Is the data in nested std::arrays guaranteed to be contiguous?

Question

Is the data in std::array<std::array<T,N>, M> guaranteed to be contiguous? For example:

#include <array>
#include <cassert>

int main()
{
    enum {M=4, N=7};
    typedef std::array<char,N> Row;
    typedef std::array<Row, M> Matrix;
    Matrix a;
    a[1][0] = 42;
    const char* data = a[0].data();

    /* 8th element of 1D data array should be the same as
       1st element of second row. */
    assert(data[7] == 42);
}

Is the assert guaranteed to succeed? Or, to put it another way, can I rely on there being no padding at the end of a Row?

EDIT: Just to be clear, for this example, I want the data of the entire matrix to be contiguous.

Answer 1

No, contiguity is not guaranteed in this case.

std::array is guaranteed to be an aggregate, and is specified in such a way that the underlying array used for storage must be the first data member of the type.

However, there is no requirement that sizeof(array<T, N>) == sizeof(T) * N, nor is there any requirement that there are no unnamed padding bytes at the end of the object or that std::array has no data members other than the underlying array storage. (Though, an implementation that included additional data members would be, at best, unusual.)

Answer 2

They are very likely contiguous. If they are not, the compiler is actively fighting you there. There's no guarantee it won't insert padding but there's hardly a reason for it.

Is the assert guaranteed to succeed?

data[7] is an out-of-bounds access (undefined behaviour). The inner array object has only seven elements, so index 7 is not valid.