How does an underscore in front of a variable in a cocoa objective-c class work?

Question

I've seen in a few iPhone examples that attributes have used an underscore _ in front of the variable. Does anyone know what this means? or how it works?

an interface file I'm using looks like:

    @interface MissionCell : UITableViewCell {
Mission *_mission;
    UILabel *_missionName;
    }

    @property (nonatomic, retain) UILabel *missionName;

    - (Mission *)mission;

I'm not sure exactly what the above does but when I try to set the mission name like:

    aMission.missionName = missionName;

I get the error: request for member 'missionName' in something not a structure or union

Answer 1

It doesn't really mean anything, it's just a convention some people use to differentiate member variables from local variables.

As for the error, it sounds like aMission has the wrong type. What it its declaration?

Answer 2

If you use the underscore prefix for your ivars (which is nothing more than a common convention, but a useful one), then you need to do 1 extra thing so the auto-generated accessor (for the property) knows which ivar to use. Specifically, in your implementation file, your synthesize should look like this:

@synthesize missionName = _missionName;

More generically, this is:

@synthesize propertyName = _ivarName;

Answer 3

It's just a convention for readability, it doesn't do anything special to the compiler. You'll see people use it on private instance variables and method names. Apple actually recommends not using the underscore (if you're not being careful you could override something in your superclass), but you shouldn't feel bad about ignoring that advice. :)