如何检测 NSString 是否为空? [英] How to detect if NSString is null?

问题描述

我有一段代码可以检测NSString 是否为NULL、nil 等.但是，它崩溃了.这是我的代码:

I have a piece of code that detects if a NSString is NULL, nil, etc. However, it crashes. Here is my code:

NSArray *resultstwo = [database executeQuery:@"SELECT * FROM processes WHERE ready='yes' LIMIT 0,1"];
for (NSDictionary *rowtwo in resultstwo) {

NSString *getCaption = [rowtwo valueForKey:@"caption"];

if (getCaption == NULL) {
theCaption = @"Photo uploaded...";
} else if (getCaption == nil) {
theCaption = @"Photo uploaded...";
} else if ([getCaption isEqualToString:@""]) {
theCaption = @"Photo uploaded...";
} else if ([getCaption isEqualToString:@" "]) {
theCaption = @"Photo uploaded...";
}

}

这是错误:

由于未捕获的异常NSInvalidArgumentException"而终止应用，原因:-[NSNull isEqualToString:]:无法识别的选择器发送到实例 0x3eba63d4'

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull isEqualToString:]: unrecognized selector sent to instance 0x3eba63d4'

我做错了吗?我需要以不同的方式来做吗?

Am I doing something wrong? Do I need to do it a different way?

推荐答案

Objective-C 对象的 NULL 值(类型 id)是 nil.

而 NULL 用于 C 指针(类型 void *).

While NULL is used for C pointers (type void *).

(最终两者都持有相同的值(0x0).但是它们的类型不同.)

(In the end both end up holding the same value (0x0). They differ in type however.)

在 Objective-C 中:

• nil (全部小写) 为空指向 Objective-C 对象的指针.
• Nil (大写) 是一个空指针到 Objective-C 类.
• NULL (all caps) 是一个指向其他任何(C 指针，即).
• [NSNull null] 是单例，用于不可能使用 nil 的情况(向/从 NSArrays eg)

• nil (all lower-case) is a null pointer to an Objective-C object.
• Nil (capitalized) is a null pointer to an Objective-C class.
• NULL (all caps) is a null pointer to anything else (C pointers, that is).
• [NSNull null] is a singleton for situations where use of nil is not possible (adding/receiving nil to/from NSArrays e.g.)

在Objective-C++中:

• 以上所有，加上:
• null (小写) 或 nullptr(C++11 或更高版本)是指向C++ 对象.

• All of the above, plus:
• null (lowercase) or nullptr (C++11 or later) is a null pointer to C++ objects.

因此要检查 nil，您应该与 nil(或分别为 NULL)进行比较明确:

So to check against nil you should either compare against nil (or NULL respectively) explicitly:

if (getCaption == nil) ...

或者让 ObjC/C隐式为你做:

if (!getCaption) ...

这是因为 C 中的每个表达式(并且 Objective-C 是其超集)都有一个隐含的布尔值:

This works as every expression in C (and with Objective-C being a superset thereof) has an implicit boolean value:

expression != 0x0 => true
expression == 0x0 => false

<小时>

现在，当检查 NSNull 时，这显然不起作用，因为 [NSNull null] 返回一个指向 NSNull 的单例实例的指针，而不是 nil，因此它不等于 0x0.

---

Now when checking for NSNull this obviously wouldn't work as [NSNull null] returns a pointer to a singleton instance of NSNull, and not nil, and therefore it is not equal to 0x0.

所以要检查 NSNull 可以使用:

So to check against NSNull one can either use:

if ((NSNull *)getCaption == [NSNull null]) ...

或(首选，见评论):

if ([getCaption isKindOfClass:[NSNull class]]) ...

<小时>

请记住，如果 getCaption 恰好是 nil，后者(使用消息调用)将返回 false，虽然正式正确，可能不是您期望/想要的.

---

Keep in mind that the latter (utilising a message call) will return false if getCaption happens to be nil, which, while formally correct, might not be what you expect/want.

因此，如果一个人(无论出于何种原因)需要同时检查 nil/NULL 和 NSNull，必须结合这两项检查:

Hence if one (for whatever reason) needed to check against both nil/NULL and NSNull, one would have to combine those two checks:

if (!getCaption || [getCaption isKindOfClass:[NSNull class]]) ...

<小时>

有关形成等效肯定检查的帮助，请参阅德摩根定律a> 和 布尔否定.

---

For help on forming equivalent positive checks see De Morgan's laws and boolean negation.

NSHipster.com 刚刚发表了一篇关于细微差别的精彩文章介于 nil、null 等之间

NSHipster.com just published a great article on the subtle differences between nil, null, etc.

这篇关于如何检测 NSString 是否为空?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！