class, 3D-Array, return style

[spamme]

Dear all,

I have class names Tensor, which contains a 3 dimensional array. The access to that array can be done like in Fortran by:

 Tensor<double> density;
[...] 
density(z,y,x) = do something; 

But I would like to have also an C/C++ access-style:

 Tensor<double> density; 
[...]
density[z][y][x] = do something; 

To enable to make things like that:

Tensor<double> density;
[...]
double **tmp; 

tmp = density[25];

The class definition:

template<class T> class Tensor : protected DimS{

 protected:
 T ***data;
 
 public:

 T& operator()(int z, int y, int x) {return data[z][y][x];};
 T operator()(int z, int y, int x) const {return data[z][y][x];};

 [...]
};

The 3D-array data is allocated by:

template<class T> T ***allocate_3D(itype nz, itype offz,
				   itype ny, itype offy,
				   itype nx, itype offx
				   )
{
  T  ***field;
  
  size_t  NZ = (size_t)(nz  + 2*offz);
  size_t  NY = (size_t)(ny  + 2*offy);
  size_t  NX = (size_t)(nx  + 2*offx);
  
  field = (T ***) calloc (NZ, sizeof(T **)) + offz;
  if( NULL == field  ){ _error("Could not allocate the field"); }
  
  field[-offz] = (T **) calloc (NZ*NY, sizeof(T *)) + offy;
  if( NULL == field[-offz]  ){free(field); _error("Could not allocate the field"); }


  field[-offz][-offy] = (T *) calloc (NZ*NY*NX, sizeof(T)) 
                       + offx;

  if( NULL == field[-offz][-offy]  ){
    free(field[-offz]); 
    free(field); 
    _error("Could not allocate the field"); 
  }
  
  for(itype i=1-offz ; i < nz+offz ; i++)  
    field[i] = field[i-1] + NY;
  
  for(itype i=1-offz; i < nz+offz ; i++) 
    field[i][-offy] = field[i-1][-offy] + NY*NX;
 
  for(itype i=-offz ; i < nz+offz ; i++)
    for(itype j=1-offy ; j < ny+offy ; j++)
      field[i][j] = field[i][j-1] + NX;

  return field;
}

Thanks a lot for help!