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今天讲Objective-C的NSValue，它可以代表更复杂的Objective-C的类型，比如NSRange，结构体类型。

Represent Other Values Using Instances of the NSValue Class

The NSNumber class is itself a subclass of the basic NSValue class, which provides an object wrapper around a single value or data item. In addition to the basic C scalar types, NSValue can also be used to represent pointers and structures.

The NSValue class offers various factory methods to create a value with a given standard structure, which makes it easy to create an instance to represent, for example, an NSRange, like the example from earlier in the chapter:

NSString *mainString = @"This is a long string";
NSRange substringRange = [mainString rangeOfString:@"long"];
NSValue *rangeValue = [NSValue valueWithRange:substringRange];

It’s also possible to create NSValue objects to represent custom structures. If you have a particular need to use a C structure (rather than an Objective-C object) to store information, like this:

typedef struct {
    int i;
    float f;
} MyIntegerFloatStruct;

you can create an NSValue instance by providing a pointer to the structure as well as an encoded Objective-C type. The @encode() compiler directive is used to create the correct Objective-C type, like this:

struct MyIntegerFloatStruct aStruct;
aStruct.i = 42;
aStruct.f = 3.14;

NSValue *structValue = [NSValue value:&aStruct
						 withObjCType:@encode(MyIntegerFloatStruct)];

The standard C reference operator (&) is used to provide the address of aStruct for the value parameter.