Prefixing property names with an underscore in Objective C [duplicate]

Solution 1:

I always use underscores. It creates a clear distinction between local variables and instance variables. It also avoids compiler warnings in the following situation:

@interface MyClass
{
    NSString *name
}

@property (nonatomic, copy) NSString *name;

- (id) initWithName:(NSString *) name;
@end

@implementation MyClass

@synthesize name;

// The following method will result in a compiler warning
// (parameter name same as ivar name)
- (id) initWithName:(NSString *) name {
   if (self = [super init]) {
      self.name = name;
   }

   return self;
}

@end

EDIT:

After having to endure downvotes and reading through the comments, let me try to make my point:

Apple recommends that ivars have the same name as their property. Apple also recommends that properties start with a lowercase letter. And Apple also recommends that local variables start with a lowercase letter.

Now you have a problem, because when you read a piece of code, and you see a variable being used, you cant' tell by the naming convention if this variable is an ivar or a local variable. That sucks. The solution is to have different naming conventions for ivars and local variables. That's just plain common sense.

The way you implement this naming convention is irrelevant. If you really want, you can simply append "_WOOHAHA" to the ivar names. I don't care (but maybe others will). The thing is that people who know what they're doing have decided to go with the "underscore prefix" for ivars. IMHO, they made the right decision, even if their own company recommends something else. (the developers I'm talking about are the people writing some major Apple frameworks and the .NET Framework classes)

In the end, code quality is more important than following a stupid rule that isn't even followed by the people preaching it.

Another remark about the code you've shown: never use retain on string properties. You should use copy instead.

For more info about copy/retain on properties, see:

NSString property: copy or retain?

Solution 2:

The naming convention for the instance variable prefixed by _ is now clearly stated by Apple in the "Coding Guidelines for Cocoa", after the revision of 2012-02-16, with its reason.

Make sure the name of the instance variable concisely describes the attribute stored. Usually, you should not access instance variables directly, instead you should use accessor methods (you do access instance variables directly in init and dealloc methods). To help to signal this, prefix instance variable names with an underscore (_), for example:

@implementation MyClass {
    BOOL _showsTitle;
}

If you synthesize the instance variable using a declared property, specify the name of the instance variable in the @synthesize statement.

@implementation MyClass
@synthesize showsTitle=_showsTitle;

https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CodingGuidelines/Articles/NamingIvarsAndTypes.html#//apple_ref/doc/uid/20001284-BAJGIIJE

The lecture in iTunes U, iPhone App Development CS193p Fall 2011 taught by Paul Hegarty at Stanford University, also explains this convention.

http://itunes.apple.com/itunes-u/ipad-iphone-application-development/id473757255

I'm aware of that this question is asked quite a while ago, but I myself had the same question and wanted to share my findings.