Is there any way to store unsigned long in core data?

Question

CoreData provides Integer 16, Integer 32 and Integer 64 storage, but doesn't support any sign qualifiers. You can store an unsigned int (32 bit) as a signed long (64 bit) and make sure the value is preserved for the full range, but an unsigned long seems to require a 128 bit signed integer to store, which of course isn't supported by CoreData. Is there any way then to store unsigned long in coreData?

Answer 1

[Previous comment promoted to answer]

Sounds like it is the bit pattern which is important to you and not the integer value per se. You can store it as a signed - just cast it as C signed<->unsigned casts don't enforce mathematical correctness and just preserve the bits. Cast it back to use it.

Follow up question:

In general yes in (Obj-)C(++) you can store an unsigned integer value into a variable with the equivalent signed integer type, and vice-versa. C casts from signed -> unsigned by definition equate to a bit-copy when using 2's complement integers and the two types are the same size. Going the other way, unsigned -> signed, is "implementation defined" - which in practice usually means a bit-copy. Clang & GCC use a bit-copy for both, but if you want to be absolutely certain you can use a union:

unsigned long r;
long l;

r = (unsigned long)l; // will always work (cast optional)

// following is l = (long)r (cast optional) without "implementation defined" risk
{ union { long sValue; unsigned long uValue; } tmp; tmp.uValue = r; l = tmp.sValue;}

But seriously I doubt anybody would! (Note: Clang at least will compile it down to a straight assignment (bit-copy).)

Answer 2

If you really need the full precision of 64 bit unsigned, you can make it transformable (check the documentation about storing Non-Standard Persistent Attributes). CoreData let's you store just about anything that way. But you probably don't need the full 64-bit precision...?!?

Answer 3

You could always convert [de]serialize it as a string. It isn't particularly clean, but it gives you the ability to store it as long as you can parse it back into an unsigned long.

Answer 4

An unsigned long is not 128bits (yet).
(or do you have a 128bits CPU?)

On a Mac, depending on your CPU architecture, it may be 32 or 64 bits.

See with:

NSLog( @"%u", sizeof( unsigned long ) );

So basically an unsigned long will be compatible will Integer32 or Integer64.