Nature of NSString

Question

Such a basic question, but i don't know the answer.What exactly the difference between:

NSString *str = @"Hello";

and

NSString *str = [[NSString alloc] initWithString:@"Hello"];

When should i use each one?

score 4 · Answer 1 · answered Apr 13 '13 at 14:14

A @"" expressions is replaced at compile time¹ with an instance of NSConstantString, which is a specialized subclass of NSString with a fixed memory layout². This also explains why NSString is the only object that can be initialized at compile time³.

A [[NSString alloc]initWithString:@""] produces a NSString instance, initializes that instance with a literal expression, and releases the instance. Therefore, the object allocation is superfluous and immediately discarded. Which is why you should always use just the literal when creating immutable strings.

_{¹ The LLVM code that rewrites the expression is RewriteModernObjC::RewriteObjCStringLiteral in RewriteModernObjC.cpp.}
_{² To see the NSConstantString definition, cmd+click it in Xcode.}
_{³ Creating compile time constants for other classes would be easy but it would require the compiler to use a specialized subclass. This would break compatibility with older Objective-C versions.}

Btw, if you release the object created with `initWithString:` the release is ignored because your object pointer is already pointed to the `NSConstantString` instance, (at least in the current implementation of Clang), but the static analyzer of Xcode will still nag you to do it because implementing `initWithString:` as a literal is a feature of the compiler, not of the language. — Jano, Apr 13 '13 at 15:20

score 2 · Accepted Answer · edited May 23 '17 at 11:43

2

When should i use each one?

NSString *str = [[NSString alloc] initWithString:@"Hello"]; //1st one

The above is redundant, and has same meaning as

NSString *str = @"Hello"; //2nd one

So always use shorter one. i.e 2nd one in my example.

EDIT:

Also see this What's the difference between NSString *s = @"string" and NSString *s = [[NSString alloc] initWithString:@"string"]?

edited May 23 '17 at 11:43

Community

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answered Apr 13 '13 at 13:31

Anoop Vaidya

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Yes, and the compiler emits a warning for the first one: `using 'initWithString:' with a literal is redundant`. – Martin R Apr 13 '13 at 13:35
@MartinR: Your view on the edit link and comparing it with ARC. – Anoop Vaidya Apr 13 '13 at 13:37

Nature of NSString

2 Answers2