Question

I am learning how to create iPhone apps and I have seen that most of the variables we create store memory addresses (pointers) instead than holding the actual value or object. I also have found out that every time you declare a variable with the pointer char (*) you know that the variable is going to hold the address and whenever you don’t use the (*) mark to declare a variable you know that it will hold the value instead than the memory location. But I don’t know when to us which. for example I have:

CGFloat someVar = [image1 alpha];  // This variable does not require *
                                   // image 1 is a: IBOutlet UIImageView

and in this other case I have to use a pointer:

UIViewController *someOtherVar = [[UIViewController alloc] init];  // this type of var requires *

It will be nice if I can know when can I use each instead of trying each until project compiles.

Answer 1

The function and method signatures in the headers and documentation will indicate what the type is.

For example, here is how the alpha property is declared for UIView:

@property(nonatomic) CGFloat alpha;

There is no * anywhere, so you know it returns CGFloat and not CGFloat*.

In contrast, the backgroundColor property is declared like this:

@property(nonatomic, copy) UIColor *backgroundColor;

so you know it will return UIColor* (a pointer).

Some things are declared with a type of id, which is always going to be a pointer to an object.

In general, Objective-C objects (types declared with @interface) will always be referenced as pointers, while primitive C types and structs will often (but not always) be passed and returned by value.