On 05/05/25 14:44 +0200, Tomasz Kamiński wrote:
This patch provides an equality operator between _Sink_iter and
default_sentinel,
that returns true, if any further writes to the _Sink_iter and underlying _Sink,
will be discared, and thus can be omitted. This operator is implemented in terms
new of _M_discarding virtual function in _Sink.
Currently only the _Padding_sink reports discarding mode of if width of sequence
characters is greater than _M_maxwidth (precision), or underlying _Sink is
discarding characters. The _M_discarding override, is separate function from
_M_ignoring, that remain annotated with [[__gnu__::__always_inline__]].
Despite having notion of maximum characters to be written (_M_max), _Iter_sink
nevers discard characters, as the total number of characters that would be
written
needs to be returned by format_to_n. This is documented in-source by providing
an
_Iter_sink::_M_discarding override, that always returns false.
The function is currently queried only by the _Padding_sinks, that may be
stacked
for example a range is formatted, with padding with being specified both for
range
itself and it's elements. The state of underlying sink is checked during
construction
and after each write (_M_sync_discarding).
libstdc++-v3/ChangeLog:
* include/std/format (__Sink_iter<_CharT>::operator==)
(__Sink<_CharT>::_M_discarding, _Iter_sink<_CharT,
_OutIter>::_M_discarding)
(_Padding_sinl<_CharT, _Out>::_M_padwidth)
(_Padding_sink<_CharT, _Out>::_M_maxwidth): Remove const.
(_Padding_sink<_CharT, _Out>::_M_sync_discarding)
(_Padding_sink<_CharT, _Out>::_M_discarding): Define.
(_Padding_sink<_CharT, _Out>::_Padding_sink(_Out, size_t, size_tt))
(_Padding_sink<_CharT, _Out>::_M_force_update):
(_Padding_sink<_CharT, _Out>::_M_flush): Call _M_sync_discarding.
(_Padding_sink<_CharT, _Out>::_Padding_sink(_Out, size_t)): Delegate.
---
libstdc++-v3/include/std/format | 66 +++++++++++++++++++++++++++------
1 file changed, 54 insertions(+), 12 deletions(-)
diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 054ce350440..08a28fc92da 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -3144,6 +3144,10 @@ namespace __format
auto
_M_reserve(size_t __n) const
{ return _M_sink->_M_reserve(__n); }
+
+ friend bool
+ operator==(_Sink_iter __it, default_sentinel_t)
+ { return __it._M_sink->_M_discarding(); }
I don't love using default_sentinel like this.
Conceptually an output iterator that compares equal to a sentinel but
is still writable seems ... odd. Did you choose this API just to avoid
introducing a member function that isn't part of the usual iterator
API?
};
// Abstract base class for type-erased character sinks.
@@ -3263,6 +3267,11 @@ namespace __format
_M_bump(size_t __n)
{ _M_next += __n; }
+ // Returns true if the _Sink is discarding incoming characters.
+ virtual bool
+ _M_discarding() const
+ { return false; }
+
public:
_Sink(const _Sink&) = delete;
_Sink& operator=(const _Sink&) = delete;
@@ -3488,6 +3497,15 @@ namespace __format
_M_count += __s.size();
}
+ bool
+ _M_discarding() const override
+ {
+ // format_to_n return total number of characters, that would be written,
+ // thus we need to see and all characters.
"see and all characters" seems wrong, does it mean "see and format all
characters"? Or somethign else?
+ // see https://eel.is/c%2B%2Bdraft/format#functions-22
These links aren't necessarily permanent, so I would prefer a "dated"
reference to a particular standard (or working paper), e.g.
// see C++20 [format.functions] p20
N.B. it was p20 in C++20, but p22 in C++23, so if you prefr to link to
the newer standard, then:
// see C++23 [format.functions] p22
+ return false;
+ }
+
public:
[[__gnu__::__always_inline__]]
explicit
@@ -3550,6 +3568,15 @@ namespace __format
}
}
+ bool
+ _M_discarding() const override
+ {
+ // format_to_n return total number of characters, that would be written,
+ // thus we need to see and all characters.
+ // see https://eel.is/c%2B%2Bdraft/format#functions-22
Same comment about "see and all" and the standard reference.
+ return false;
+ }
+
typename _Sink<_CharT>::_Reservation
_M_reserve(size_t __n) final
{
@@ -3636,17 +3663,15 @@ namespace __format
template<typename _Out, typename _CharT>
class _Padding_sink : public _Str_sink<_CharT>
{
- const size_t _M_padwidth;
- const size_t _M_maxwidth;
+ size_t _M_padwidth;
+ size_t _M_maxwidth;
_Out _M_out;
size_t _M_printwidth;
[[__gnu__::__always_inline__]]
bool
_M_ignoring() const
- {
- return _M_printwidth >= _M_maxwidth;
- }
+ { return _M_printwidth >= _M_maxwidth; }
[[__gnu__::__always_inline__]]
bool
@@ -3659,12 +3684,21 @@ namespace __format
return false;
}
+ void
+ _M_sync_discarding()
+ {
+ if constexpr (is_same_v<_Out, _Sink_iter<_CharT>>)
+ if (_M_out == default_sentinel)
+ _M_maxwidth = _M_printwidth;
+ }
+
void
_M_flush()
{
span<_CharT> __new = this->_M_used();
basic_string_view<_CharT> __str(__new.data(), __new.size());
_M_out = __format::__write(std::move(_M_out), __str);
+ _M_sync_discarding();
this->_M_rewind();
}
@@ -3682,7 +3716,10 @@ namespace __format
// We have more characters than padidng, no padding is needed,
// write direclty to _M_out.
if (_M_printwidth >= _M_padwidth)
- _M_out = __format::__write(std::move(_M_out), __str);
+ {
+ _M_out = __format::__write(std::move(_M_out), __str);
+ _M_sync_discarding();
+ }
// We reached _M_maxwidth that is smaller than _M_padwidth.
// Store the prefix sequence in _M_seq, and free _M_buf.
else
@@ -3718,6 +3755,10 @@ namespace __format
_Str_sink<_CharT>::_M_overflow();
}
+ bool
+ _M_discarding() const override
+ { return _M_ignoring(); }
+
typename _Sink<_CharT>::_Reservation
_M_reserve(size_t __n) override
{
@@ -3752,15 +3793,16 @@ namespace __format
public:
[[__gnu__::__always_inline__]]
- explicit _Padding_sink(_Out __out, size_t __padwidth)
- : _M_padwidth(__padwidth), _M_maxwidth(-1),
+ explicit
+ _Padding_sink(_Out __out, size_t __padwidth, size_t __maxwidth)
+ : _M_padwidth(__padwidth), _M_maxwidth(__maxwidth),
_M_out(std::move(__out)), _M_printwidth(0)
- { }
+ { _M_sync_discarding(); }
[[__gnu__::__always_inline__]]
- explicit _Padding_sink(_Out __out, size_t __padwidth, size_t __maxwidth)
- : _M_padwidth(__padwidth), _M_maxwidth(__maxwidth),
- _M_out(std::move(__out)), _M_printwidth(0)
+ explicit
+ _Padding_sink(_Out __out, size_t __padwidth)
+ : _Padding_sink(std::move(__out), __padwidth, (size_t)-1)
{ }
_Out
--
2.49.0
