Re: [Boost-users] [fusion] memory layout of fusion::vector vs boost::tuple/boost::array

Subject: Re: [Boost-users] [fusion] memory layout of fusion::vector vs boost::tuple/boost::array
From: alfC (alfredo.correa_at_[hidden])
Date: 2010-10-09 06:11:06

On Oct 9, 2:24 am, alfC <alfredo.cor..._at_[hidden]> wrote:
> On Oct 8, 8:00 pm, OvermindDL1 <overmind..._at_[hidden]> wrote:
>
>
>
> > On Fri, Oct 8, 2010 at 6:57 PM, Joel de Guzman
>
> > <j..._at_[hidden]> wrote:
> > > On 10/9/2010 2:44 AM, alfC wrote:
>
> > >> On Oct 6, 5:33 pm, Joel de Guzman<j..._at_[hidden]>  wrote:
>
> > >>> On 10/7/2010 3:53 AM, alfC wrote:
>
> > >>>> Hi,
>
> > >>>>   I am trying to convert a boost::array into a fusion::vector.
> > >>>> reinterpreting the memory from boost::array. It works for tuples but
> > >>>> not fusion::vector, why is that. Is there a way to make it work? ( I
> > >>>> was hoping that the memory layout is the same to make conversion from
> > >>>> one to the other very easy.)
>
> > >>> Don't do that. It will *never* be guaranteed to work even if it works
> > >>> now. The memory layout of fusion::vector is an internal implementation
> > >>> detail and can change anytime.
>
> > >> The question is: is it guaranteed for boost::tuples<double,
> > >> double, ...>?
> > >> (it seems so, just by the PODness of tuples of PODs in its stated
> > >> design.) If so, then it is a *feature* of boost::tuple.
>
> > > No it is not. 1) It is not documented 2) It is not a POD
> > > 3) Anything with a reinterpret_cast is not guaranteed to work
> > > (you can only rely on reinterpret_cast only when casting
> > > back from a lost type e.g. through type erasure).
>
> > > Disregarding any of that is playing with fire.
>
> > The previous posts a few posts ago seems overly complicated, I would
> > just do this (suitably wrapped into a function to handle it, maybe
> > named copy):
>
> > // Proper includes here...
>
> > struct set1stTo2nd
> > {
> >     template<typename T>
> >     void operator()(T& t) const
> >     {
> >         using namespace boost::fusion;
> >         deref(begin(t)) = deref(next(begin(t)));
> >     }};
>
> > typedef boost::array<int,3> arrType;
> > typedef boost::fusion::vector<int,int,int> vecType;
> > typedef boost::fusion::vector<arrType&,vecType&> zipSeqType;
>
> > arrType arr(1,1,1);
> > vecType vec(5,5,5);
>
> > for_each(zipSeqType(arr,vec), set1stTo2nd());
> > assert(arr == make_vector(5,5,5));

FINALLY! I got it. I feel better now.

It turns out that the correct line of code is

for_each(
        boost::fusion::zip_view<zipSeqType>(zipSeqType(arr2,vec1)),
        set1stTo2nd()
);

you wrote the correct typedef but then didn't apply the zip_view
function.

(Although, related to the original question, I am still a bit worried
about too much unnecessary copy which seems unavoidable since the
memory layout of array<double, N> and fusion::vector<double*N> is
different.)

below is the full program that *works*:

#include<boost/array.hpp>
#include <boost/fusion/container.hpp>
#include <boost/fusion/include/container.hpp>
#include <boost/fusion/adapted/boost_array.hpp>
#include <boost/fusion/include/boost_array.hpp>
#include <boost/fusion/algorithm/iteration/for_each.hpp>
#include <boost/fusion/include/for_each.hpp>
#include <boost/fusion/algorithm/transformation/zip.hpp>
#include <boost/fusion/include/zip.hpp>
#include <boost/fusion/view/zip_view.hpp>
#include <boost/fusion/include/zip_view.hpp>

#include<iostream>
using std::clog; using std::endl;

using namespace boost::fusion;
struct set1stTo2nd
{
    template<typename T>
    void operator()(T t) const
    {
        deref(boost::fusion::begin(t)) =
deref(boost::fusion::next(boost::fusion::begin(t)));
    }
};

typedef boost::array<double,3> arrType;
typedef boost::fusion::vector<double,double,double> vecType;
typedef boost::fusion::vector<arrType&,vecType&> zipSeqType;

int main(){
        boost::array<double, 3> arr1={{1.0, 2.1, 3.2}};
        clog << "arr1: " << arr1[0] << ", " <<  arr1[1] << ", " << arr1[2] <<
endl;

  boost::fusion::vector<double, double, double> vec1(arr1);
        clog << "vec1: " << at_c<0>(vec1) << ", "<< at_c<1>(vec1) << ", " <<
at_c<2>(vec1) << endl;

        boost::array<double, 3> arr2;
        for_each(
                boost::fusion::zip_view<zipSeqType>(zipSeqType(arr2,vec1)),
                set1stTo2nd()
        );
        clog << "arr2: " << at_c<0>(arr2) << ", "<< at_c<1>(arr2) << ", " <<
at_c<2>(arr2) << endl;
        return 0;
}

prints:
arr1: 1, 2.1, 3.2
vec1: 1, 2.1, 3.2
arr2: 1, 2.1, 3.2

Have a good weekend,
Alfredo