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

On Thu, Oct 7, 2010 at 3:33 PM, OvermindDL1 wrote:

On Wed, Oct 6, 2010 at 10:54 PM, 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,

...

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 to vector: I found the feature of the library that allows conversion from array to vector. Just by including #include #include

boost::array ba={{1.0, 2.1, 3.2}}; vector bfv1(ba); //works.

I still can the other way around. boost::array ba0(bfv1); //doesn't work ideas?

great, I also managed to solve the original problem on how to convert from boost::array to fusion::vector 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::vectorsi::length, quantitysi::time, quantitysi::mass > vector_of_q; vector_of_q fvq( transform( ba, // boost::array //

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{ 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 to array 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 to array. 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/boo... 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 #include ... boost::array boostarr={{1.0, 2.1, 3.2}}; vector fusionvec(boostarr); //now works I still can't do the other way around: boost::array boostarr2(fusionvec); //doesn't work ideas? _______________________________________________

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