out)), _M_term(__term),
> + _M_prev_escape(true), _M_out_discards(false)
> + {
> + _M_out = __format::__write(std::move(_M_out), _Esc::_S_term(_M_term));
> + _M_sync_discarding();
> + }
> +
> + _Out
> + _M_finish()
> + {
> + if (_M_out_discards)
> + return std::move(_M_out);
> +
> + if (!this->_M_used().empty())
> + {
> + _M_write();
> + if constexpr (__unicode::__literal_encoding_is_unicode<_CharT>())
> + if (auto __rem = this->_M_used(); !__rem.empty())
> + {
> + basic_string_view<_CharT> __str(__rem.data(), __rem.size());
> + _M_out = __format::__write_escape_seqs(std::move(_M_out), __str);
> + }
> + }
> + return __format::__write(std::move(_M_out), _Esc::_S_term(_M_term));
> + }
> + };
> +
> enum class _Arg_t : unsigned char {
> _Arg_none, _Arg_bool, _Arg_c, _Arg_i, _Arg_u, _Arg_ll, _Arg_ull,
> _Arg_flt, _Arg_dbl, _Arg_ldbl, _Arg_str, _Arg_sv, _Arg_ptr, _Arg_handle,
> diff --git a/libstdc++-v3/testsuite/std/format/ranges/string.cc b/libstdc++-v3/testsuite/std/format/ranges/string.cc
> index 99e5eaf411f..a7d584f8e42 100644
> --- a/libstdc++-v3/testsuite/std/format/ranges/string.cc
> +++ b/libstdc++-v3/testsuite/std/format/ranges/string.cc
> @@ -279,6 +279,93 @@ void test_padding()
> VERIFY( strip_prefix(resv, 46, '*') );
> VERIFY( strip_quotes(resv) );
> VERIFY( resv == in );
> +
> + // width is 5, size is 15
> + in = "\u2160\u2161\u2162\u2163\u2164";
> + in += in; // width is 10, size is 30
> + in += in; // width is 20, size is 60
> + in += in; // width is 40, size is 120
> + in += in; // width is 80, size is 240
> + in += in; // width is 160, size is 480
> +
> + lc.assign_range(in);
> +
> + resv = res = std::format("{:s}", lc);
> + VERIFY( resv == in );
> +
> + resv = res = std::format("{:*>10s}", lc);
> + VERIFY( resv == in );
> +
> + resv = res = std::format("{:*>200s}", lc);
> + VERIFY( strip_prefix(resv, 40, '*') );
> + VERIFY( resv == in );
> +
> + resv = res = std::format("{:?s}", lc);
> + VERIFY( strip_quotes(resv) );
> + VERIFY( resv == in );
> +
> + resv = res = std::format("{:*>10?s}", lc);
> + VERIFY( strip_quotes(resv) );
> + VERIFY( resv == in );
> +
> + resv = res = std::format("{:*>200?s}", lc);
> + VERIFY( strip_prefix(resv, 38, '*') );
> + VERIFY( strip_quotes(resv) );
> + VERIFY( resv == in );
> +}
> +
> +void test_escaping()
> +{
> + std::string res;
> + std::string_view resv;
> +
> + const std::string_view input =
> + "\t\n\r\\\""
> + "\u008a" // Cc, Control, Line Tabulation Set,
> + "\u00ad" // Cf, Format, Soft Hyphen
> + "\u1d3d" // Lm, Modifier letter, Modifier Letter Capital Ou
> + "\u00a0" // Zs, Space Separator, No-Break Space (NBSP)
> + "\u2029" // Zp, Paragraph Separator, Paragraph Separator
> + "\U0001f984" // So, Other Symbol, Unicorn Face
> + ;
> + const std::string_view output =
> + R"(\t\n\r\\\")"
> + R"(\u{8a})"
> + R"(\u{ad})"
> + "\u1d3d"
> + R"(\u{a0})"
> + R"(\u{2029})"
> + "\U0001f984";
> +
> + std::forward_list<char> lc(std::from_range, input);
> + resv = res = std::format("{:s}", lc);
> + VERIFY( resv == input );
> + resv = res = std::format("{:?s}", lc);
> + VERIFY( strip_quotes(resv) );
> + VERIFY( resv == output );
> +
> + // width is 5, size is 15
> + std::string in = "\u2160\u2161\u2162\u2163\u2164";
> + in += in; // width is 10, size is 30
> + in += in; // width is 20, size is 60
> + in += in; // width is 40, size is 120
> + in += in; // width is 80, size is 240
> + in += in; // width is 160, size is 480
> + std::string_view inv = in;
> +
> + // last charcter is incomplete
> + lc.assign_range(inv.substr(0, 479));
> +
> + // non-debug format, chars copied as is
> + resv = res = std::format("{:s}", lc);
> + VERIFY( resv == inv.substr(0, 479) );
> +
> + // debug-format, incomplete code-point sequence is copied
> + resv = res = std::format("{:?s}", lc);
> + VERIFY( strip_quotes(resv) );
> + VERIFY( resv.substr(0, 477) == inv.substr(0, 477) );
> + resv.remove_prefix(477);
> + VERIFY( resv == R"(\x{e2}\x{85})" );
> }
>
> int main()
> @@ -287,4 +374,6 @@ int main()
> test_outputs<char>();
> test_outputs<wchar_t>();
> test_nested();
> + test_padding();
> + test_escaping();
> }
> --
> 2.51.0
>
>
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