https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109400
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> Additionally, std::ranges::advance is not specialized for
> std::istreambuf_iterator so just increments character by character. The
> ranges::advance(i, n) form should use std::advance. The ranges::advance(i,
> sentinel) form will already be optimal when the sentinel is the same type as
> the iterator, but will be slow when using default_sentinel_t.
Oops, assignable_from<istreambuf_iterator<char>&, default_sentinel_t> is
satisfied, via the non-explicit istreambuf_iterator(default_sentinel_t)
constructor, so it already works efficiently with default_sentinel.
> The
> ranges::advance(i, n, sentinel) form could be done efficiently, and has the
> advantage that it's not undefined if EOF is reached before advancing n times.
This could still be improved though.
Something like this:
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -42,6 +42,17 @@
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+template<typename> class char_traits;
+template<typename, typename> class istreambuf_iterator;
+template<typename _Tp>
+ static constexpr bool __is_istreambuf_iter = false;
+
+template<typename _CharT>
+ static constexpr bool
+ __is_istreambuf_iter<istreambuf_iterator<_CharT, char_traits<_CharT>>>
+ = std::__is_char<_CharT>::__value;
+
namespace ranges
{
template<typename>
@@ -737,6 +748,8 @@ namespace ranges
while (++__n);
}
}
+ else if constexpr (__is_istreambuf_iter<_It>)
+ std::advance(__it, __n);
else
{
// cannot decrement a non-bidirectional iterator
@@ -791,6 +804,11 @@ namespace ranges
return __n;
else if (__n > 0)
{
+ if constexpr (__is_istreambuf_iter<_It>)
+ {
+ // TODO need access to __it._M_sbuf
+ }
+
iter_difference_t<_It> __m = 0;
do
{
But it seems to me that it would be helpful if ranges::advance was a
customization point, so that program-defined iterators could provide an
optimized implementation too.
