NULL和nil是否等效? [英] Are NULL and nil equivalent?

问题描述

实际上我的问题是:null和nil是否相等?

Actually my question here is: are null and nil equivalent or not?

我有一个例子，但是当它们相等时我会感到困惑.

I have an example but I am confused when they are equal when they are not.

NSNull *nullValue = [NSNull null];
NSArray *arrayWithNull = [NSArray arrayWithObject:nullValue];
NSLog(@"arrayWithNull: %@", arrayWithNull);
id aValue = [arrayWithNull objectAtIndex:0];

if (aValue == nil) {
    NSLog(@"equals nil");
} else if (aValue == [NSNull null]) {
    NSLog(@"equals NSNull instance");
    if ([aValue isEqual:nil]) {
        NSLog(@"isEqual:nil");
    }
}

在上述情况下，它表明null和nil都不相等，并且显示等于NSNull实例"

Here in the above case it shows that both null and nil are not equal and it displays "equals NSNull instance"

NSString *str=NULL;
id str1=nil;
if(str1 == str)
{
   printf("\n IS EQUAL........");

}
else
{
    printf("\n NOT EQUAL........");
}

在第二种情况下，它显示两者相等，并且显示"IS EQUAL".

And in the second case it shows both are equal and it displays "IS EQUAL".

任何人的帮助将不胜感激.

Anyone's help will be much appreciated.

谢谢你， 结束.

推荐答案

nil和NULL本质上是相同的，nil类似于(NSObject *)0，而NULL更像是(void *)0.但是两个都是整数值为零的指针.您可以将邮件发送到nil而不会引发错误.

nil and NULL are essentially the same, nil is something like (NSObject *)0, while NULL is more like (void *)0. But both are pointers with an integer value of zero. You can send messages to nil without raising an error.

NSNull和NULL(当然是nil)是不同的.您只能将NSNull用作帮助程序，以将empty对象添加到NSArray或另一个容器类中，因为您不能将nil添加到它们中.因此，取而代之的是，使用[NSNull null]作为替换，并且必须检查数组元素是否为NSNull，而不是它是否为nil(它永远不会等于nil).

NSNull and NULL (or nil, of course) are different things, however. You just use NSNull as a helper to add an empty object to an NSArray or another container class, since you can't add nil to them. So instead, you use [NSNull null] as a replacement, and you have to check if an array element is NSNull, not if it's nil (it will never be equal to nil).

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