A C++ question

I saw some code in ACE:

class ACE_Export ACE_U_LongLong
{
   ....
   ....

union
      {
        struct
          {
#     if defined (ACE_BIG_ENDIAN)
            /// High 32 bits.
            ACE_UINT32 hi_;

            /// Low 32 bits.
            ACE_UINT32 lo_;
#     else
            /// Low 32 bits.
            ACE_UINT32 lo_;

            /// High 32 bits.
            ACE_UINT32 hi_;
#     endif /* ! ACE_BIG_ENDIAN */

          } data_;

        /// To ensure alignment on 8-byte boundary.
        /**
         * @note "double" isn't usually usable with
         *       ACE_LACKS_FLOATING_POINT, but this seems OK.
         */
        double for_alignment_;
      };
   ....
};

I don't understand why for_alignment_ can ensure aligment on 8-byte 
boundary. Anyone can tell me?

Thanks!

ChinaGirl from Canada





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