Why aren't these NSString instance variables allocated? - ios

Question

first post here. I was reading through an Objective-C tutorial earlier, and I saw that they had made a couple of NSString instance variables like this:
#implementation MyViewController {
NSString *stringOne;
NSString *stringTwo;
NSString *stringThree;
NSString *stringFour;
NSString *stringFive;
}
And then simply used them in ViewDidLoad like this:
- (void)viewDidLoad
{
[super viewDidLoad];
stringOne = #"Hello.";
stringTwo = #"Goodbye.";
stringThree = #"Can't think of anything else to say.";
stringFour = #"Help...";
stringFive = #"Pheww, done.";
}
How have they done this without instantiating the string? Why does this work? Surely you'd have to do something like stringOne = [NSString stringFromString:#"Hello."]; to properly alloc and init the object before you could simply do stringOne= #"Hello.";.
Sorry if this a dumb question, but I find these little things throw me.
Thanks,
Mike

Answer 1

From the Apple String Programming Guide:
Creating Strings
The simplest way to create a string object in source code is to use the Objective-C #"..." construct:
NSString *temp = #"Contrafibularity";
Note that, when creating a string constant in this fashion, you should use UTF-8 characters. Such an object is created at compile time and exists throughout your program’s execution. The compiler makes such object constants unique on a per-module basis, and they’re never deallocated. You can also send messages directly to a string constant as you do any other string:
BOOL same = [#"comparison" isEqualToString:myString];

Answer 2

String constants like #"Hello" are already allocated and initialized for you by the compiler.

Answer 3

Just remember this basic thing:-
NSString *string = ...
This is a pointer to an object, "not an object"!
Therefore, the statement: NSString *string = #"Hello"; assigns the address of #"Hello" object to the pointer string.
#"Hello" is interpreted as a constant string by the compiler and the compiler itself allocates the memory for it.
Similarly, the statement
NSObject *myObject = somethingElse;
assigns the address of somethingElse to pointer myObject, and that somethingElse should already be allocated and initialised.
Therefore, the statement: NSObject *myObject = [[NSObject alloc] init]; allocates and initializes a NSObject object at a particular memory location and assigns its address to myObject.
Hence, myObject contains address of an object in memory, for ex: 0x4324234.
Just see that we are not writing "Hello" but #"Hello", this # symbol before the string literal tells the compiler that this is an object and it returns the address.
I hope this would answer your question and clear your doubts. :)

Answer 4

actually this can be said "syntactic sugar". there are some other type of NS object that can be creatable without allocation or formatting.
e.g:
NSNumber *intNumber1 = #42;
NSNumber *intNumber2 = [NSNumber numberWithInt:42];
NSNumber *doubleNumber1 = #3.1415926;
NSNumber *doubleNumber2 = [NSNumber numberWithDouble:3.1415926];
NSNumber *charNumber1 = #'A';
NSNumber *charNumber2 = [NSNumber numberWithChar:'A'];
NSNumber *boolNumber1 = #YES;
NSNumber *boolNumber2 = [NSNumber numberWithBool:YES];
NSNumber *unsignedIntNumber1 = #256u;
NSNumber *unsignedIntNumber2 = [NSNumber numberWithUnsignedInt:256u];
NSNumber *floatNumber1 = #2.718f;
NSNumber *floatNumber2 = [NSNumber numberWithFloat:2.718f];
// an array with string and number literals
NSArray *array1 = #[#"foo", #42, #"bar", #3.14];
// and the old way
NSArray *array2 = [NSArray arrayWithObjects:#"foo",
[NSNumber numberWithInt:42],
#"bar",
[NSNumber numberWithDouble:3.14],
nil];
// a dictionary literal
NSDictionary *dictionary1 = #{ #1: #"red", #2: #"green", #3: #"blue" };
// old style
NSDictionary *dictionary2 = [NSDictionary dictionaryWithObjectsAndKeys:#"red", #1,
#"green", #2,
#"blue", #3,
nil];
for more information, see "Something wonderful: new Objective-C literal syntax".