On Feb 21, 2014, at 3:17 PM, [email protected] wrote:
> I have an NSNumber containing 5.
> The NSNumber instance is a key in a dictionary collection.
> In order to obtain the value I need to pass a pointer to a memory location
> containing 5 to Mono.
> However not all keys in the dictionary need be NSNumber instances, they may
> be NSStrings.
> If this case to obtain the value I need to pass a pointer to a memory
> location containing a Mono data structure.
>
> So:
> [DBObject_key -monoPtr] returns a (void *).
> [NSString_key -monoPtr] returns a MonoObject *
> [NSNumber_key -monoPtr] returns say an int*.
>
> So I want to be able to send the same message to any class and get an
> appropriate void* pointer.
This is an unreasonable and nonsensical requirement. NSNumber can hold
floating point values and integers of varying sizes. There's no one
representation of its content, so there's no way for any recipient of a
supposed "pointer to that content" (however that might be obtained) to
interpret the pointer.
If you have some specific representation in mind, then don't use NSNumber. Use
a custom class wrapping the specific representation you want to use.
> This usage pattern, within a generated code environment too, makes the
> imposition of an lvalue that I can take an address from undesirable.
However undesirable it may be, it's a consequence of you wanting an address for
things which don't have an address. The thing has to be stored, at least
temporarily, in storage which has a proper address (and a predictable size and
semantic interpretation).
> Plus it also needs to be an NSNumber, not a custom NSObject subclass.
Why?
If you really, really want to try to do this _and_ you can pick a single
in-memory representation for all NSNumber content values, you could do
something similar to what NSString does for -UTF8String. You'll notice that
that method's returned string has the lifetime of an autoreleased object, not
the original string object. I believe that it just creates an autoreleased
NSData holding the UTF-8-encoded bytes and returns that NSData's -bytes. So,
you'd do something like:
@interface NSNumber (mugginsoft_MonoPtr)
- (void*) monoPtr;
@end
@interface NSNumber (mugginsoft_MonoPtr)
- (void*) monoPtr
{
// NOTE: This falls down for signed integer values and
floating-point values.
// You could pick another type but it would have similar
problems.
unsigned long long value = [self unsignedLongLongValue];
return [[NSData dataWithBytes:&value length:sizeof(value)] bytes];
}
@end
You may need to do this in a translation unit with ARC disabled so that the
NSData really will end up in the autorelease pool rather than having a lifetime
precisely restricted to your method.
Regards,
Ken
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