请注意，如果创建的对象在创建后可以更改，则如果该对象插入到集合中，则哈希值不能更改。实际上，这意味着哈希值必须从初始对象创建时开始固定。更多信息请参阅Apple的NSObject协议的-hash方法文档:

If a mutable object is added to a collection that uses hash values to determine the object’s position in the collection, the value returned by the hash method of the object must not change while the object is in the collection. Therefore, either the hash method must not rely on any of the object’s internal state information or you must make sure the object’s internal state information does not change while the object is in the collection. Thus, for example, a mutable dictionary can be put in a hash table but you must not change it while it is in there. (Note that it can be difficult to know whether or not a given object is in a collection.)

对我来说，这听起来完全是无稽之谈，因为它可能会有效地降低哈希查找的效率，但我认为最好还是谨慎行事，并遵循文档所说的。

开始

 NSUInteger prime = 31;
 NSUInteger result = 1;

然后对于每一个原始元素

 result = prime * result + var

对于对象，你用0表示nil，否则它们的hashcode。

 result = prime * result + [var hash];

对于布尔值，使用两个不同的值

 result = prime * result + ((var)?1231:1237);

解释与归因

这不是tcurdt的作品，评论要求更多的解释，所以我相信编辑归因是公平的。

This algorithm was popularized in the book "Effective Java", and the relevant chapter can currently be found online here. That book popularized the algorithm, which is now a default in a number of Java applications (including Eclipse). It derived, however, from an even older implementation which is variously attributed to Dan Bernstein or Chris Torek. That older algorithm originally floated around on Usenet, and certain attribution is difficult. For example, there is some interesting commentary in this Apache code (search for their names) that references the original source.

最重要的是，这是一个非常古老，简单的哈希算法。它不是性能最好的，甚至在数学上也没有被证明是一个“好”算法。但它很简单，而且很多人长期使用它，效果很好，所以它有很大的历史支持。

记住，你只需要在isEqual为真时提供相等的哈希值。当isEqual为false时，散列不一定是不相等的，尽管假设它是不相等的。因此:

保持哈希简单。选择一个(或几个)成员变量是最有特色的。

例如，对于CLPlacemark，只有名称就足够了。是的，有2或3个不同的CLPlacemark具有完全相同的名称，但这是罕见的。使用这个散列。

@interface CLPlacemark (equal)
- (BOOL)isEqual:(CLPlacemark*)other;
@end

@implementation CLPlacemark (equal)

...

-(NSUInteger) hash
{
    return self.name.hash;
}


@end

注意，我没有指定城市、国家等。名字就足够了。也许是名称和CLLocation。

散列应该是均匀分布的。所以你可以使用^ (xor号)来组合几个成员变量

这就像

hash = self.member1.hash ^ self.member2.hash ^ self.member3.hash

这样哈希将被均匀分布。

Hash must be O(1), and not O(n)

那么在数组中要做什么呢?

再次,简单。你不必hash数组的所有成员。足以散列第一个元素，最后一个元素，计数，也许还有一些中间元素，就这样。

Sorry if I risk sounding a complete boffin here but... ...nobody bothered mentioning that to follow 'best practices' you should definitely not specify an equals method that would NOT take into account all data owned by your target object, e.g whatever data is aggregated to your object, versus an associate of it, should be taken into account when implementing equals. If you don't want to take, say 'age' into account in a comparison, then you should write a comparator and use that to perform your comparisons instead of isEqual:.

如果您定义了一个isEqual:方法来任意执行相等比较，那么一旦您忘记了equals解释中的“扭曲”，您就会冒这个方法被其他开发人员甚至您自己误用的风险。

因此，虽然这是一个关于哈希的很好的问答，你通常不需要重新定义哈希方法，你可能应该定义一个特别的比较器。

这并没有直接回答你的问题，但我之前已经使用MurmurHash来生成哈希:MurmurHash

我想我应该解释一下原因:低语是非常快的……

结合@tcurdt的答案和@oscar-gomez的答案来获取属性名，我们可以为isEqual和hash创建一个简单的解决方案:

NSArray *PropertyNamesFromObject(id object)
{
    unsigned int propertyCount = 0;
    objc_property_t * properties = class_copyPropertyList([object class], &propertyCount);
    NSMutableArray *propertyNames = [NSMutableArray arrayWithCapacity:propertyCount];

    for (unsigned int i = 0; i < propertyCount; ++i) {
        objc_property_t property = properties[i];
        const char * name = property_getName(property);
        NSString *propertyName = [NSString stringWithUTF8String:name];
        [propertyNames addObject:propertyName];
    }
    free(properties);
    return propertyNames;
}

BOOL IsEqualObjects(id object1, id object2)
{
    if (object1 == object2)
        return YES;
    if (!object1 || ![object2 isKindOfClass:[object1 class]])
        return NO;

    NSArray *propertyNames = PropertyNamesFromObject(object1);
    for (NSString *propertyName in propertyNames) {
        if (([object1 valueForKey:propertyName] != [object2 valueForKey:propertyName])
            && (![[object1 valueForKey:propertyName] isEqual:[object2 valueForKey:propertyName]])) return NO;
    }

    return YES;
}

NSUInteger MagicHash(id object)
{
    NSUInteger prime = 31;
    NSUInteger result = 1;

    NSArray *propertyNames = PropertyNamesFromObject(object);

    for (NSString *propertyName in propertyNames) {
        id value = [object valueForKey:propertyName];
        result = prime * result + [value hash];
    }

    return result;
}

现在，在你的自定义类中，你可以很容易地实现isEqual:和hash:

- (NSUInteger)hash
{
    return MagicHash(self);
}

- (BOOL)isEqual:(id)other
{
    return IsEqualObjects(self, other);
}