What is the difference between methods that are declared with - and methods that are declared with +
e.g
- (void)methodname
+ (void)methodname
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Methods prefixed with - are instance methods. This means they can only be invoked on an instance of a class, eg:
[myStringInstance length];
Methods prefixed with + are class methods. This means they can be called on Classes, without needing an instance, eg:
[NSString stringWithString:@"Hello World"];
Jasarien
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1Just curious: why this choice of notation? +/- feels a bit hackish to me (probably because it reminds me of how perl uses '@', '#', '$' to mean *something*) – Anthony Kong Feb 12 '10 at 13:44
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2I can't comment on the choice of notation, because I simply don't know... Sorry. Maybe someone else will know? – Jasarien Feb 12 '10 at 13:55
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2UML uses +/- sure. Also UML uses +/- to annotate methods. However other than that UML's use and Obj-C use are completely different. – deft_code Sep 03 '10 at 21:16
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@FrançoisBeausoleil I think uml uses +/- signs to for public/private attribute or functions. (http://www.tutorialspoint.com/images/notation_class.jpg) – Mehdi Karamosly Oct 22 '13 at 19:33
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+(void)methodname is a class variable and -(void)methodname is object variable.
Lets say you make a utility class that has a method to reverse string. The class you call MYUtility.
If you use +, like
+ (NSString *)reverse:(NSString *)stringToReverse
You could use it directly like
NSString *reversed = [MYUtility stringToReverse:@"I Love objective C"];
if you used a -, like
- (NSString *)reverse:(NSString *)stringToReverse
You have to use :
MYUtility *myUtil = [[MYUtility alloc] init];
NSString *reversed = [myUtil stringToReverse:@"There are many ways to do the same thing"];
With the class based function, you just call directly, but you don't have access to any local variables besides #defines that you can do because the class isn't instantiated.
But with the - (NSString you must instantiate the class before use, and you have access to all local variables.
This isn't a pick one thing and stay with it, many classes have both, just look at the header file for NSString, it is littered with + functions and - functions.
Abhinay Reddy Keesara
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minus are instance methods (only accessible via an instantiated object)
plus are class methods (like in Java Math.abs(), you can use it without an instantited object)
Tim
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According to this page:
Instance methods begin with - and class level methods begin with +
See this SO question for more information.
Community
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Kaleb Brasee
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The first is an instance method and the second is a class method. You should read Apple's Objective-C documentation to learn about the difference.
Stefan Arentz
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