Re: [boost] Re: Formal Review in progress

From: Peder Holt (peder.holt_at_[hidden])
Date: 2005-05-28 06:01:36

On 5/27/05, Peder Holt <peder.holt_at_[hidden]> wrote:
> 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

I did some tests on VC8 beta, and it compiles the testcase in half the
time compared to the present implementation of typeof_impl. As far as
I have been able to test, it also works around the compiler bug for
VC8 discussed earlier.

Here is a more detailed sketch of the proposed solution:

namespace boost {namespace type_of{
    template<typename T>
    struct encode_result {
        typedef typename encode_type<BOOST_TYPEOF_VECTOR(0)<>,
T>::type encoded_type;
        typedef typename mpl::size<encoded_type>::type size;
        char (&value0)[...];
        char (&value1)[...];
        char (&value2)[...];
        ...
        char (&value50)[...];
    };

    template<class T>
    encode_result<T> typeof_result(const T&);
}}
#define BOOST_TYPEOF_AT(n, expr)
sizeof(boost::type_of::typeof_result(expr).BOOST_PP_CAT(value,n))

Peder.

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