Difference between "+" and "-" before function name in Objective-C

What is the diffe开发者_如何学Gorence between "+" and "-" before the function name interface declaration in an Objective-C program. Example:

- (void)continueSpeaking;

+ (NSArray *)availableVoices;

What's the difference?

+ defines a class method

Class methods belong to the class itself, not instances of the class.

Example: [AppDelegate someMethod]

- defines an instance method

Example [[[UIApplication sharedApplication] delegate] someMethod]

One way to describe the difference is that - methods operate on objects, while + methods operate on the class itself.

Say your class was named MyClass, and you created an instance of it and stored it into a variable called myInstance:

- (void)continueSpeaking can be called like so: [myInstance continueSpeaking].

However, the method + (NSArray *)availableVoices can only be called like so: [MyClass availableVoices]