Assigning values to union in C++

[pranava]

Hello I have the following C++ code

typedef union UUID {
unsigned char byte[16]; /**< Array of 16 bytes. */
unsigned int ll[2]; /**< Array of two 64-bit words. */
} UUID;

struct EntryHeader {
UUID uuid; /**< UUID to which entry pertains. */
};

#define UUID_SELF {0,0,0,2}

int main(void)
{
struct EntryHeader entry;
entry.uuid = UUID_SELF;
}
The compiler complains thus

$ g++ union.cpp
union.cpp: In function ‘int main()’:
union.cpp:15:17: warning: extended initializer lists only available with -std=c++0x or -std=gnu++0x
union.cpp:15:17: warning: extended initializer lists only available with -std=c++0x or -std=gnu++0x
union.cpp:15:17: error: no match for ‘operator=’ in ‘entry.EntryHeader::uuid = {0, 0, 0, 2}’
union.cpp:1:20: note: candidate is: UUID& UUID:perator=(const UUID&)

How do I go about assigning values to this union in C++. Any help would be appreciated.

Thanks
-Pranava

[treuss]

You need to decide, which part of your union you want to assign values to:

  EntryHeader entry;
  entry.uuid.ll = {0u, 4294967295u};
  entry.uuid.byte = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};

Note that array assignments are only supported by C++-11, not by C++-98.

Another note: Are you sure your compiler uses 64 bits for unsigned ints?

[pranava]

Thanks, but that too didn't work. Someone else suggested this that worked ok.

Use the compiler instead of the preprocessor.

1
2
//#define UUID_SELF {0,0,0,2}
const UUID UUID_SELF = {0,0,0,2} ;


And then, either:
1
2
EntryHeader entry ;
entry.uuid = UUID_SELF ;

or: EntryHeader entry = { UUID_SELF /*, .... */ } ;

[OReubens]

That works but it's not the same...

The answer is that in C++ you can only INITIALIZE the first 'member' of a union.
You can at any time ASSIGN any other 'member' of a union.
You can't assign initialiser lists (which is what you were trying to do)

It may not seem like a big deal, but when you are dealing with consts, it can be problematic, since you can't assign consts.

union uu
{
	long l2[2];
	char c8[8];
};

class UU
{
public:
	uu u;
	int i;
};

int main()
{
	uu u = { 12,56 };      // Initializes the l2 portion of u with 12 and 56.
	UU U = { 12,56, 5 }; // initializes the l2 portion of u with 12 and 56 and i with 5
}

If you flip l2 and c8 around in the uu union and don't change the rest of the code...

union uu
{
	char c8[8]; // Now c8 if first
	long l2[2];
};

class UU
{
public:
	uu u;
	int i;
};

int main()
{
	uu u = { 12,56 };      // Initializes the c8 portion of u with 12 and 56, the remaining 6 c8's are aggregated to 0.
	UU U = { 12,56, 5 }; // initializes the c8 portion of u with 12, 56 and 5, the remaining 5 c8's and i are aggregated to 0


	// In this case you can 'force' a break by adding extra brackets.
	UU U2 = { {12,56}, 5 }; // Initialize the c8 portion of 8 with 12 and 56, aggregates the remaining 6 to 0, and initialises i with 5.
}