[Boost-users] Efficient "[][]" operator overloading (encapsulating a MultiArray)

Subject: [Boost-users] Efficient "[][]" operator overloading (encapsulating a MultiArray)
From: Jiří Vyskočil (svzj_at_[hidden])
Date: 2013-04-09 10:36:29

Hello,

I need to encapsulate a 3D MultiArray inside another class "GridArray". 
The reason is that GridArray will have another implementation not using 
MultiArray and it seems much easier to provide an abstration over 
MultiArray, than to reimplement its interface for the other implementation.

I need to keep the [][][] operator access to the structure. I created a 
class like this, but it is slow:

template<class T> class GridArray<T,3> {
public:
     GridArray(int x, int y, int z):
         _array(boost::extents[x][y][z]) { }
     boost::multi_array<T, 3> _array;
     struct SliceRef {
         struct ColRef {
             SliceRef &_parent;
             int _x, _y;
             ColRef(SliceRef &p, const int x, const int y):
                 _parent(p), _x(x), _y(y) {}
             T& operator[](const int z) {
                 return _parent._parent._array[_x][_y][z];
             }
         };
         GridArray<T,3> &_parent;
         int _x;
         SliceRef(GridArray<T,3> &p, const int x):
             _parent(p), _x(x) {}
         ColRef operator[](const int y) {
             return ColRef(*this,_x,y);
         }
     };
     SliceRef operator[](const int x){
         return SliceRef(*this,x);
     }
};

I am unable to write a minimal test case - if I compare using 
multi_array directly vs. using this GridArray class in a simple test 
program (compiled with -O2), the performance is the same. But the real 
code shows about 20% slowdown - it's a numerical simulaiton, I create 
several arrays at the beginning and then do computations with them. The 
rest of the code hasn't been touched - I only chanded the constructors 
for the arrays and replaced a header which contained some typedefs with 
a header containing this class.

Do you have any idea what might be going on? Is there a better way to 
write an interface for successive [] operators?

Thanks,
Jiri