Memory layout question

Question

Do these two structs have the same memory layout? (C++)

struct A
{
   int x;
   char y;
   double z;
};

struct B
{
   A a;
};

Further can I access x, y, z members if I manually cast an object of this to an A?

struct C
{
   A a;
   int b;
};

Thanks in advance.

EDIT:

What if they were classes instead of structs?

Answer 1

Yes and yes. The latter is commonly used for emulating OO inheritance in C.

Answer 2

Yes, that'll work. Depending on compiler structure packing settings, it may not work with members other than the first.

Answer 3

You can verify this yourself by checking field offsets relative to the start of an instance of each.

A aObj;
B bObj;
C cObj;

int xOffset1 = &aObj.x - &aObj;
int xOffset2 = &bObj.a.x - &bObj;

ASSERT(xOffset1 == xOffset2);

and so on

Answer 4

$9.2/16- "Two standard-layout struct (Clause 9) types are layout-compatible if they have the same number of non-static data members and corresponding non-static data members (in declaration order) have layout-compatible types (3.9)."

So the answer is 'yes'