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: Christopher Schmidt (mr.chr.schmidt_at_[hidden])
Date: 2010-10-09 05:46:44

Am 09.10.2010 11:24, schrieb alfC:
> 
> 
> On Oct 8, 8:00 pm, OvermindDL1 <overmind..._at_[hidden]> wrote:
[snip]
>> 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));
> 
> Thank you very much for the code. Unfortunately it doesn't work,
> compiles but it doesn't copy the vector into the array.
> Below is the full program, it leaves the array intact.
> 
> It doesn't make sense either, the for_each argument  has only two
> elements, the array and the vector. The for_each will just first "do"
> the array and then "do" the vector, but it just seems to assign the
> first element of each to the second but not copying values from the
> vector to the array.
> 
> (I still remember the time when I could copy one an array of doubles
> to the other.) Below is the full program if you want to give it a try:
> 
> #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<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(zipSeqType(arr2,vec1), set1stTo2nd());

fusion::zip(arr2,vec1) should be used here, or
fusion::zip_view<zipSeqType>(zipSeqType(arr2,vec1)) .

http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/algorithm/transformation/functions/zip.html
http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/view/zip_view.html

> 	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: 4.85589e-270, 4.85589e-270, 4.90963e-270

-Christopher