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: OvermindDL1 (overminddl1_at_[hidden])
Date: 2010-10-09 10:53:33

On Sat, Oct 9, 2010 at 4:11 AM, alfC <alfredo.correa_at_[hidden]> wrote:
>
>
> On Oct 9, 2:24Â am, alfC <alfredo.cor..._at_[hidden]> wrote:
>> On Oct 8, 8:00Â 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]> Â wrote:
>>
>> > >>> On 10/7/2010 3:53 AM, alfC wrote:
>>
>> > >>>> Hi,
>>
>> > >>>> Â  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
>> > {
>> > Â  Â  template<typename T>
>> > Â  Â  void operator()(T& t) const
>> > Â  Â  {
>> > Â  Â  Â  Â  using namespace boost::fusion;
>> > Â  Â  Â  Â  deref(begin(t)) = deref(next(begin(t)));
>> > Â  Â  }};
>>
>> > 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(
> Â  Â  Â  Â boost::fusion::zip_view<zipSeqType>(zipSeqType(arr2,vec1)),
> Â  Â  Â  Â 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
> {
> Â  Â template<typename T>
> Â  Â void operator()(T t) const
> Â  Â {
> Â  Â  Â  Â deref(boost::fusion::begin(t)) =
> deref(boost::fusion::next(boost::fusion::begin(t)));
> Â  Â }
> };
>
> typedef boost::array<double,3> arrType;
> typedef boost::fusion::vector<double,double,double> vecType;
> typedef boost::fusion::vector<arrType&,vecType&> zipSeqType;
>
> int main(){
> Â  Â  Â  Â boost::array<double, 3> arr1={{1.0, 2.1, 3.2}};
> Â  Â  Â  Â clog << "arr1: " << arr1[0] << ", " << Â arr1[1] << ", " << arr1[2] <<
> endl;
>
> Â boost::fusion::vector<double, double, double> vec1(arr1);
> Â  Â  Â  Â clog << "vec1: " << at_c<0>(vec1) << ", "<< at_c<1>(vec1) << ", " <<
> at_c<2>(vec1) << endl;
>
> Â  Â  Â  Â boost::array<double, 3> arr2;
> Â  Â  Â  Â for_each(
> Â  Â  Â  Â  Â  Â  Â  Â boost::fusion::zip_view<zipSeqType>(zipSeqType(arr2,vec1)),
> Â  Â  Â  Â  Â  Â  Â  Â set1stTo2nd()
> Â  Â  Â  Â );
> Â  Â  Â  Â clog << "arr2: " << at_c<0>(arr2) << ", "<< at_c<1>(arr2) << ", " <<
> at_c<2>(arr2) << endl;
> Â  Â  Â  Â 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

Yeah, my mistake, busy at work and not paying attention, I meant to
have the fusion::zip there.  :)