Sunday, March 25, 2007

Forgotton friend: pointer/reference to an array

The name of an array "degenerates" into a pointer to the first element of the array. For example,
int Array [10];
int *p = Array;

there is a quite a bit loss of information here. Specifically, such a decay loses its type information and specifically, its dimensions. The type of Array is "An integer array of 10 integers." So we lost the word "array" and we also lost the length (10) of the array.

An advantage of using pointer to an array is that this type information is retained. int (*q)[10] = &Array; It declares a pointer q to to an array of 10 integers. So whats the big deal? Compiler has this information and it is happy to let us take advantage of it.

template <int p, int q, int r>
int average (int (&array)[p][q][r]) { ... }

main () {
int q [][2][2] = {
{ {1, 2}, {3, 4} },
{ {5, 6}, {7, 8} }
};
average (a); /// This call knows the dimensions of the array.
}
This type information can be exploited to implement the average function using template meta-programming techniques! This can be useful in generative programming as well.