[Objective-C]为现有对象增加额外的实例变量/数据

想到要如何为所有的对象增加实例变量吗？ 使用Category可以很方便地为现有的类增加方法，但却无法直接增加实例变量(有为此使用查表法的，也算曲线救国吧)。不过从Mac OS X v10.6开始，系统提供了Associative References，这个问题就很容易解决了。

我根据Objective-C Reference中的示例修改了一下，直接上代码了。重点是其中objc_setAssociatedObject的使用, 并且其中Key是一个地址，而不是字串。

#import <Foundation/Foundation.h>#import <objc/runtime.h> int main (int argc, const char * argv[]) {     @autoreleasepool {/*Seciton 0. 关联数据的Key和Value*/    static char overviewKey;static const char *myOwnKey = "VideoProperty\0";static const char intValueKey = 'i';     NSArray *array = [[NSArray alloc]            initWithObjects:@ "One", @"Two", @"Three", nil];    // For the purposes of illustration, use initWithFormat: to ensure    // we get a deallocatable string    NSString *overview = [[NSString alloc]            initWithFormat:@"%@", @"First three numbers"];NSString *videoKeyValue = @"This is a video";NSNumber *intValue = [[NSNumber alloc]initWithInt:5];/*Section 1. 关联数据设置部分*/    objc_setAssociatedObject (            array,            &overviewKey,            overview,            OBJC_ASSOCIATION_RETAIN        );        [overview release];objc_setAssociatedObject (    array,    myOwnKey,    videoKeyValue,    OBJC_ASSOCIATION_RETAIN);objc_setAssociatedObject (    array,    &intValueKey,    intValue,    OBJC_ASSOCIATION_RETAIN);     /*Section 3. 关联数据查询部分*/    NSString *associatedObject =  (NSString *) objc_getAssociatedObject (array, &overviewKey);    NSLog(@"associatedObject: %@", associatedObject);NSString *associatedObject2 = (NSString *) objc_getAssociatedObject(array, myOwnKey);NSLog(@"Video Key value is %@", associatedObject2);NSString *assObject3 = (NSString *) objc_getAssociatedObject(array, &myOwnKey);if( assObject3 ){NSLog(@"不会进入这里! assObject3 应当为nil!");}else{NSLog(@"OK. 通过myOwnKey的地址是得不到数据的!");}    NSNumber *assKeyValue = (NSNumber *) objc_getAssociatedObject(array, &intValueKey); NSLog(@"Int value is %d",[assKeyValue intValue]);/*Section 3. 关联数据清理部分*/    objc_setAssociatedObject (            array,            &overviewKey,            nil,            OBJC_ASSOCIATION_ASSIGN        );objc_setAssociatedObject (    array,    myOwnKey,    nil,    OBJC_ASSOCIATION_ASSIGN);objc_setAssociatedObject (    array,    &intValueKey,    nil,    OBJC_ASSOCIATION_ASSIGN);        [array release];     }    return 0;}

clang -o associates -g -x objective-c++ -Wall associates.mm -framework Foundation -lobjc