Why is my union's size bigger than I expected?

Question

When I print the size of a union like this:

union u {
  char c[5];
  int i;
} un;

using this:

int _tmain(int argc, _TCHAR*          


        

Answer 1

The alignment of your union must be the largest alignment of any of its members. This is 4. Therefore, the size of the union must be aligned to that size. It could have been 5 (as c is the largest member of the union), but because the alignment of the union as a whole is 4, the size of the union is padded to 8.

Note that this is just for VC++. The standard does not specifically require it. Though it does allow implementations to pad types as needed, which VC++ does. GCC could do something different, and there could be compile-time switches you could employ to change this behavior.