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

问题描述

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

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'，正在终止应用程序，原因:'-[NSNull isEqualToString:]:无法识别的选择器已发送到实例0x3eba63d4'

我做错什么了吗?我需要用其他方式做吗?

解决方案

Objective-C对象(类型id)的NULL值为nil.

NULL用于 C指针(类型void *).

(最后，两者最终都拥有相同的值(0x0).但是它们的类型不同.)

在 Objective-C 中:

• nil (全部小写)为空 指向 Objective-C对象的指针.
• Nil (大写)是空指针 到 Objective-C类.
• NULL (全部大写)是指向的空指针 其他任何(即 C指针).
• [NSNull null]是 singleton (单例)，用于无法使用nil的情况(例如，在NSArray中添加nil或从中接收nil)

在 Objective-C ++ 中:

• 以上所有，以及:
• null (小写字母)或nullptr( C ++ 11 或更高版本)是指向 C ++对象的空指针.

---

因此要与nil进行比较，您应该与nil(或分别为NULL)进行比较明确:

if (getCaption == nil) ...

或让 ObjC / C 为您进行隐含的操作:

if (!getCaption) ...

这适用于 C 中的每个表达式(并且 Objective-C 是其超集)具有隐式布尔值:

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

---

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

因此要检查NSNull，可以使用:

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

或(首选，请参见评论):

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

---

请记住，如果getCaption恰好是nil，则后者(使用消息调用)将返回false，尽管在​​形式上正确，但可能并非您期望/想要的.

因此，如果出于某种原因(无论出于何种原因)需要进行检查 nil/NULL 和 NSNull，则必须将这两项检查结合起来:

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

---

有关形成同等肯定检查的帮助，请参见德摩根定律和布尔求反.

编辑: NSHipster.com 刚刚发表了一篇有关细微差别的文章在nil，null等之间.

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...";
}

}

And here's the error:

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

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

解决方案

The NULL value for Objective-C objects (type id) is nil.

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

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

In Objective-C:

• 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.)

In Objective-C++:

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

---

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

if (getCaption == nil) ...

or let ObjC / C do it implicitly for you:

if (!getCaption) ...

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

---

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.

So to check against NSNull one can either use:

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

or (preferred, see comments):

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

---

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.

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]]) ...

---

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

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

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