C union assignment

I have union that represent some structures that all uint32 type but with different bit-fields. can i do assignment of one union to other like this:

typedef union foo_u
{
   // raw
   uint32_t foo32;

   // interpretation 1
   struct
   {
      uint16_t a;
      uint16_t b;
   } foo_flavor1;

   //interpretation 2
   struct
   {
      uint32_t a        : 16;
      uint32_t b        : 12;
      uint32_t c        : 4;
   } foo_flavor2;
} foo;
foo a;
foo b;
a.foo32 = 10;
b.foo32 = 30;
b=a;

or i have to do the assignment like this:

b.foo32 = a.foo32;