[boost] Performance problem in iostreams::stream

From: Jerker Öhman (jerker.ohman_at_[hidden])
Date: 2007-01-16 04:58:33

I posted this to boost.user but didn't get any reply so I will try here 
instead.

The problem (as far as I can tell) is that a buffered iostreams::stream 
dumps it's internal buffers every time you call seek or tell, even if 
you seek to the current position.

You end up in this function.

template<typename T, typename Tr, typename Alloc, typename Mode> 
typename indirect_streambuf<T, Tr, Alloc, Mode>::pos_type 
indirect_streambuf<T, Tr, Alloc, Mode>::seek_impl
       (stream_offset off, BOOST_IOS::seekdir way, BOOST_IOS::openmode 
which)
{
       if (pptr() != 0)
           this->BOOST_IOSTREAMS_PUBSYNC(); // sync() confuses VisualAge 6.
       if (way == BOOST_IOS::cur && gptr())
           off -= static_cast<off_type>(egptr() - gptr());
       setg(0, 0, 0);
       setp(0, 0);
       return obj().seek(off, way, which, next_);
}

I am no expert in iostreams but I think that the naive idea would be to 
check if the buffer already contains the position we are searching for 
and if so, adjust the buffers accordingly (at least if the stream is 
binary). With the current implementation you can't really use buffered 
streams if you seek a lot in them (which, perhaps is the reason you want 
to use buffered streams in the first place).