I had totally forgotten about it myself. The occasion to take a fresh
look at it.
Here is a new version considering 1st pubseekoff call returned value to
find out if it can be used. I removed the check/comparison with 2nd call
result as it's not usable. In new 4_neg.cc test case if I ask to advance
istreambuf_iterator by 100000 the returned pos_type is showing 100000
offset even if it is obviously false. You need to read next byte to see
that it is eof.
libstdc++: Implement std::advance for istreambuf_iterator using
pubseekoff
If advance increment is smaller than input buffer size just advance
in this
buffer thanks to __safe_gbump. If increment is larger check for
seekoff support
and use it accordingly. Otherwise fallback on current
__safe_gbump/underflow loop.
libstdc++-v3/ChangeLog:
* include/bits/streambuf_iterator.h (advance): Re-implement
using streambuf pubseekoff/seekoff when supported.
*
testsuite/25_algorithms/advance/istreambuf_iterators/char/4_neg.cc: New
test case.
Ok to commit when back in stage 1 ?
François
On 31/03/2023 23:03, Jonathan Wakely wrote:
On Tue, 15 Oct 2019 at 21:20, François Dumont wrote:
Here is an update to set _M_sbuf to null if the user advance too much.
Note that in this case the streambuf remains un-modified which is
different from the current implementation. I think it is another
enhancement.
I also change the Debug assertion message for something more dedicated
to std::advance algo.
François
On 10/14/19 10:12 PM, François Dumont wrote:
The same way I proposed to review std::copy overload for
istreambuf_iterator we can implement std::advance using pubseekoff.
It is both a cleaner implementation and avoids yet another friend
declaration.
Looks like I never sent my review of this one, it's been sitting in my
draft mails for years, sorry.
It looks like this will fail if the streambuf doesn't support seeking.
The default behaviour for seekoff is to return -1, in which case
you'll get -1 for both calls to pubseekoff, and new_pos - cur_pos will
be zero, which is not equal to n, so you set the istreambuf_iterator
to end-of-stream. That seems wrong, we could still advance using the
old code (or just call ++ in a loop!)
* include/std/streambuf
(advance(istreambuf_iterator<>&, _Distance)): Remove friend
declaration.
* include/bits/streambuf_iterator.h (__copy_move_a2): Re-implement
using
streambuf pubseekoff.
Tested under Linux x86_64.
Ok to commit ?
François
diff --git a/libstdc++-v3/include/bits/streambuf_iterator.h b/libstdc++-v3/include/bits/streambuf_iterator.h
index 32285a668fc..4527660e191 100644
--- a/libstdc++-v3/include/bits/streambuf_iterator.h
+++ b/libstdc++-v3/include/bits/streambuf_iterator.h
@@ -479,37 +479,59 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__glibcxx_assert(__n > 0);
__glibcxx_requires_cond(!__i._M_at_eof(),
- _M_message(__gnu_debug::__msg_inc_istreambuf)
- ._M_iterator(__i));
+ _M_message(__gnu_debug::__msg_advance_oob)
+ ._M_iterator(__i)
+ ._M_integer(__n));
typedef istreambuf_iterator<_CharT> __is_iterator_type;
typedef typename __is_iterator_type::traits_type traits_type;
typedef typename __is_iterator_type::streambuf_type streambuf_type;
typedef typename traits_type::int_type int_type;
+ typedef typename traits_type::off_type off_type;
+ typedef typename streambuf_type::pos_type pos_type;
const int_type __eof = traits_type::eof();
+ const pos_type __npos = pos_type(off_type(-1));
streambuf_type* __sb = __i._M_sbuf;
- while (__n > 0)
- {
streamsize __size = __sb->egptr() - __sb->gptr();
- if (__size > __n)
- {
+ if (__size >= __n)
__sb->__safe_gbump(__n);
- break;
- }
-
+ else
+ {
+ // Check for seekoff support.
+ if (__sb->pubseekoff(0, ios_base::cur, ios_base::in) != __npos)
+ __sb->pubseekoff(__n, ios_base::cur, ios_base::in);
+ else
+ {
+ _GLIBCXX_DEBUG_ONLY(_Distance __n_orig = __n);
+ for (;;)
+ {
__sb->__safe_gbump(__size);
__n -= __size;
+
+ if (__n == 0)
+ break;
+
if (traits_type::eq_int_type(__sb->underflow(), __eof))
{
- __glibcxx_requires_cond(__n == 0,
- _M_message(__gnu_debug::__msg_inc_istreambuf)
- ._M_iterator(__i));
+ __i._M_sbuf = 0;
break;
}
+
+ __size = __sb->egptr() - __sb->gptr();
+ if (__size > __n)
+ __size = __n;
+ }
+
+ _GLIBCXX_DEBUG_ONLY(__n = __n_orig);
+ }
}
__i._M_c = __eof;
+ __glibcxx_requires_cond(!__i._M_at_eof(),
+ _M_message(__gnu_debug::__msg_advance_oob)
+ ._M_iterator(__i)
+ ._M_integer(__n));
}
/// @} group iterators
diff --git a/libstdc++-v3/testsuite/25_algorithms/advance/istreambuf_iterators/char/4_neg.cc b/libstdc++-v3/testsuite/25_algorithms/advance/istreambuf_iterators/char/4_neg.cc
new file mode 100644
index 00000000000..88ccaeff499
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/advance/istreambuf_iterators/char/4_neg.cc
@@ -0,0 +1,65 @@
+// { dg-require-fileio "" }
+// { dg-do run { xfail *-*-* } }
+// { dg-require-debug-mode "" }
+
+#include <iterator>
+#include <fstream>
+#include <algorithm>
+
+#include <testsuite_hooks.h>
+
+void test01()
+{
+ using namespace std;
+
+ typedef istreambuf_iterator<char> in_iterator_type;
+
+ unsigned found = 0;
+
+ {
+ ifstream fbuf("istream_unformatted-1.txt");
+
+ in_iterator_type beg(fbuf);
+ in_iterator_type end;
+
+ for (;;)
+ {
+ beg = find(beg, end, '1');
+ if (beg == end)
+ break;
+
+ ++found;
+ VERIFY( *beg == '1' );
+
+ advance(beg, 9);
+ VERIFY( *beg == '0' );
+ }
+ }
+
+ {
+ ifstream fbuf("istream_unformatted-1.txt");
+
+ in_iterator_type beg(fbuf);
+ in_iterator_type end;
+
+ beg = find(beg, end, '1');
+ VERIFY( beg != end );
+ VERIFY( *beg == '1' );
+
+ advance(beg, 9);
+ VERIFY( *beg == '0' );
+
+ unsigned line_length = 10;
+ while (*++beg != '1')
+ ++line_length;
+
+ // Try to jump directly to the end + 1 through advance.
+ advance(beg, (found - 2) * line_length + 9 + 2);
+ }
+}
+
+int main()
+{
+ test01();
+ return 0;
+}
