How do I access internal members of a union?

Question

I have a union that is defined like this:

typedef union {
  enum {
    REVISION = 0,
    CURRENT_VERSION = REVISION
  };

  enum FLAGS{
    FLAG_DEFAULT               = 0x00000000,
    FLAG_EOD                   = 0x00000001,
    FLAG_OUTOFORDER            = 0x00000002
  };

  CHAR _filler[32];

  struct INTERNAL_STRUCTURE {
    UINT16 type;
    UINT16 flags;
  };
}CORRHDR

How do I access the member's of INTERNAL_STRUCTURE from my code?

I've thought I could just do this:

CORRHDR hdr;
hdr.INTERNAL_STRUCTURE.type = 1;

I'm wrong. I can see the enums in the union, but nothing else. Could someone explain the structure (or benefit) of this type to me?

Answer 1

You have declared the type called INTERNAL_STRUCTURE, but not an actual instance of that type. Try this:

typedef union {
  //...
  CHAR _filler[32];
  struct {
    UINT16 type;
    UINT16 flags;
  } INTERNAL_STRUCTURE;
}CORRHDR;

Then to access the field:

CORRHDR ch;
printf("%u\n", ch.INTERNAL_STRUCTURE.type);

Answer 2

You need to define an object of type INTERNAL_STRUCTURE before you can try to access it. Your union as of now does not have one. Try:

struct INTERNAL_STRUCTURE {
    UINT16 type;
    UINT16 flags;
  };
struct INTERNAL_STRUCTURE is;

...

Answer 3

You defined a type named INTERNAL_STUCTURE

not an instance named INTERNAL_STRUCTURE.

typedef union {
  enum {
    REVISION = 0,
    CURRENT_VERSION = REVISION
  };

  enum FLAGS{
    FLAG_DEFAULT               = 0x00000000,
    FLAG_EOD                   = 0x00000001,
    FLAG_OUTOFORDER            = 0x00000002
  };

  CHAR _filler[32];

  struct {
    UINT16 type;
    UINT16 flags;
  } INTERNAL_STRUCTURE;
}CORRHDR;

Answer 4

Also, you define your struct but don't use it!

You need to define a variable of that struct type:

union Blob
{
  CHAR _filler[32];

  struct INTERNAL_STRUCTURE {
    UINT16 type;
    UINT16 flags;
  } myStruct;
};

CORRHDR hdr;
hdr.myStruct.type = 1;