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

On Fri, Oct 8, 2010 at 6:57 PM, Joel de Guzman wrote:

On 10/9/2010 2:44 AM, alfC wrote:

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On Oct 6, 5:33 pm, Joel de Guzman  wrote:

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On 10/7/2010 3:53 AM, alfC wrote:

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Hi,

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  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? (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 arrType; typedef boost::fusion::vector vecType; typedef boost::fusion::vector 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.