----- Original Message ----- From: "David Abrahams" <dave@boostpro.com> To: boost-users@lists.boost.org Subject: Re: [Boost-users] metafunction invocation performance Date: Thu, 26 Feb 2009 10:02:20 -0500

On Jul 4, 2008, at 7:36 AM, Hicham Mouline wrote:

...

The solution to this one is provided by Joaquin, in his reply below:

I tried compiling your code with MSVC++ 8.0 and it is indeed incredibly slow. After playing a little with the code I realized that MPL views are the culprit here: for some reason it is much faster to fold on a vector-based computation rather than a view:

I learned an interesting and somewhat depressing fact from Doug Gregor recently. Most compilers don't use a canonical representation of types. That is, in most compilers, int* and add_pointer<int>::type occupy separate pieces of storage, and the complier jumps through some complicated hoops when it needs to evaluate them for sameness. Of course that also means that vector<int*> and vector<add_pointer<int>::type> are represented separately, etc. That can totally skewer the gains of memoization, among other things.f

Apparently recent versions of GCC have canonical type representation, and so does Clang (for what that's worth -- no C++ yet), but I betcha MS doesn't, and that would help explain why views cost more.

I can try to experiment with this. I could try the view version versus the non-view version provided my Joaquin in g++. What minimum version of g++ ? regards,