Question

I was trying to answer another question about the == operator and I created this code:

NSString *aString = @"Hello";
NSString *bString = aString;
NSString *cString = @"Hello";

if (aString == bString)
    NSLog(@"CHECK 1");

if (bString == cString)
    NSLog(@"CHECK 2");

if ([aString isEqual:bString])
    NSLog(@"CHECK 3");

if ([aString isEqual:cString])
    NSLog(@"CHECK 4");

NSLog(@"%i", aString);
NSLog(@"%i", bString);
NSLog(@"%i", cString);

But was surprised at the results:

Equal[6599:10b] CHECK 1
Equal[6599:10b] CHECK 2
Equal[6599:10b] CHECK 3
Equal[6599:10b] CHECK 4
Equal[6599:10b] 8240
Equal[6599:10b] 8240
Equal[6599:10b] 8240

Is there some compiler trickery going on here?

Answer 1

There is clearly string uniquing going on, at least within a single compilation unit. I recommend you take a brief tour through man gcc during which you visit all uses of “string”. You’ll find a few options that are directly relevant to literal NSStrings and their toll-free-bridged counterparts, CFStrings:

• -fconstant-string-class=class-name sets the name of the class used to instantiate @"..." literals. It defaults to NSConstantString unless you’re using the GNU runtime. (If you don’t know if you are, you aren’t.)
• -fconstant-cfstrings enables use of a builtin to create CFStrings when you write CFSTR(...).

You can disable uniquing for C string literals using -fwritable-strings, though this option is deprecated. I couldn’t come up with a combination of options that would stop the uniquing of NSString literals in an Objective-C file. (Anyone want to speak to Pascal string literals?)

You see -fconstant-cfstrings coming into play in CFString.h‘s definition of the CFSTR() macro used to create CFString literals:

    #ifdef __CONSTANT_CFSTRINGS__
    #define CFSTR(cStr)  ((CFStringRef) __builtin___CFStringMakeConstantString ("" cStr ""))
    #else
    #define CFSTR(cStr)  __CFStringMakeConstantString("" cStr "")
    #endif

If you look at the implementation of the non-builtin __CFStringMakeConstantString() in CFString.c, you’ll see that the function does indeed perform uniquing using a very large CFMutableDictionary:

    if ((result = (CFStringRef)CFDictionaryGetValue(constantStringTable, cStr))) {
        __CFSpinUnlock(&_CFSTRLock);
    }
    // . . .
    return result;

See also responses to the question, “What’s the difference between a string constant and a string literal?”