Do array elements count as a common initial sequence?

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Do elements of arrays count as a common initial sequence?

struct arr4 { int arr[4]; };
struct arr2 { int arr[2]; };

union U
{
    arr4 _arr4;
    arr2 _arr2;
};

U u;
u._arr4.arr[0] = 0; //write to active
u._arr2.arr[0]; //read from inactive

According to this cppreference page:

In a standard-layout union with an active member of non-union class type T1, it is permitted to read a non-static data member m of another union member of non-union class type T2 provided m is part of the common initial sequence of T1 and T2....

Would this be legal, or would it also be illegal type punning?

Answer 1

C++11 says (9.2):

If a standard-layout union contains two or more standard-layout structs that share a common initial sequence, and if the standard-layout union object currently contains one of these standard-layout structs, it is permitted to inspect the common initial part of any of them. Two standard-layout structs share a common initial sequence if corresponding members have layout-compatible types and either neither member is a bit-field or both are bit-fields with the same width for a sequence of one or more initial members.

As to whether arrays of different size form a valid common initial sequence, 3.9 says:

If two types T1 and T2 are the same type, then T1 and T2 are layout-compatible types

These arrays are not the same type, so this doesn't apply. There is no special further exception for arrays, so the arrays may not be layout-compatible and do not form a common initial sequence.

In practice, though, I know of a compiler (GCC) which:

• ignores the "common initial sequence" rule, and
• allows type punning anyway, but only when accesses are "via the union type" (as in your example), in which case the "common initial sequence" rule is obeyed indirectly (because a "common initial sequence" implies a common initial layout on the architectures the compiler supports).

I suspect many other compilers take a similar approach. In your example, where you type-pun via the union object, such compilers will give you the expected result - reading from the inactive member should give you value written via the inactive member.