Feb 28 2009
Multidimensional arrays in C++
Dynamic, multidimensional arrays are not supported directly by C++. If you create such an array in following way:
array = new int*[_size];
for( unsigned int i = 0; i < _size; i++ )
array[i] = new int[_size];
you are in fact creating one dimensional array of pointers to other arrays. Main two disadvantages of this approach:
-
Initialization can take a while (image really big 2-3 dimensional array, lets say: 16384 x 16384: how many times memory allocation is being performed? 1 + 16384. Quite a lot, right?
-
dynamic arrays created like in the code above can not be used as a parameters for memcmp, memcpy, memset & similar functions. the memory allocated is not a continuous memory block.
As I needed a 2-dimensional array that occupies continuous block of memory – I started googling :) I have found a great solution by David Maisonave (Axter) (code, header). The code above could be rewritten as follows:
Pixel** Initialize2dArray( int size )
{
Pixel **array2d;
Pixel *temp;
array2d = new Pixel*[size];
temp = new Pixel[size * size];
for( int i = 0; i < size; i++ )
{
array2d[i] = temp;
temp += size;
}
return array2d;
}
void Deinitialize2dArray( Pixel** array2d )
{
delete [] *array2d;
delete [] array2d;
}
How does it work? The idea is to put in every ‘cell’ of the array a pointer that will point to a part of the ‘temp’ array. The parts are of the ‘width’ size. So, when you type array[2][3] it actually selects the data that is indexed 2 * width + 3 = exactly what you’re expecting, right? But beware, there is a little difference when using array created in described way and all those mem* functions I wrote at the beginning, instead of writing:
memcpy( array2d, source, sizeof(int) * size * size );
you actually need to write:
memcpy( &array2d[0][0], source, sizeof(int) * size * size );
It is great, right? And, of course, you could try using std::vector (it allocates continuous block of memory – correct me if I am wrong) or Boost Multi Array instead – but if you’re worrying about performance… I think the presented solution is the best.
