Exemple where TYPE_ALIGNMENT() fails

Question

Hi,

I have a question relating to alignment in c/c++. In http://stackoverflow.com/questions/364483/determining-the-alignment-of-c-c-structures-in-relation-to-its-members Michael Burr posted this Makro:

#define TYPE_ALIGNMENT( t ) offsetof( struct { char x; t test; }, test )

in the comments someone wrote this might fail with non POD typs. Can someone give me an code example where this fails?

Answer 1

offsetof is only specified to work for POD types. If a class contains any data members that are not POD, the class itself is not POD. So, if t in your example is a non-POD type, it is not guaranteed to work.

From the C++ standard (18.1/5):

The macro offsetof accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union.

So, if you use offsetof on a non-POD type, the results are undefined.