Objective-C语法混乱

Question

Could anyone tell me the difference between

NSString* string;

And

NSString* string = [NSString string];

Answer 1

Just like in C,

NSString *string;

declares the variable (pointer) string but gives it no value. This means you can't use the variable until you've initialized it, for example by doing string = @"foo"; .

Note:

• If this is an instance variable it actually will be initialized automatically — to nil .
• If you use the variable somewhere in your code without initializing it in every possible branch ( if conditions, etc.) then the compiler will complain. To avoid this you could just set it to an empty string or nil to start with.

---

The line

NSString *string = [NSString string];

uses the +string method of the NSString class to create an empty string. You could also use @"" .

(Further note: I recommend using NSString *string instead of NSString* string because the former hides a syntax idiosyncrasy: NSString *string1, *string2; is the correct way to declare multiple pointer variables on one line.)

Answer 2

NSString *string;

The first your just declaring that there will be a variable that is an NSString , named string , but not initializing it.

You might do this when you want a string to contain one of a range of values, determined by some condition:

NSString *string;

if (condition) {
    string = @"This condition";
} else {
    string = @"That condition";
}

NSString *string = [NSString string];

The second you are doing all of the above, with the additional step of initializing your string variable to an empty string.

You might do this when you want a string to contain a specific single value:

NSString *string = @"There are no conditions"

Answer 3

NSString *string;

Creates a pointer but not a string object (instance). It may be initialized to nil or not depending on where it is an compiler options. In other words it creates a place (memory storage) for a pointer to an NSString bot no NSString object.

NSString *string = [NSString string];

Creates an empty string object (instance) and a pointer to it.