Re: Passing multidimensional arrays to functions

"kanze" <>
6 Jun 2006 18:05:02 -0400
<> wrote:

A multi-dimensional array A is to be allocated at runtime and
passed to a function. I am struggling with two cases:

1) All but the first dimension of A are known at compile time.
For example, the array is complex<double> A[N][6][6]. I can
allocate the storage using 'new', but

     complex<double> *A = new complex<double> [N*6*6];

doesn't work, because now the type of A does not match my
function prototype, complex<double> A[][6][6].

So use the types consistently:

    complex<double>(* A)[6][6] = new complex<double>[N][6][6] ;

What you have declared is an array[] of array[6] of array[6]
complex<double>. As a parameter to a function, this is
converted to a pointer to array[6] of array[6] of
complex<double>. In a new expression, it means that array new
is used (since the top element of the type is an array), which
means that instead of returning a pointer to the allocated
object (of type T*, if the object type is T), the return type is
also constructed by converting the topmost array type to a
pointer, and becomes pointer to array[6] of array[6] of

Because array has precedence over pointer in a type expression,
you need to put the pointer in parentheses with the name of the
declared object.

2) Other dimensions of A are unknown at compile time. For
example, complex<double> A[N1][2][N3]. Here my function
prototype contains complex<double> *A, and within the function
I do the subscript offset calculation i*s2+j*s3+k. I am
passing s2 (2 in this example) and s3 (N3 here) to the
function. I allocate using

     complex<double> *A = new complex<double> [N1*2*N3];

and the compiler accepts this. However, it would be preferable
to be able to address array elements within the called
function as A[i][j][k], rather than as A[i*s2+j*s3+k].

A class. It's rather simple to arrange for operator[] to return
a proxy, on which operator[] is also overloaded.

Can someone suggest a straightforward way to deal with these
two cases, preferably one that doesn't involve installing a
new class library?

Well, for the second problem, you either install a new class
library, or write your own. The first isn't a problem, but I'd
still go with a class based solution.

James Kanze GABI Software
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