Obj-C experiment: String equality

main.m

#import <Foundation/Foundation.h>

#define SAMPLE @"one sample string value"

NSString *addressOf(id object) {
    return [NSString stringWithFormat:@"%p", object];
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        {
            // Using the same string literal produces the same instance
            NSString *one = SAMPLE;
            NSString *two = SAMPLE;
            
            assert([one isEqualToString:two]);
            assert([one isEqual:two]);
            assert(one == two);
            assert([addressOf(one) isEqualToString:addressOf(two)]);
        }
        
        {
            // Using `copy` produces the same instance
            NSString *one = SAMPLE;
            NSString *two = one.copy;
            
            assert([one isEqualToString:two]);
            assert([one isEqual:two]);
            assert(one == two);
            assert([addressOf(one) isEqualToString:addressOf(two)]);
        }
        
        {
            // Grant's example
            NSString *str = @"TestStr";
            // Compiler warning: Direct comparison of a string literal has undefined behavior
            assert(str == @"TestStr"); // (but it still works!)
        }
        
        {
            // Force creating different instances for the same contents
            NSString *one = SAMPLE;
            NSString *two = [@"one sample " stringByAppendingString:@"string value"];
            
            assert([one isEqualToString:two]);
            assert([one isEqual:two]);
            // !
            assert(one != two);
            // !
            assert(![addressOf(one) isEqualToString:addressOf(two)]);
            
            // Now we can test the array behavior
            NSArray *arr = @[one, two];
            assert([arr containsObject:one]);
            assert([arr containsObject:two]);
            assert([arr containsObject:SAMPLE]);
        }
        
        NSLog(@"Survived with no failures!");
    }
    return 0;
}