NSObject

1. NSObject 的结构体

http://www.cocoachina.com/ios/20160503/16060.html

struct objc_object {
    isa_t isa; // 指向 类对象
};

struct objc_class : objc_object {
    isa_t isa;
    Class superclass;
    cache_t cache;
    class_data_bits_t bits;
};

isa_t 结构体