On 04/10/19 07:01 +0200, François Dumont wrote:
On 9/27/19 1:00 PM, Jonathan Wakely wrote:
On 19/07/19 23:37 +0200, François Dumont wrote:
It sounds reasonable to overload std::copy_n for istreambuf_iterator.
I wonder whether it's worth doing:
#if __cplusplus >= 201703L
if constexpr (is_same_v<_OutputIterator, _CharT*>)
return __result + __it._M_sbuf->sgetn(__result, __n);
else
{
#endif
...
#if __cplusplus >= 201703L
}
#endif
We could extend that to also work for basic_string<_CharT>::iterator
and vector<_CharT>::iterator too if we wanted.
I'm not sure if it will perform any better than the code below (it's
approximately equivalent) but it should result in smaller binaries, as we
wouldn't be instantiating the code below when outputting to a pointer
or contiguous iterator.
We don't need to do that now, it can be a separate patch later (if we
do it at all).
Very good remark, I hadn't check streambuf to find out if there were
better to do. For me it is the streambuf method to target for an
std::copy_n overload.
So here is a new proposal much simpler. I see no reason to enable it
only for char types, is there ?
The reason to only do this for char-like types and std::char_traits is
that users could specialize istreambuf_iterator on their own types or
their own traits, and the specialization would not declare __copy_n_a
as a friend, and would not have the _M_sbuf member.
So I think we should still limit the optimisation to __is_char types
and std::char_traits. In practice that will help for all reasonable
cases, and unreasonable users who specialize the class can still
compile, but don't get the optimisation.
Ideally I'd also like to see tests where the input buffer is larger
than the size being read, e.g. read 5 chars from "123456" and verify
we don't read the '6'.
In test01 I am doing something like that.
Thanks.
Also, these tests don't exercise the code path that causes an
underflow. It would be good to use an ifstream to read from one of the
files in the testsuite/data directory, and read a large amount of data
(more than fits in a filebuf's read area) so that the underflow logic
is tested.
With this new proposal I don't need to do it, I'am counting on sgetn tests.
Agreed.
OK for trunk with the __is_char<_CharT> and char_traits<_CharT>
constraint restored. Thanks!
