¿Qué es NULL en Objective-C?

What is NULL in Objective-C?

[NSNull null] is an object that’s meant to stand in for nil in situations where nil isn’t allowed. For example, you can’t have a nil value in an NSArray. So if you need to represent a “nil”, you can use [NSNull null] .

nothing
Objective-C builds on C’s representation of nothing by adding nil . nil is an object pointer to nothing. Although semantically distinct from NULL , they are technically equivalent.

nil is a frequently used and important predeclared identifier in Go. It is the literal representation of zero values of many kinds of types. Many new Go programmers with experiences of some other popular languages may view nil as the counterpart of null (or NULL ) in other languages.

In Java, null is a reserved word (keyword) for literal values. It seems like a keyword, but actually, it is a literal similar to true and false. The reserved word null is case sensitive and we cannot write null as Null or NULL, the compiler will not recognize them and give an error.

If you treat it is a null string like nil , then test (object. length == 0) . object. length will return 0 if object == nil , so this test covers nil objects and strings with length 0.

id is a data type of object identifiers in Objective-C, which can be use for an object of any type no matter what class does it have. id is the final super type of all objects.

In general use, nil (a contraction of Latin “nihil”) means “nothing” or the absence of something. Sometimes nil is used to mean the number zero (0). In certain programming languages such as Pascal , Smalltalk , or LISP (list processing), nil represents the absence of an object, or the empty pointer value.

NSNull is different from nil or NULL, in that it is an actual object, rather than a zero value. Additionally, in Foundation/NSObjCRuntime.h, Nil is defined as a class pointer to nothing. This lesser-known title-case cousin of nil doesn’t show up much very often, but it’s at least worth noting.

NSNull defines a class method ( +null ) that returns the singleton NSNull object. NSNull represents nothing with an actual object rather than a zero value like nil or NULL. NSNull is used throughout Foundation and other frameworks to skirt around the limitations of collections like NSArray and NSDictionary not being able to contain nil values.

C represents nothing as 0 for primitive values and NULL for pointers ( which is equivalent to 0 in a pointer context). The only time you see NULL in Objective-C code is when interacting with low-level C APIs like Core Foundation or Core Graphics.

Newly- alloc ‘d NSObject s start life with their contents set to 0 . For this reason, it’s unnecessary to set an object’s non-primitive properties to nil in its initializer. Perhaps the most notable behavior of nil , however, is that it can have messages sent to it without a problem.