NSString and NSMutableString fun

mutablestrings.m

#import <Foundation/Foundation.h>

int main(int argc, char *argv[]) {
	@autoreleasepool {
		NSMutableString *a_mutable = [[NSMutableString alloc] initWithString:@"foo"];
		NSString *a_cast_immutable = (NSString *)a_mutable;
		
		assert(a_mutable == a_cast_immutable);
		
		NSMutableString *b = [a_cast_immutable mutableCopy];
		
		NSLog(@"original mutable: %p ; %@", a_mutable, a_mutable);               // original mutable: 0x7f8eba40a220 ; foo
		NSLog(@"       immutable: %p ; %@", a_cast_immutable, a_cast_immutable); //        immutable: 0x7f8eba40a220 ; foo
		NSLog(@"    mutable copy: %p ; %@", b, b);                               //     mutable copy: 0x7f8eba40a2b0 ; foo
		
		[a_mutable appendString:@"bar"];
		NSLog(@"appended \"bar\".");
		
		assert(a_mutable == a_cast_immutable);
		
		NSLog(@"original mutable: %p ; %@", a_mutable, a_mutable);               // original mutable: 0x7f8eba40a220 ; foobar
		NSLog(@"       immutable: %p ; %@", a_cast_immutable, a_cast_immutable); //        immutable: 0x7f8eba40a220 ; foobar
		NSLog(@"    mutable copy: %p ; %@", b, b);                               //     mutable copy: 0x7f8eba40a2b0 ; foo
	}
}

notes.md

I knew casting an NSMutableString to NSString to make it "immutable" would result in the same pointer, of course; that the compiler and any users of the NSString would see it as immutable, but that underneath it would still be mutable.

First, this test shows that aliasing can be a big problem in this case; that's something we should be cognizant of.

But calling mutableCopy on an NSString that under the hood knows it is already an NSMutableString returns a new NSMutableString instance. This makes sense: this isn't to avoid aliasing issues, but just because we explicitly asked for a copy.

output

2013-07-17 16:06:59.900 mutablestrings[3138:507] original mutable: 0x7f8eba40a220 ; foo
2013-07-17 16:06:59.902 mutablestrings[3138:507]        immutable: 0x7f8eba40a220 ; foo
2013-07-17 16:06:59.902 mutablestrings[3138:507]     mutable copy: 0x7f8eba40a2b0 ; foo
2013-07-17 16:06:59.902 mutablestrings[3138:507] appended "bar".
2013-07-17 16:06:59.902 mutablestrings[3138:507] original mutable: 0x7f8eba40a220 ; foobar
2013-07-17 16:06:59.903 mutablestrings[3138:507]        immutable: 0x7f8eba40a220 ; foobar
2013-07-17 16:06:59.903 mutablestrings[3138:507]     mutable copy: 0x7f8eba40a2b0 ; foo