Question

Can any one tell the difference between:

NSString* myStr = @"Some value";

and

NSString* myStr = [[NSString alloc] initWithString:@""];

why i should alloc a string even i can give some value directly..?

Answer 1

From Apple’s 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 = @"/tmp/scratch";

Note that, when creating a string constant in this fashion, you should avoid using anything but 7-bit ASCII 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, though you can retain and release them as you do any other object.

The “compiler makes such object constants unique […]” suggests to me that string literals are interned for each module. As far as I know, interning and object lifetime & memory management are the main differences between the two approaches.

As for why one might use the latter, if for some reason you wanted myStr to not be a unique, interned string, then you might try the -initWithString: approach, though the interned string would still exist, using -copy would be simpler (as NR4TR points out) and (as Yuji notes in the comments) the resulting string will likely be the exact same one as the string literal, despite what Apple’s documentation suggests.