Why is istream/ostream slow

Question

At 50:40 of http://channel9.msdn.com/Events/GoingNative/2013/Writing-Quick-Code-in-Cpp-Quickly Andrei Alexandrescu makes a joke about how not efficient/slow istream is.

I had an issue in the past with ostream being slow and fwrite being significantly faster (reducing many seconds when running the main loop once) but I never understood why nor looked into it.

What makes istream and ostream slow in C++? or at least slow compared to other things (like fread/fget, fwrite) which would equally satisfied the needs.

Answer 1

Actually, IOStreams don't have to be slow! It is a matter of implementing them in a reasonable way to make them fast, though. Most standard C++ library don't seem to pay too much attention to implement IOStreams. A long time ago when my CXXRT was still maintained it was about as fast as stdio - when used correctly!

Note that there are few performance traps for users laid out with IOStreams, however. The following guidelines apply to all IOStream implementations but especially to those which are tailored to be fast:

1. When using std::cin, std::cout, etc. you need to call std::sync_with_stdio(false)! Without this call, any use of the standard stream objects is required to synchronize with C's standard streams. Of course, when using std::sync_with_stdio(false) it is assumed that you don't mix std::cin with stdin, std::cout with stdout, etc.
2. Do not use std::endl as it mandates many unnecessary flushes of any buffer. Likewise, don't set std::ios_base::unitbuf or use std::flush unnecessarily.
3. When creating your own stream buffers (OK, few users do), make sure they do use an internal buffer! Processing individual characters jumps through multiple conditions and a virtual function which makes it hideously slow.