Multidimensional array indexing using pointer to elements

Question

As far as I know, multidimensional array on stack will occupy continuous memory in row order. Is it undefined behavior to index multidimensional array using a pointer to ele

Answer 1

I believe the behavior in your example is technically undefined.

The standard has no concept of a multidimensional array. What you've actually declared is an "array of 5 arrays of 4 ints". That is a[0] and a[1] are actually two different arrays of 4 ints, both of which are contained in the array a. What this means is that a[0][0] and a[1][0] are not elements of the same array.

[expr.add]/4 says the following (emphasis mine)

When an expression that has integral type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integral expression. In other words, if the expression P points to the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i + n-th and i − n-th elements of the array object, provided they exist. Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined

So, since p[11] expands to *(p + 11) and since p and p + 11 are not elements of the same array (one is an element of a[0] and the other is more than one element past the end of a[0]), the behavior of that addition is undefined.

I would, however, be very surprised to find any implementation where such an addition resulted in anything other than the one you expect.