What does -> mean?

I'm a relative newbie to Objective-C (only studied Arron Hillegras's book) and am confused by the following snippit of code I've found in one of Apple's code examples, in particular what does the -> mean?

-(id) initWi开发者_开发技巧thNumbers:(NSArray *)numbers
{
    self = [super init];
    if (self != nil)
    { 
       self->_numbers = [numbers copy];
    }
    return self;
}

In the header file _numbers is declared as

NSNumber * _number;

( the underscore has some significance from what I recall reading somewhere but that too eludes me at the moment.

Thanks Robin

-> is a normal C operator for accessing the members of a pointer to a struct; the . operator is for accessing members of a struct. Thus:

a->b

is translated to

(*a).b

Since Objective-C objects are pointers to structs underneath it all, this works for accessing instance variables.

You use -> when you want to access an ivar. Like C structures you will use a . or -> (in pointers to structs) in Objective-C objects you can use -> but is not necessary since you can access them directly.

Hence:

self->_numbers = [numbers copy];

and

_numbers = [numbers copy];

are the same

You want to use -> when you want to access that ivar explicitly.

Be aware that in Objective-C you can use . but only when its a property. You can use -> regardless that.

It's usually called the 'arrow' operator. It allows you to access the instance variables of an object (or of a struct) using a reference or pointer to the instance. It's common syntax with C and C++.

I'm struggling to find a nice write up, but you might find this one informative.

As to the underscore - commonly they mean "private"; according to the Coding Guidelines for Cocoa - Naming Basics:

Avoid the use of the underscore character as a prefix meaning private, especially in methods. Apple reserves the use of this convention. Use by third parties could result in name-space collisions; they might unwittingly override an existing private method with one of their own, with disastrous consequences.

That indicates a reference to -in this case- an instance variable of an object. Self refers to the object itself, and by writing self->_numbers, you refer to the variable that is part of the class instead of a global variable named _numbers.

Are you sure this is not mentioned in your book?

It's an operator which roots from C Programming Language. And since Objective-C is compatible with C programming, you may see developers using traditional C-style programming.

"->" is used to access a elements of a pointer. And since the "objects" in Objective-C are just pointers (denoted by *), e.g. NSNumber *number; , you can use this notation to access their elements.

I never used this notation in Objective-C since the dot notation in Objective-C is accepted. If "self" has a synthesized property "number" then self.number should be the same as self->number (this is true ONLY in Objective-C.