在比较期间,每次都不会调用覆盖isEqual方法

Question

我有一个叫做的课

Contact;

在Contact我有(simple version to test, no hash yet)

- (BOOL)isEqual:(id)other {
    if (other == self)
        return YES;
    if (!other || ![other isKindOfClass:[self class]])
        return NO;
    return NO;
}

我做:

NSMutableArray *arr = [[NSMutableArray alloc] init];
Contact *contact = [Contact new];
[arr addObject:contact]

// I expect my isEqual to be called here but it does not get called
[arr containsObject:contact] // this evaluates to true somehow!!!

但是,如果我添加另一个要键入的对象NSString,则会调用它来比较String对象而不是联系对象.意思是

[arr addObject:@""] // so now arr has two elements

// here I expect two calls to isEqual but only one gets there
// when comparing string object against Contact
[arr containsObject:contact]

为什么isEqual在我上面提到的情况下没有被调用？

Answer 1

请阅读参考库中isEqual: NSObject协议中的讨论.

您会发现,对于集合内部的对象(例如NSArray),hash可能会用于确定两个对象是否实际相同.如果两个指针实际指向同一个对象,则无需检查是否相等 - 因此isEqual:永远不会被调用.

参考库建议的解决方案也是hash在子类中实现的.