类簇

类簇的实质就是一个抽象工厂，不要继承一个类簇类

id someClass = [NSString alloc]; // 返回的对象类型：NSPlaceholderString
NSString *string1 = [[NSString alloc] init]; // 返回的对象类型：__NSCFConstantString
NSString *string2 = [[NSString alloc] initWithFormat:@"string2"]; // 返回的对象类型：__NSCFString

http://limboy.me/ios/2014/01/04/class-cluster.html
http://www.cocoachina.com/ios/20141219/10696.html
http://blog.csdn.net/itianyi/article/details/41350255
https://developer.apple.com/library/ios/documentation/General/Conceptual/CocoaEncyclopedia/ClassClusters/ClassClusters.html