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

Question

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

#include 
#include 

        

Answer 1

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.

Answer 2

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.)