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

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, N> myaxis; //creates axis in a stack allocated data structure. regular_grid my_grid(axis_values); regular_grid my_grid(axis_values);

template class regular_grid { boost::array< boost::array, 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, N> axis_; regular_grid(const boost::array< boost::array, 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, 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 > class regular_grid { typedef boost::array< boost::array, N > array_type; typedef typename boost::mpl::apply::type storage_type; storage_type axis_; // use axis }; typedef boost::add_referenceboost::mpl::_1 > reference_policy; typedef boost::mpl::identity<> copy_policy; regular_grid<2, 3, reference_policy> g; In Christ, Steven Watanabe