2012/2/19 Sturla Molden <sturla@molden.no>
Den 19.02.2012 10:28, skrev Mark Wiebe:
Particular styles of using templates can cause this, yes. To properly do this kind of advanced C++ library work, it's important to think about the big-O notation behavior of your template instantiations, not just the big-O notation of run-time. C++ templates have a turing-complete language (which is said to be quite similar to haskell, but spelled vastly different) running at compile time in them. This is what gives template meta-programming in C++ great power, but since templates weren't designed for this style of programming originally, template meta-programming is not very easy.
The problem with metaprogramming is that we are doing manually the work that belongs to the compiler. Blitz++ was supposed to be a library that "thought like a compiler". But then compilers just got better. Today, it is no longer possible for a numerical library programmer to outsmart an optimizing C++ compiler. All metaprogramming can do today is produce error messages noone can understand. And the resulting code will often be slower because the compiler has less opportunities to do its work.
As I've said, the compiler is pretty much stupid. It cannot do what Blitzz++ did, or what Eigen is currently doing, mainly because of the basis different languages (C or C++). -- Information System Engineer, Ph.D. Blog: http://matt.eifelle.com LinkedIn: http://www.linkedin.com/in/matthieubrucher