how to mask members in a union

i have a union

union filter_row
  {
        MAC_Filter MAC;
        IP_Filter IP;
    开发者_运维百科    TCP_Filter TCP;
        UDP_Filter UDP;
        ICMP_Filter ICMP;
        ARP_Filter ARP;
  };

The members of this union are structure....how to mask the members of structures... Do I have to initialize them to all 1's initially???

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All members of a union share the same storage space, enough to accommodate the alignment and size of the largest and most restrictive of the members. Normally, you'd also require a member in the structure to indicate what union member to use.