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: Alfredo Correa (alfredo.correa_at_[hidden])
Date: 2010-10-07 19:42:18

On Thu, Oct 7, 2010 at 3:33 PM, OvermindDL1 <overminddl1_at_[hidden]> wrote:

> On Wed, Oct 6, 2010 at 10:54 PM, alfC <alfredo.correa_at_[hidden]> 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.
> >>
> >
> > It is not only not guaranteed, it does not work at present. Any two
> > cents why is that fusion::vector may (do?) implement a non obvious
> > memory layout?
> >
> > I solved the problem of array<double, N> to vector<double ...>:
> > I found the feature of the library that allows conversion from
> > array<double, N> to vector<double*N ... >. Just by including
> > #include <boost/fusion/adapted/boost_array.hpp>
> > #include <boost/fusion/include/boost_array.hpp>
> >
> > boost::array<double, 3> ba={{1.0, 2.1, 3.2}};
> > vector<double, double, double> bfv1(ba); //works.
> >
> > I still can the other way around.
> > boost::array<double, 3> ba0(bfv1); //doesn't work
> > ideas?
> >
> > great, I also managed to solve the original problem on how to convert
> > from
> > boost::array<double, N> to
> > fusion::vector<quantity<T1>, quantity<T2>, ... quantity<TN> >
> > thanks to your other posting suggesting using fusion::transform
> >
> > quantity<T> can be constructed from a double with
> > quantity<T>::from_value(double)
> >
> > The line of code that does this is
> >
> > typedef boost::fusion::vector<quantity<si::length>,
> > quantity<si::time>, quantity<si::mass> > vector_of_q;
> > vector_of_q fvq(
> >        transform(
> >                ba, // boost::array<double, 3> //
> incld"/fusion/array.hpp"!!
> >                vector_of_q(),
> >                from_value()
> >        )
> > );
> >
> > where "from_value" is defined by
> >
> > struct from_value{ // binary fusion transformation
> >        template <typename SignatureT> struct result;
> >        template <typename Q>
> >        struct result<from_value(double const&, Q const&)>{
> >                typedef Q type;
> >        };
> >        template<typename Q>
> >        Q operator()(double const& value, Q const&) const{
> >                return Q::from_value(value);
> >        }
> > };
> >
> > I had to use the *binary* version of transform because the type of the
> > result depends on the destination fusion::vector.
> >
> > The other way around from vector<quantity<T1> ... quantity<TN> > to
> > array<double, N> should be easier (because the destination type is
> > always double by using the quantity<T>::value() ) but I didn't managed
> > because I didn't find the way to convert from vector<double*N> to
> > array<double, N>. ideas?
> >
> > Thank you for making fusion, it is an incredible library once you
> > realize what it is for.
> > Alfredo
>
>
> You do know that based on this:
>
> http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/adapted/boost__array.html
> boost::array is already adapted as a fusion container and can be used
> directly, as soon as you include the proper hpp that is?
>

yes, thank you, that what I wrote in my last post . I can make this work

#include <boost/fusion/adapted/boost_array.hpp>
#include <boost/fusion/include/boost_array.hpp>
...
boost::array<double, 3> boostarr={{1.0, 2.1, 3.2}};
vector<double, double, double> fusionvec(boostarr); //now works

I still can't do the other way around:

boost::array<double, 3> boostarr2(fusionvec); //doesn't work
ideas?

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