C++ alignment and arrays

Question

• I have some type T that I explicitly specify as x-aligned
• x > sizeof(T)
• x > any implementation fundamental alignment
• (ex: x is page or cache aligned)

Suppose I now have: T arr[y], where arr is x-aligned (either by being allocated on the stack, or in data, or by an x-aligned heap allocation)

Then at least some of arr[1],...,arr[y-1] are not x-aligned.

Correct? (In fact, it must be correct if sizeof(T) does not change with extended alignment specification)

Note1: This is not the same question as How is an array aligned in C++ compared to a type contained?. This question asks about the alignment of the array itself, not of the individual elements inside.

Note2: This question: Does alignas affect the value of sizeof? is essentially what I'm asking - but for extended-alignment.

Note3: https://stackoverflow.com/a/4638295/7226419 Is an authoritative answer to the question (that sizeof(T) includes any padding necessary to satisfy alignment requirements for having all T's in an array of T's properly aligned.

Answer 1

If type T is x-aligned, every object of type T is x-aligned, including any array elements. In particular, this means x > sizeof(T) cannot possibly hold.

A quick test with a couple of modern compilers confirms:

#include <iostream>

struct alignas(16) overaligned {};
struct unaligned {};

template <class T> void sizes()
{
    T m, marr[2];
    std::cout << sizeof(m) << " " << sizeof(marr) << std::endl;
}

int main ()
{
    sizes<unaligned>();
    sizes<overaligned>();
}

Output:

1 2
16 32