2D array vs array of arrays

Question

What is the difference between a 2D array and an array of arrays?

I have read comments, such as @Dave's, that seem to differentiate between the two.

This breaks if he's using 2d arrays, or pointer-to-array types, rather than an array of arrays. – Dave

I always thought that both referred to:

int arr_arr[][];

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EDIT: @FutureReader, you may wish to see How do I use arrays in C++?

Answer 1

There are four different concepts here.

• The two-dimensional array: int arr[][]. It cannot be resized in any direction, and is contiguous. Indexing it is the same as ((int*)arr)[y*w + x]. Must be allocated statically.
• The pointer-to array: int (*arr)[]. It can be resized only to add more rows, and is contiguous. Indexing it is the same as ((int*)arr)[y*w + x]. Must be allocated dynamically, but can be freed free(x);
• The pointer-to-pointer: int **arr. It can be resized in any direction, and isn't necessarily square. Usually allocated dynamically, not necessarily contiguous, and freeing is dependent on its construction. Indexing is the same as *(*(arr+y)+x).
• The array-of-pointers: int *arr[]. It can be resized only to add more columns, and isn't necessarily square. Resizing and freeing also depends on construction. Indexing is the same as *(*(arr+y)+x).

Every one of these can be used arr[y][x], leading to the confusion.

Answer 2

A 2 dimensional array is by definition an array of arrays.

What Dave was saying is that in that context, there are different semantics between the definition of a 2D array like this:

int x[][];

this:

int *x[];

or this:

int **x;