NSSet-Pitfalls.m

#import <Foundation/Foundation.h>

// Two things to know about NSSet:
//
// - NSSet assumes its elements will never mutate.  It breaks if they do.
// - Unlike NSDictionary, NSSet does not copy its unique objects.  It stores
//   the original pointers.
//
// Bottom line: DO NOT MUTATE OBJECTS THAT ARE ELEMENTS OF AN NSSET.  The
// code below demonstrates the kind of bugs you could run into if you
// disregard this guideline.  The output I got is at the bottom of the file.
//
// Ways to avoid these problems:
//
// - Keep set elements simple.  For example, use a set of database row ids
//   instead of entire database row objects.
// - Copy the values you add, e.g. `[string1 copy]` instead of `string1`.
//   This example has the side benefit that `copy` returns an immutable
//   string, so you can't accidentally mutate the set element.


// One way to break an NSSet, simply by mutating one element.
void demo1() {
	NSMutableString *string1 = [NSMutableString stringWithString:@"abc"];
	NSMutableString *string2 = [NSMutableString stringWithString:@"abcdef"];
	NSLog(@"Strings:");
	NSLog(@"%x %@", string1, string1);
	NSLog(@"%x %@", string2, string2);

	// Unlike NSDictionary, NSSet does not copy.  The pointers listed below
	// are the same as above.
	NSSet *set = [NSSet setWithObjects:string1, string2, nil];
	NSLog(@"Set:");
	for (NSString *str in set) {
		NSLog(@"%x %@", str, str);
	}
	NSLog(@"set count = %d", [set count]);
	NSLog(@"contains %@? %d", string1, [set containsObject:string1]);
	
	// Simply mutate one element to be "abcxxx".  According to the output, the
	// set does not contain "abcxxx", even though that value is listed when you
	// enumerate the set.
	[string1 appendString:@"xxx"];
	NSLog(@"MUTATED string1");
	NSLog(@"Strings:");
	NSLog(@"%x %@", string1, string1);
	NSLog(@"%x %@", string2, string2);

	NSLog(@"Set:");
	for (NSString *str in set) {
		NSLog(@"%x %@", str, str);
	}
	NSLog(@"set count = %d", [set count]);
	for (NSString *str in @[@"abc", @"abcxxx"]) {
		NSLog(@"contains %@? %d", str, [set containsObject:str]);
	}
}


// Variation of demo1() that only has one element in the set.  In this case
// the output *does* correctly report after mutation that the set contains
// "abcxxx".  This could burn you if you never tested your code with sets
// containing more than one element.
void demo2() {
	NSMutableString *string1 = [NSMutableString stringWithString:@"abc"];
	NSLog(@"Strings:");
	NSLog(@"%x %@", string1, string1);

	// Unlike NSDictionary, NSSet does not copy.  The pointers listed below
	// are the same as above.
	NSSet *set = [NSSet setWithObjects:string1, nil];
	NSLog(@"Set:");
	for (NSString *str in set) {
		NSLog(@"%x %@", str, str);
	}
	NSLog(@"set count = %d", [set count]);
	NSLog(@"contains %@? %d", string1, [set containsObject:string1]);
	
	// Mutate one element to be "abcxxx".
	[string1 appendString:@"xxx"];
	NSLog(@"MUTATED string1");
	NSLog(@"Strings:");
	NSLog(@"%x %@", string1, string1);

	NSLog(@"Set:");
	for (NSString *str in set) {
		NSLog(@"%x %@", str, str);
	}
	NSLog(@"set count = %d", [set count]);
	for (NSString *str in @[@"abc", @"abcxxx"]) {
		NSLog(@"contains %@? %d", str, [set containsObject:str]);
	}
}


// Another way to break an NSSet, by mutating one element to equal another.
void demo3() {
	NSMutableString *string1 = [NSMutableString stringWithString:@"abc"];
	NSMutableString *string2 = [NSMutableString stringWithString:@"abcdef"];
	NSLog(@"Strings:");
	NSLog(@"%x %@", string1, string1);
	NSLog(@"%x %@", string2, string2);

	// Unlike NSDictionary, NSSet does not copy.  The pointers listed below
	// are the same as above.
	NSSet *set = [NSSet setWithObjects:string1, string2, nil];
	NSLog(@"Set:");
	for (NSString *str in set) {
		NSLog(@"%x %@", str, str);
	}
	NSLog(@"set count = %d", [set count]);
	NSLog(@"contains %@? %d", string1, [set containsObject:string1]);
	
	// Mutate string1 to match string2.  According to the output, the set's
	// count is still 2, and enumerating it lists 2 elements, even though it
	// contains only one unique value.
	[string1 appendString:@"def"];
	NSLog(@"MUTATED string1 to match string2");
	NSLog(@"Strings:");
	NSLog(@"%x %@", string1, string1);
	NSLog(@"%x %@", string2, string2);

	NSLog(@"Set:");
	for (NSString *str in set) {
		NSLog(@"%x %@", str, str);
	}
	NSLog(@"set count = %d", [set count]);
	NSLog(@"contains abc? %d", [set containsObject:@"abc"]);
	NSLog(@"contains abcdef? %d", [set containsObject:@"abcdef"]);
}

int main(int argc, char *argv[]) {
	@autoreleasepool {
		NSLog(@"---- demo1 ----");
		demo1();
		NSLog(@"\n\n\n");
		NSLog(@"---- demo2 ----");
		demo2();
		NSLog(@"\n\n\n");
		NSLog(@"---- demo3 ----");
		demo3();
	}
}


// The output I got:
//
//	---- demo1 ----
//	Strings:
//	67400520 abc
//	674007c0 abcdef
//	Set:
//	67400520 abc
//	674007c0 abcdef
//	set count = 2
//	contains abc? 1
//	MUTATED string1
//	Strings:
//	67400520 abcxxx
//	674007c0 abcdef
//	Set:
//	67400520 abcxxx
//	674007c0 abcdef
//	set count = 2
//	contains abc? 0
//	contains abcxxx? 0
//
//
//
//	---- demo2 ----
//	Strings:
//	677042a0 abc
//	Set:
//	677042a0 abc
//	set count = 1
//	contains abc? 1
//	MUTATED string1
//	Strings:
//	677042a0 abcxxx
//	Set:
//	677042a0 abcxxx
//	set count = 1
//	contains abc? 0
//	contains abcxxx? 1
//
//
//
//	---- demo3 ----
//	Strings:
//	67706370 abc
//	677063b0 abcdef
//	Set:
//	67706370 abc
//	677063b0 abcdef
//	set count = 2
//	contains abc? 1
//	MUTATED string1 to match string2
//	Strings:
//	67706370 abcdef
//	677063b0 abcdef
//	Set:
//	67706370 abcdef
//	677063b0 abcdef
//	set count = 2
//	contains abc? 0
//	contains abcdef? 1