在ObjectC中初始化空字符串？

Question

Someone use the following to initialize the NSstring

NSString *astring = [[NSString alloc] init];

I am wondering why not just use

NSString *atring = nil or NSString *astring = @""

Answer 1

There is no semantic difference between NSString *astring = [[NSString alloc] init]; and NSString *astring = @""; - but NSString *astring = nil; is completely different. The first two produce a reference to an immutable string value, the last indicates the absence of a value.

Whether the various ways of generating an zero-length string produce different objects is entirely an implementation detail. The code:

NSString *a = [[NSString alloc] init];
NSString *b = [NSString new];
NSString *c = @"";
NSString *d = [NSString stringWithString:@""];
NSLog(@"%p, %p, %p, %p, %p", a, b, c, d, @""); // %p = print the value of the reference itself

outputs (the exact values will vary):

0x7fff7100c190, 0x7fff7100c190, 0x1000028d0, 0x1000028d0, 0x1000028d0

showing only 2 zero-length string objects were created - one for @"" and one for alloc / init . As the strings are immutable such sharing is safe, but in general you should not rely on it and try to compare strings using reference comparison ( == ).

Answer 2

NSString *atring = nil

is different -- it's a nil pointer, not an empty string.

NSString *astring = @""

is almost the same, if you change it to something like

NSString* astring=[@"" retain];

It's one of the things that "don't matter"; he or she simply used one way. Probably for no particular reason at all.

Answer 3

NSString *atring = nil; is simply setting the pointer to nil and does nothing other than ensure that pointer is set to nil;

NSString *astring = @""; is a shorthand literal and is the equivalent of [NSString stringWithString:@""];

On another point I don't know why you would want to initialize a string to nothing if its not mutable since you won't be able to change it later without overriding it.