Pass-by-value argument in message expression is undefined - iphone

Question

I'm developing an iPhone application and I getting that warning at method:
NSNumber *latitudeValue;
NSNumber *longitudeValue;
[self obtainLatitude:latitudeValue longitude:longitudeValue];
The method is declared as follows:
- (void) obtainLatitude:(NSNumber *)latitudeValue longitude:(NSNumber *)longitudeValue {
NSNumberFormatter * f = [[NSNumberFormatter alloc] init];
[f setNumberStyle:NSNumberFormatterDecimalStyle];
latitudeValue = [f numberFromString:[latitude.text stringByReplacingOccurrencesOfString:#"," withString:#"."]];
longitudeValue = [f numberFromString:[longitude.text stringByReplacingOccurrencesOfString:#"," withString:#"."]];
[f release];
}
As you can see, I'm trying to calculate latitudeValue and longitudeValue calling obtainLatitude:longitude: but I'm doing something wrong.
How can I fix that error?

Answer 1

Elfred's answer works, but pass-by-reference for non-NSError** parameters is pretty uncommon. As well, coordinates -- numeric values, in general -- are most typically stored in regular old C types in structures because, comparatively, an NSNumber is quite a bit of overhead (no big deal for a few of 'em, would be a problem if you have a few dozen, hundred, or thousands of coordinates).
Something like:
struct MyLocation {
CGFloat latitude;
CGFloat longitude;
};
typedef struct MyLocation MyLocation;
Then:
- (MyLocation) mapCoordinates {
MyLocation parsedLocation;
parsedLocation.latitude = ....;
parsedLocation.longitude = ....;
return parsedLocation;
}
Something like the above would be more typical in an iPhone/Cocoa program.
As Dave points out, you really don't need to define your own type for this. Use CLLocationCoordinate2D or CLLocation.

Answer 2

You are indeed passing the pointers by value, so when you reassign them, that just takes effect inside thee method. One alternative is to do the following:
- (void) obtainLatitude:(NSNumber **)latitudeValue longitude:(NSNumber **)longitudeValue {
NSNumberFormatter * f = [[NSNumberFormatter alloc] init];
[f setNumberStyle:NSNumberFormatterDecimalStyle];
*latitudeValue = [f numberFromString:[latitude.text stringByReplacingOccurrencesOfString:#"," withString:#"."]];
*longitudeValue = [f numberFromString:[longitude.text stringByReplacingOccurrencesOfString:#"," withString:#"."]];
[f release];
}
then your call would look like:
NSNumber *latitudeValue;
NSNumber *longitudeValue;
[self obtainLatitude:&latitudeValue longitude:&longitudeValue];