Question

I’m new to iOS development and I’ve been seeing the following in several tutorials as well as when Xcode autogenerates code for me when subclassing one of my classes. Let’s say I have the following .h and .m files

#import <UIKit/UIKit.h>

@interface Class : NSObject {
    NSArray *_infos;
    NSString *_context;
}

@property (nonatomic, retain) NSArray *infos;
@property (nonatomic, retain) NSString *context;

@end

#import "Class.h"

@implementation Class

@synthesize infos = _infos;
@synthesize context = _context;

@end

And then consider this which is how I would normally do it:

#import <UIKit/UIKit.h>

@interface Class : NSObject {
    NSArray *infos;
    NSString *context;
}

@property (nonatomic, retain) NSArray *infos;
@property (nonatomic, retain) NSString *context;

@end

#import "Class.h"

@implementation Class

@synthesize infos;
@synthesize context;

@end

What is the difference? From the notation I can just infer that they’re just declaring the variables as private, but how does it work? If I’m correct.

Answer 1

It’s a silly naming convention. Use it if you want to, leave it if you don’t.

The advantage is that a method argument/local variable named context does not conflict with the ivar _context. It has little to do with privacy, since you can just specify @private to make the ivars private (or just @synthesize them in the first place).

The disadvantage is that you have underscores everywhere, and underscores are occasionally a bit special in C (though an underscore followed by a lowercase letter is only reserved in file scope, so you should be fine provided the ivar starts with a lowercase letter…). Where necessary, I stick an underscore at the end which feels less dirty.

Sidenote: A few people use method names beginning with an underscore to mean “private”, but this convention is reserved by Apple.