On 09/09/20 22:12 +0200, François Dumont via Libstdc++ wrote:
libstdc++: Use only public basic_streambuf methods in std::advance overload
std::advance overload for istreambuf_iterator can be implemented using
basic_streambuf public pubseekoff method so that it doesn't have to be
basic_streambuf friend.
libstdc++-v3/ChangeLog:
       * include/std/streambuf
       (advance(istreambuf_iterator<>&, _Distance)): Remove friend
declaration.
       * include/bits/streambuf_iterator.h
       (advance(istreambuf_iterator<>&, _Distance)): Re-implement using
       streambuf pubseekoff.
       *
testsuite/25_algorithms/advance/istreambuf_iterators/char/3.cc: New
       test.
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 8712b90edd6..afe967e5f03 100644
--- a/libstdc++-v3/include/bits/streambuf_iterator.h
+++ b/libstdc++-v3/include/bits/streambuf_iterator.h
@@ -453,37 +453,26 @@ _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));
-
- 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;
- const int_type __eof = traits_type::eof();
-
- streambuf_type* __sb = __i._M_sbuf;
- while (__n > 0)
+ _M_message(__gnu_debug::__msg_advance_oob)
+ ._M_iterator(__i)
+ ._M_integer(__n));
+
+ typedef basic_streambuf<_CharT> __streambuf_t;
+ typedef typename __streambuf_t::pos_type __pos_t;
+ __pos_t __cur_pos
+ = __i._M_sbuf->pubseekoff(0, ios_base::cur, ios_base::in);
+ __pos_t __new_pos
+ = __i._M_sbuf->pubseekoff(__n, ios_base::cur, ios_base::in);
Two seeks might be considerably more expensive than the existing code.
For example, advancing by __n=1 is very cheap if the get area already
contains data, we just bump a pointer. Two seeks does virtual calls
to the derived streambuf which does ... something? We don't know, it
depends on the streambuf. It might even fail and return
pos_type(off_type(-1)).
I don't think this is an improvement.
