Objective-c in Nil/nil/null/nsnull

Transferred from: http://nshipster.cn/nil/

Understanding the concept of "non-existent" is not only a philosophical problem, but also a practical one. We are the inhabitants of the physical universe, and the reason lies in the uncertainty of the existence of the logical universe. As a physical embodiment of a logical system, the computer faces a thorny problem, that is, how to use the existence of expression does not exist .

In Objective-c, there are several different kinds of non-existent . The reason for this is to go back to a frequently mentioned nshipster, explaining how objective-c bridges in C's program paradigm and in object-oriented paradigms inspired by Smalltalk.

C 0 is used as a raw value that does not exist , and NULL as a pointer (this is equivalent to in the pointer environment 0 ).

Objective-c is added on the basis that the expression of C does not exist nil . nilis a pointer to an object that does not exist. Although it NULL is semantically different, they are technically equal.

At the framework level, the foundation defines a NSNull class method that +null returns a separate NSNull object. NSNullas nil well as NULL different, because it is an actual object instead of a 0 value.

In addition, in foundation/nsobjcruntime.h, it Nil is defined as a pointer to a class that is zero-oriented. This little- nil known uppercase cousin doesn't often appear, but it's worth noting at least.

About nilSome of the things

The content that was just been 分配 NSObject set to 0 . This means that all pointers to other objects from that object nil start, so init self.(association) = nil it is not necessary to set such expressions in the method.

Perhaps the nil most notable behavior is that although it is zero, it can still have messages sent to it.

In other languages, such as C + +, this will cause your program to crash, but in Objective-c, the nil call method returns a value of 0. This greatly simplifies the expression because it avoids nil checking it before use:

// 举个例子，这个表达...if (name != nil && [name isEqualToString:@"Steve"]) { ... }// …可以被简化为：if ([name isEqualToString:@"steve"]) { ... }

Knowing nil how to work in Objective-c allows you to turn this convenience into a feature rather than a bug lurking in your app. Be sure to avoid nil situations where values are not needed, either by checking or returning early to quiet failures, or by adding one NSParameterAssert to throw an exception.

NSNull: There is no

NSNullis widely used in foundation and other frameworks to resolve defects such as NSArray and NSDictionary the like and such collections cannot have nil values. You can NSNull encapsulate boxing as valid values to NULL achieve the purpose of nil using them in the collection:

NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionary];mutableDictionary[@"someKey"] = [NSNull null]; // Sets value of NSNull singleton for `someKey`NSLog(@"Keys: %@", [mutableDictionary allKeys]); // @[@"someKey"]

In general, the four expressions here do not have values that every objective-c programmer should know:

NULL
(void *) 0
The literal 0 value of the C pointer

Nil
(ID) 0
The literal 0 value of the Objective-c object

Nil
(Class) 0
The literal 0 value of the Objective-c class

NSNull
[NSNull NULL]
A separate object used to represent a value of 0

Objective-c in Nil/nil/null/nsnull