Question

I’m teaching myself Objective-C from a book (Cocoa programming for mac OS X) and am about halfway through however I have two questions that aren’t answered or defined in the book.

1. When defining class methods what is the difference between (assuming there in a .h file):

   – (int) population;

   + (int) population;

The way I see it at the moment is that - methods require the class to be allocated and initialized first however + can be called statically without requiring allocation and initialization. E.g. (in a function in another class)

// Using - Earth *world = [[Earth alloc] init]; int population = [world population];  // Using + int population = [Earth population]; 

If that is correct, when should I use static methods and are they’re any disadvantages with doing so.

2. When defining a var in either a function paramater or as an actual var in a function, does the use of * mean the var will be an object? e.g. (again in a header file.)

   – (void) setPopulation: (NSNumber *) population; //Use of * as population is of NSNumber

   – (void) setPopulation: (int) population; // population isn’t a class so doesn’t need *

Sorry if any of my terms don’t make sense in the land of Objective-C such as static methods, etc. I’m a PHP and Ruby Programmer.

Answer 1

The -/+ in method declarations for Objective-C simply denote whether the method is a class method or an instance method. For example, with Objective-C, you cannot send an instance a message that was declared as a class method. For example:

@interface MyObject : NSObject -(void)myInstanceMethod; +(void)myClassMethod; @end  // ...  MyObject* obj = [[MyObject alloc] init];  [obj myInstanceMethod];      // this is okay [obj myClassMethod];         // this will fail [[obj class] myClassMethod]; // this is okay [MyObject myClassMethod];    // this is okay [MyObject myInstanceMethod]; // this will fail 

As to the second part of your question, Objective-C is a strict super-set of C. It adds classes but they are really C data structures whose implementations are hidden from you by the Objective-C runtime. Because of this, classes are always represented as pointers. In C, the * means that the variable is being declared as a pointer to some memory address. You can use pointers with primitive types in C as well, but Objective-C objects must always be referred to by pointers.

There are many great tutorials/introductions to pointers out there. I would suggest simply googling for C tutorial and pointers to learn more.