Re: [boost] Re: Formal Review in progress

From: Peder Holt (peder.holt_at_[hidden])
Date: 2005-05-27 13:07:46

On 5/27/05, David Abrahams <dave_at_[hidden]> wrote:
> Alexander Nasonov <alnsn-boost_at_[hidden]> writes:
> 
> > Peder Holt <peder.holt <at> gmail.com> writes:
> >
> >> On 5/27/05, Alexander Nasonov <alnsn-boost <at> yandex.ru> wrote:
> >> > Sizes may become very huge, though.
> >>
> >> If you replace your function with:
> >> char (&foo())[900][800][700][600];
> >> you will get an overflow when calculating e.g. sizeof(foo()) /
> >> sizeof(foo()[0]),
> >> and the result will be garbage.
> >
> > That's right.
> > Huge arrays can be replaced with tuples.
> >
> > tuple<char(&)[9], char(&)[8], char(&)[7], char(&)[6]> foo();
> 
> Why not an mpl::vector for that matter?
> 
> It's easy enough to extract each dimension of an array with partial
> specialization.  Just do the representation backwards:
> 
> char (& foo(  whatever ) ) [CODEN][CODEN-1]...[CODE1][CODE0]
> 
> Steve Dewhurst was here before us.
> 
> > sizeof(get<0>(foo())); sizeof(get<1>(foo())); sizeof(get<2>(foo()));
> > sizeof(get<3>(foo()));
> >
> > Not sure if it compiles with boost::tuple.
> 
> Inefficient at compile time.  get<N> is O(N) each time, thus the above
> is O(N^2); that's part of what's behind the fusion library.

I made a version of typeof_impl that uses a single function invocation
and an additional encode struct invocation (in addition to
encode_type)

Instead of taking the size of a function, 
I take the size of a member of the struct returned by a function.

Current version:
Access element 1: sizeof(at<1>(expr))

Proposed version:
Access element 1: sizeof(typeof_result(expr).value1)

If this is in any way better performance-wise than the current
implementation, I do not know.

Peder

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