Re: [Boost-users] Help with (possibly MPL) pattern for a viewer that may contain a copy of statically allocated data

Subject: Re: [Boost-users] Help with (possibly MPL) pattern for a viewer that may contain a copy of statically allocated data
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-01-24 15:19:07

AMDG

Jesse Perla wrote:
> I am wondering what the best pattern/approach for a data structure 
> that may contain either a reference or a copy of some data used in a 
> constructor.  I would love the opportunity to have the data structure 
> capable of conditionally storing the data for the data copy on the 
> stack.  Are there any fancy generic tricks/patterns to do this using 
> boost::mpl, etc.?
>
> So to put in some pseudocode notes for discussion to get the point 
> across of what I think I want:
>
> boost::array< boost::array<double, M>, N> myaxis; //creates axis in a 
> stack allocated data structure.
> regular_grid<M, N, COPYDATA> my_grid(axis_values);
> regular_grid<M, N, REFDATA> my_grid(axis_values);
>
> template<int M, int N, int ConstructionPolicy>
> class regular_grid
> {
>      boost::array< boost::array<double, M>, N>& axis_ref_; //Reference 
> to the axis, either static or actual reference.  All code in the class 
> would use this, not axis_
>
>     STATIC_IF(ConstructionPolicy == COPYDATA)
>      {
>             boost::array< boost::array<double, M>, N> axis_;
>             regular_grid(const boost::array< boost::array<double, M>, 
> N>& axis) : axis_(axis), axis_ref_(axis_) //constructor makes copy, 
> sets ref to copy.
>            {
>            }
>
>      }
>     STATIC_ELSE(ConstructionPolicy == REFDATA)
>     {
>             regular_grid(const boost::array< boost::array<double, M>, 
> N>& axis) : axis_ref_(axis) {} //constructor just uses the ref.
>     }
> };
>
> Is this possible?  any ideas on code to look at to see how to do it?

If you don't need to decide at runtime whether to store a copy or a 
reference, then
you can avoid the reference when you have a copy using static polymorphism:

template<int M, int N, class Storage = boost::mpl::identity<> >
class regular_grid {
    typedef boost::array< boost::array<double, M>, N > array_type;
    typedef typename boost::mpl::apply<Storage, array_type>::type 
storage_type;
    storage_type axis_;
    // use axis
};

typedef boost::add_reference<boost::add_const<boost::mpl::_1> > 
reference_policy;
typedef boost::mpl::identity<> copy_policy;

regular_grid<2, 3,  reference_policy> g;

In Christ,
Steven Watanabe