Pass-by-value argument in message expression is undefined

Question

Hello.

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

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];

Answer 2

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.