page 261 +(id) sharedAddressBook

wilfordbrimley · April 4th, 2006, 01:38 AM

relevant code:

Code:

+(id)sharedAddressBook {
    static AddressBook *sharedAddressBook = nil;
    if ( ! sharedAddressBook ) {
        // Load from file if the file exists
        NSFileManager *fm = [NSFileManager defaultManager];
        if ( [fm fileExistsAtPath:AddressBookFilePath] ) {
            sharedAddressBook = [[AddressBook alloc] initWithFile:AddressBookFilePath];
        }
        else { // Create a new AddressBook
            sharedAddressBook = [[AddressBook alloc] init];
        }
    }
    return sharedAddressBook;
}

it confuses me that every call to sharedAddressBook starts with:
static AddressBook *sharedAddressBook = nil;
if ( ! sharedAddressBook ) {
...
}
this seems like it will always be evaluated, and the address book will be reinitialized every time...

please let me know what I'm missing here.

mtrent · June 3rd, 2006, 01:02 PM

What you are missing here is the role of the "static" keyword. The explanation on Page 264 should clear things up for you:

"A static variable called sharedAddressBook is declared, and initialized to nil. Being declared static means that it will not disappear when the method returns, but will remain for the life of the program. The if statement checks to see if the variable is nil; if not, it simply returns teh AddressBook to the calling code. If the variable is nil, a new AddressBook must be created."

So, the if statement *will* always be evaluated -- on that point you are correct. It's just that the value of sharedAddressBook will be nil only on the first call to the function. Then sharedAddressBook will be assigned a non-nil value, and that value will be remembered for the life of the program.

Hope that helps!