[boost] Re: mpl::begin<mpl::vector<...> > performance problem

From: David Abrahams (dave_at_[hidden])
Date: 2005-03-01 10:40:43

Aleksey Gurtovoy <agurtovoy_at_[hidden]> writes:

> I haven't had a chance to look at it in detail yet, but I'm pretty
> sure that the penatly comes from simply _instantiating_ the vector
> type with a huge number of elements. 'begin' just happened to be the
> first metafunction that causes that in Arkadiy's code. Of course that
> doesn't mean that it's not an issue, especially since we guarantee
> that "predefined" content specification has amortized constant
> time complexity. At the very least, if we find out that there is
> nothing we can do, the docs would need to be fixed.

According to the tests we did for "C++ Template Metaprogramming," 

   there is no cost associated with argument arity except on GCC and
   Comeau, where the cost rises linearly with N

So, on those compilers, expect that first instantiation of a long
vector template will be expensive.  That said, I see no reason why we
need to be causing any instantiation.  Everything *could* be done by
partial specialization matching, though it might be verbose to account
for the numbered and unnumbered forms.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com