Adding nil article

2013-01-07-nil.md

@@ -0,0 +1,87 @@
+---
+layout: post
+title: "nil / Nil / NULL / NSNull"
+
+ref: "https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/NumbersandValues/Articles/Null.html"
+published: true
+---
+
+Understanding the concept of nothingness is as much a philosophical issue as it is a pragmatic one. We are inhabitants of a universe of _somethings_, yet reason in a logical universe of existential uncertainties. As a physical manifestation of a logical system, computers are faced with the intractable problem of how to represent _nothing_ with _something_.
+
+---
+
+In Objective-C, there are several different varieties of _nothing_. The reason for this goes back to [a common NSHipster refrain](http://nshipster.com/ns_enum-ns_options/), of how Objective-C bridges the procedural paradigm of C with Smalltalk-inspired object-oriented paradigm.
+
+C represents _nothing_ as `0` for primitive values, and `NULL` for pointers ([which is equivalent to `0` in a pointer context](http://c-faq.com/null/nullor0.html)).
+
+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 to one another.
+
+On the framework level, Foundation defines `NSNull`, which defines a class method, `+null`, which returns the singleton `NSNull` object. `NSNull` is different from `nil` or `NULL`, in that it is an actual object, rather than a zero value. 
+
+Additionally, in [Foundation/NSObjCRuntime.h](https://gist.github.com/4469665), `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.
+
+## There's Something About `nil`
+
+Newly-`alloc`'d `NSObject`s start life with [their contents set to `0`](https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html). This means that all pointers that object has to other objects begin as `nil`, so it's unnecessary to, for instance, set `self.(association) = nil` in `init` methods.
+
+Perhaps the most notable behavior of `nil`, though, is that it can have messages sent to it. 
+
+In other languages, like C++, this would crash your program, but in Objective-C, invoking a method on `nil` returns a zero value. This greatly simplifies expressions, as it obviates the need to check for `nil` before doing anything:
+
+    // For example, this expression...
+    if (name != nil && [name isEqualToString:@"Steve"]) { ... }
+
+    // ...can be simplified to:
+    if ([name isEqualToString:@"steve"]) { ... }
+
+Being aware of how `nil` works in Objective-C allows this convenience to be a feature, and not a lurking bug in your application. Make sure to guard against cases where `nil` values are unwanted, either by checking and returning early to fail silently, or adding a `NSParameterAssert` to throw an exception.
+
+## `NSNull`: Something for Nothing
+
+`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. You can think of `NSNull` as effectively [boxing][1] the `NULL` or `nil` value so that it can be used in collections:
+
+    NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionary];
+
+    mutableDictionary[@"someKey"] = nil; // Removes value for `someKey`
+    NSLog(@"Keys: %@", [mutableDictionary allKeys]); // @[]
+
+    mutableDictionary[@"someKey"] = [NSNull null]; // Sets value of NSNull singleton for `someKey`
+    NSLog(@"Keys: %@", [mutableDictionary allKeys]); // @[@"someKey"]
+
+---
+
+So to recap, here are the four values representing _nothing_ that every Objective-C programmer should know about:
+
+<table>
+  <thead>
+    <tr>
+      <th>Symbol</th>
+      <th>Value</th>
+      <th>Meaning</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><tt>NULL</tt></td>
+      <td><tt>(void *)0</tt></td>
+      <td>literal null value for C pointers</td>
+    </tr>
+    <tr>
+      <td><tt>nil</tt></td>
+      <td><tt>(id)0</tt></td>
+      <td>literal null value for Objective-C objects</td>
+    </tr>
+    <tr>
+      <td><tt>Nil</tt></td>
+      <td><tt>(Class)0</tt></td>
+      <td>literal null value for Objective-C classes</td>
+    </tr>
+    <tr>
+      <td><tt>NSNull</tt></td>
+      <td><tt>[NSNull null]</tt></td>
+      <td>singleton object used to represent null</td>
+    </tr>
+  </tbody>
+</table>
+
+[1]: http://en.wikipedia.org/wiki/Object_type_(object-oriented_programming)#Boxing