自定义类实现NSCopying协议

@interface Fraction:NSObject<NSCopying>

-(id)copyWithZone:(NSZone*)zone

{

　　Fraction *newFract = [[Fraction allocWithZone:zone]init];//如果Fraction类有子类，

　　//应改为 id newFract = [[self class] allocWithZone:zone] init];//

　　[newFract setTo:numerator over:denominator];

　　return newFract;

}