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: 2010-10-08 23:00:41

On Fri, Oct 8, 2010 at 6:57 PM, Joel de Guzman
<joel_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
{
    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));

Nice and simple; there might be a better way, but these kind of things
suit me perfectly, and who knows, there might be a Phoenix way to do
set1stTo2nd instead of the functor.