Possible Duplicate:
How does an underscore in front of a variable in a cocoa objective-c class work?
I've seen this at Apple, inside UIPickerView.h:
id<UIPickerViewDataSource> _dataSource;
why's that underscore there? Does it have a special meaning? A Convention I must know about?
A lot of people use this for private variables, to differentiate between private variables and public variables within an object.
It is a completely optional way of working.
@synthesize var = _var; also (gently) guides you toward using accessors and dot notation constructively. –
Actinouranium What you're seeing is the use of underlines to distinguish between instance variables and properties. So a class declaration might be:
@interface Foo {
NSString* _label;
....
}
@property (nonatomic, retain) NSString* label; // notice: no underline
Then in the implementation file you would have:
@synthesize label=_label; // the property is matched with the ivar
Now when inside the implementation, if you want to access the instance variable directly you could just use _label but to go through the property accessor methods (which take care of retain/releases and a bunch of other book-keeping tasks) you would use self.label. From the outside, you would always want to go through the {object}.label property.
The other way is to do without the underline and just use:
NSString* label;
@property (nonatomic, retain) NSString* label;
...
@synthesize label;
It works the same but then it might confuse someone reading the code and trying to keep track of label vs self.label. I personally find the Apple convention (with underlines) a bit easier to read but it's a matter of preference.
As people said already _someVar was used to say that a variable was private. It was a simple matter of convention and doesn't really matter.
One other use, taking a trip in the wayback machine in C a _function() represented a function that wasn't platform portable and __function() represented a function that wasn't compiler portable. So, in the standard C library, you will sometimes see a variable with a _ or __ infront of the name, this is what those functions represent.
It's sometimes used to denote private variables. More generally it just means "this variable is different somehow".
can it be that... (jogging memory)...
i vaguely remember reading an ADC document explaining that apple reserves the usage of underscore-prefixed member variables? and that 3rd party developers are discouraged from using this convention so as to avoid collisions?
|K<
I use underscores to denote that a variable is a member, similar to the 'm' prefix in Hungarian notation (which notation I despise thoroughly, but that's another story). Sure you get colour-coding editors these days, but my opinion is that the prefix makes you think about the variable being a member/instance one before you type it, not just sometime afterwards when it gets colour-coded by your editor.
Generally it's to denote that a variable should not be directly touched by a developer. It's not really a requirement but it's good practice if you can't avoid having a public variable in a class that you don't want messed with.
I choose to use underscores for ivars because I often come across the following situation:
@interface MyClass:NSObject
{
NSUInteger count;
}
@property(nonatomic) NSUInteger count;
-(id) initWithCount(NSUInteger) count;
@end
(...)
@implementation MyClass
@synthesize count;
-(id) initWithCount(NSUInteger) count
{
if((self = [super init])){
self.count = count; // Parameter name collides with ivar name!
}
return self;
}
@end
So I do this:
@interface MyClass:NSObject
{
NSUInteger _count;
}
@property(nonatomic) NSUInteger count;
-(id) initWithCount(NSUInteger) count;
@end
(...)
@implementation MyClass
@synthesize count = _count;
-(id) initWithCount(NSUInteger) count
{
if((self = [super init])){
_count = count; // No name collision
}
return self;
}
@end
Of course, I could alternatively change the parameter name to "newCount" or "aCount" (I hate that one). I think it's a matter of taste.
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