NSSet isEqualToSet behaves apparently non-deterministically

Originator:ibrahimshaath
Number:rdar://35604025 Date Originated:November 16 2017
Status:Closed Resolved:
Product:iOS / Foundation Product Version:
Classification:Bug Reproducible:Always
 
Summary: NSSets containing NSObjects which (in terms of their implementation of isEqual) are identical do not always evaluate isEqualToSet: as true.

Steps to Reproduce:
Run the attached trivial project on an iOS simulator in Xcode 9.1; observe the console output.

Expected Results:
From the sets as built, one would expect equality to be determined as true every time through the loop.

Actual Results:
The determination of equality is inconsistent. It varies per run and fails an enormous majority of the time.

Comments

https://developer.apple.com/documentation/objectivec/nsobjectprotocol/1418795-isequal The key paragraph: If two objects are equal, they must have the same hash value. This last point is particularly important if you define isEqual(_:) in a subclass and intend to put instances of that subclass into a collection. Make sure you also define hash in your subclass.

By ibrahimshaath at Nov. 17, 2017, 1:26 a.m. (reply...)

// Sorry, first time using this tool, and I didn't realise I can't attach a code sample. To reproduce, replace the ViewController.m in a brand new project with the following:

import "ViewController.h"

pragma mark - UltraObject

@interface UltraObject : NSObject @property (nonatomic, readonly) BOOL truthiness; - (instancetype)initWithTruthiness:(BOOL)truthiness; @end

@interface UltraObject () @property (nonatomic, assign) BOOL truthiness; @end

@implementation UltraObject

  • (instancetype)initWithTruthiness:(BOOL)truthiness { if (self = [super init]) { _truthiness = truthiness; } return self; }

  • (BOOL)isEqual:(id)object { if (object == self) { return YES; } if (object == nil) { return NO; } if ([object isKindOfClass:[self class]] == NO) { return NO; } BOOL thisTruthiness = self.truthiness; BOOL thatTruthiness = ((UltraObject *)object).truthiness; return thisTruthiness == thatTruthiness; }

@end

pragma mark - ViewController

@implementation ViewController

  • (void)viewDidLoad { [super viewDidLoad];

    NSUInteger equal = 0; NSUInteger unequal = 0;

    for (NSUInteger i = 0; i <= 1000000; i++) { 

    UltraObject *truthyA = [[UltraObject alloc] initWithTruthiness:YES];
UltraObject *falsyA = [[UltraObject alloc] initWithTruthiness:NO];
NSSet<UltraObject *> *setA = [NSSet setWithObjects:truthyA, falsyA, nil];

UltraObject *truthyB = [[UltraObject alloc] initWithTruthiness:YES];
UltraObject *falsyB = [[UltraObject alloc] initWithTruthiness:NO];
NSSet<UltraObject *> *setB = [NSSet setWithObjects:truthyB, falsyB, nil];

if ([setA isEqualToSet:setB]) {
    equal++;
} else {
    unequal++;
}

    }

    NSLog(@"Equal: %@", @(equal)); NSLog(@"Unequal: %@", @(unequal)); }

@end

By ibrahimshaath at Nov. 17, 2017, 1:10 a.m. (reply...)

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