C/C++: Is this undefined behavior? (2D arrays)

Question

Is it undefined behavior if I go through the elements of a 2D array in the following manner?

int v[5][5], i;

for (i = 0; i < 5*5; ++i) {
     v[i] = i;
}

Then again, does it even compile? (I can't try it right now, I'm not at home.) If it doesn't, then imagine I somehow acquired a pointer to the first element and using taht instead of v[i].

Answer 1

Accessing elements of a multidimensional array from a pointer to the first element is Undefined Behavior (UB) for the elements that are not part of the first array.

Given T array[n], array[i] is a straight trip to UB-land for all i >= n. Even when T is U[m]. Even if it's through a pointer. It's true there are strong requirements on arrays (e.g. sizeof(int[N]) == N*sizeof(int)), as mentioned by others, but no exception is explicitly made so nothing can be done about it.

I don't have an official reference because as far as I can tell the C++ standard leaves the details to the C89 standard and I'm not familiar with either the C89 or C99 standard. Instead I have a reference to the comp.lang.c FAQ:

[...] according to an official interpretation, the behavior of accessing (&array[0][0])[x] is not defined for x >= NCOLUMNS.