在Objective-C中检查空字符串的正确方法是什么？ [英] What is the right way to check for a null string in Objective-C?

问题描述

我在我的iPhone应用程式中使用这个程式。

  if（title == nil）{
 // do 
} 
  

但它抛出一些异常，控制台显示标题是 （null）。

所以我现在使用这个：

  if（title == nil || [title isKindOfClass：[NSNull class]]）{
 // do something 
} 
  

有什么区别，什么是确定字符串是否为空的最好方法？

解决方案

正如其他人已经指出的，在Cocoa / Objective C下有许多种类的null。但是还要注意的是[title isKindOfClass：[NSNull class]]是无意义的，因为[NSNull null]被记录为一个单例，所以你可以只检查指针相等。请参见可可主题：使用空。 p>

所以一个很好的测试可能是：

  if id）[NSNull null] || title.length == 0）title = @Something;注意如何使用这样一个事实，即使title是nil，title.length将返回0/0。 nil / false，即在这种情况下为0，所以你不必有特殊情况。这是新的Objective C的人有麻烦习惯，特别是来自其他语言的消息/方法调用nil崩溃。
 
I was using this in my iPhone app
if (title == nil) {
    // do something
}
but it throws some exception, and the console shows that the title is "(null)".

So I'm using this now:
if (title == nil || [title isKindOfClass:[NSNull class]]) {
    //do something
}
What is the difference, and what is the best way to determine whether a string is null?
解决方案
As others have pointed out, there are many kinds of "null" under Cocoa/Objective C.  But one further thing to note is that [title isKindOfClass:[NSNull class]] is pointlessly complex since [NSNull null] is documented to be a singleton so you can just check for pointer equality. See Topics for Cocoa: Using Null.

So a good test might be:
if (title == (id)[NSNull null] || title.length == 0 ) title = @"Something";
Note how you can use the fact that even if title is nil, title.length will return 0/nil/false, ie 0 in this case, so you do not have to special case it.  This is something that people who are new to Objective C have trouble getting used to, especially coming form other languages where messages/method calls to nil crash.

                        
这篇关于在Objective-C中检查空字符串的正确方法是什么？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！