Re: [Boost-users] Tuples: Can someone help me generalize this?

Subject: Re: [Boost-users] Tuples: Can someone help me generalize this?
From: Derrick Hathaway (d.hathaway_at_[hidden])
Date: 2008-09-16 22:22:23

> AMDG
>
> Derrick Hathaway wrote:
>> Hello, boost-users,
>> what I am trying to accomplish is a generic collection of N objects  
>> of type Type. I need for the N objects to be tightly packed and I  
>> would like the collection to be constructible in this way:
>>
>> uniform_vector<3, float> my_point(1.0f, 2.1f, 3.2f);
>>
>> I'm very new to the template (meta)programming scene and I would  
>> like to get better at it, so any advice you could give that would  
>> be instructive in that regard would be much appreciated. The code  
>> below is what I have come up with.
>>
>> Question 1: Am I reinventing the wheel at all? In other words is  
>> there something I can just typedef to get this functionality?
>>
>> Question 2: If not, how could I generalize this? Is there a  
>> template metaprogramming way? I'm sure it can be done with the  
>> preprocessor but I'm no good with macros... but then again I'm not  
>> much good with templates either at least not by a boost mailing  
>> list standard anyway. :)
>
> Here's some template code that creates an appropriate tuple type.
>
> #include <boost/mpl/transform.hpp>
> #include <boost/mpl/always.hpp>
> #include <boost/mpl/range_c.hpp>
> #include <boost/mpl/back_inserter.hpp>
> #include <boost/fusion/include/vector.hpp>
> #include <boost/fusion/include/mpl.hpp>
>
> #include <iostream>
>
> // create a sequence of 5 elements
> typedef boost::mpl::range_c<int, 0, 5> range;
> // convert all the elements to float and make the result a fusion  
> vector.
> typedef boost::mpl::transform<range, boost::mpl::always<float>,
>   boost::mpl::back_inserter<boost::fusion::vector<> > >::type  
> tuple_type;
>
> int main() {
>   tuple_type t(0, 1, 2, 3, 4);
>   std::cout << typeid(tuple_type).name() << std::endl;
> }
>
> In Christ,
> Steven Watanabe

Thank you, Sir, for your quick response. I just got around to testing  
it, and I've run into a snag. a fusion::vector is larger than the sum  
of it's parts, and I would like something that is not. In other words:

sizeof(vector<float, float>) != sizeof(float) + sizeof(float)

A tuple seems to fit, as I described earlier, but I'm not sure how to  
generalize this. I've thought that there might be a way to generalize  
a tuple to provide an interface like my_tuple<N, Type> using the cons.  
Any thoughts?

Thanks again. Derrick