Re: Tail padding in base classes vs. tail padding in class members.

I am a bit puzzled by the following. Suppose this is my base class:

     class A { int *snafu; int thing; };

Compiled for a 64-bit architecture with GCC 4.x, the above is going to
have size of 16 bytes (including the 4 bytes of padding after
'thing'). My derived class is this:

     class B : public A { int foo; };

This class also ends up having size of 16 bytes, since 'foo' gets
packed into the tail padding of the base class A. Everything fine,
just as anybody would want.

Sometimes, instead of inheritance, it might be more appropriate to put
an instance of A as a data member in a class:

     class C { A boo; int foo; };

The above ends up taking 24 bytes. The compiler does NOT pack 'foo'
into the tail padding of the instance of A. I end up with 4 bytes of
wasted tail padding of A, and extra 4 bytes of tail padding in C.

Search as I might, I saw no good explanation for that refusal to reuse
the padding in class members, either in the C++ standard nor anywhere
on the web, so I thought I would turn to this newsgroup for some
education. Can someone tell me WHY is this happening?