一、iOS中_cmd
The _cmd variable is a hidden argument passed to every method that is the current selector
源码：
- (void)tryMessage
{
SEL currentSel = _cmd; // 通过_cmd关键字取到当前函数对应的SEL
NSLog(@"currentSel is ：%s",sel_getName(currentSel)); // C代码结果
NSLog(@"currentSel is ：%@",NSStringFromSelector(_cmd)); //O-C代码结果
}
得到结果：
-- :::] currentSel is ：tryMessage -- :::] currentSel is ：tryMessage
二、Class 东东
第一遇到Class的时候，我记得是这里：
@interface NSObject <NSObject> {
Class isa OBJC_ISA_AVAILABILITY;
}
#if !OBJC_TYPES_DEFINED
/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;
/// Represents an instance of a class.
struct objc_object {
Class isa OBJC_ISA_AVAILABILITY;
};
/// A pointer to an instance of a class.
typedef struct objc_object *id;
#endif
struct objc_class {
Class isa OBJC_ISA_AVAILABILITY;
#if !__OBJC2__
Class super_class OBJC2_UNAVAILABLE;
const char *name OBJC2_UNAVAILABLE;
long version OBJC2_UNAVAILABLE;
long info OBJC2_UNAVAILABLE;
long instance_size OBJC2_UNAVAILABLE;
struct objc_ivar_list *ivars OBJC2_UNAVAILABLE;
struct objc_method_list **methodLists OBJC2_UNAVAILABLE;
struct objc_cache *cache OBJC2_UNAVAILABLE;
struct objc_protocol_list *protocols OBJC2_UNAVAILABLE;
#endif
} OBJC2_UNAVAILABLE;
objc_class结构体
objc_class和objc_object分别是类结构体和对象结构体，其中objc_object都有一个只想对象本身所属的类的isa指针，用她来处理很多事情。id代表ObjC对象，获取id对象的类；
三、IMP(Method Impletiontations)
/// A pointer to the function of a method implementation. #if !OBJC_OLD_DISPATCH_PROTOTYPES typedef void (*IMP)(void /* id, SEL, ... */ ); #else typedef id (*IMP)(id, SEL, ...); #endif
显而易见，IMP是一个函数指针，这是由编辑器生成的，当你发起一个ObjC消息之后，最终她会执行的那个代码，就是由这个函数指针指定的。