[PATH 2/3] libstdc++: Simplify std::advance istreambuf_iterator overload

[Jonathan Wakely]

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:
>
>Â Â Â Â Â Â Â  * include/std/streambuf
>Â Â Â Â Â Â Â  (advance(istreambuf_iterator<>&, _Distance)): Remove friend 
>declaration.
>Â Â Â Â Â Â Â  * include/bits/streambuf_iterator.h
>Â Â Â Â Â Â Â  (advance(istreambuf_iterator<>&, _Distance)): Re-implement using
>Â Â Â Â Â Â Â  streambuf pubseekoff.
>Â Â Â Â Â Â Â  * 
>testsuite/25_algorithms/advance/istreambuf_iterators/char/3.cc: New
>Â Â Â Â Â Â Â  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.