Objective-C:NULL，nil和@“"之间有什么区别? [英] Objective-C: What's the difference between NULL, nil and @""?

问题描述

正如标题所说，NULL，nil和@""有什么区别?

As the title says, what's the difference between NULL, nil and @"" ?

例如，如果我要检查字典中的字符串是否为空.

For example, if I want to check a string in a dictionary is empty.

我应该使用哪种情况?

if ([dictionary objectForKey:@"aString"] == nil)

或

if [[dictionary objectForKey:@"aString"] isEqualToString:@""]

或

if ([dictionary objectForKey:@"aString"] == NULL)

哪个是对的?

推荐答案

nil是object-c对象的NULL模拟.实际上，它们是相同的:

nil is NULL's analog for objective-c objects. Actually they're the same:

 //MacTypes.h
 #define nil NULL

所以

if ([dictionary valueForKey:@"aString"]==nil)
if ([dictionary valueForKey:@"aString"]==NULL)

都检查字典中是否存在特定的键，尽管第一行在检查Objective-C类型时更正确.

both check if the specific key is present in a dictionary, although 1st line is more correct as it checks objective-c types.

关于:

if ([[dictionary valueForKey:@"aString"] isEqualToString:@""])

此行检查字典中是否存在带有"aString"键的条目，并将该条目与空字符串进行比较.结果将是以下之一:

This line checks if there's an entry in dictionary with "aString" key and compares that entry with empty string. The result will be one of the following:

如果您的密钥中没有这样的条目，则
• 为假
• 如果键的条目为空且该条目为空字符串，则为true
如果您的密钥对象存在并且不响应-isEqualToString:消息，
• 可能会崩溃

• is false if there's no such entry for your key
• is true if there's entry for your key and that entry is empty string
• may crash if object for your key exists and does not respond to -isEqualToString: message

因此，根据您的需要，您必须使用第一行，或者如果您需要同时检查条目是否存在以及它是否为空字符串，那么您需要两个条件:

So depending on your needs you must use 1st line, or if you need to combine both checking if entry exists and it is not an empty string then you need 2 conditions:

if ([dictionary valueForKey:@"aString"]==nil ||
               [[dictionary valueForKey:@"aString"] isEqualToString:@""])

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