Example where TYPE_ALIGNMENT() fails

I have a question relating to alignment in C/C++. In Determining the alignment of C/C++ structures in relation to its members Michael Burr posted this macro:

#define TYPE_ALIGNMENT( t ) offsetof( struct { char x; t test; 开发者_Python百科}, test )

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

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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.