As part of playing the role of Abraham Lincoln in theeponymous 2012 biopic, Daniel Day-Lewis would insist on walking through a refrigerator box marked “TIME MACHINE” anytime he entered or left the set.True Story.

While the ends certainly justify the means when it comes to Mr. Day-Lewis’ method acting, one can’t help but marvel at the absurdity of a magical cardboard box used as a nexus between the modern and historical worlds.

Yet, this isexactly what we do as Objective-C programmers.

…well notexactly, but there is a box involved.

As mentionedtime andagain in NSHipster, what makes Objective-C such a curiosity is the way it merges the old, procedural world of C with the modern Object-Oriented influences of Smalltalk. When done correctly, this tension can be exploited to craft semantically rich software without sacrificing performance. But bridging that gap between old and new is a miasma of casts, bridges, and of course, boxes.

Boxing is the process of encapsulating scalars (int,double,BOOL, etc.) and value types (struct,enum) with an object container, and is primarily used to store those values in collection objects—namely arrays and dictionaries.

NSNumber is often used to box scalars, but in Foundation, the reigning featherweight champion of boxing isNSValue.

NSValue is a simple container for a single C or Objective-C data value. It can hold scalars and value types, as well as pointers and object IDs.

While boxing is an admittedly dry subject matter, there are two methods in particular that are worth noting:+valueWithBytes:objCType:, which serves as a primer for working withNSValue, and+valueWithNonretainedObject:, which is surprisingly useful for being relatively unknown.

valueWithBytes:objCType:

+valueWithBytes:objCType:Creates and returns an NSValue object that contains a given value, which is interpreted as being of a given Objective-C type.

• value: The value for the newNSValue object.
• type: The Objective-C type of value.type should be created with the Objective-C@encode() compiler directive; it should not be hard-coded as a C string.

@encode was discussed inour rundown of the myriad@ Compiler Directives:

@encode(): Returns thetype encoding of a type. This type value can be used as the first argument encode inNSCoder -encodeValueOfObjCType:at.

The subject of type encodings would make fora great article of its own, but the main takeaway is that it serves as a terse, human-readable representation of the structure of a type. For example,

typedefstructexample{idanObject;char*aString;intanInt;}Example;

…has the encoding:

{example=@*i}

NSValue uses this type encoding to create the necessary data structures to represent these values internally. Neat!

valueWithNonretainedObject:

+valueWithNonretainedObject:Creates and returns an NSValue object that contains a given object.

anObject: The value for the new object.

If you already knew aboutvalueWithNonretainedObject, you should be nodding with a knowing grin. If you didn’t, you’re likely staring incredulously with mouth agape. Or if not, you will be soon.

In short,valueWithNonretainedObject: allows objects to be added to a collection, without the need for satisfying<NSCopying>.

It’s something that comes up occasionally, working with objects that can’t be directly added to anNSArray orNSDictionary. Without knowing aboutvalueWithNonretainedObject:, this would be something that would throw off your entire game—especially if you’re just starting out in Objective-C.

But now you’re in the know. You won’t be stifled by such pedestrian concerns. You need not grope around for answers withNSPointerArray orNSMapTable. Today is a new day.

Having unpacked all of this wisdom aboutNSValue, you can now face that cruel divide between procedural and object-oriented; C and Smalltalk. Because everything is easy with a magic box.