@interface PSSub : PS
@end
@implementation PSSub
- (instancetype)init{
self = [super init];
if (self) {
NSLog(@"%@", NSStringFromClass([self class]));
NSLog(@"%@", NSStringFromClass([super class]));
}
return self;
}
@end
以上代码打印结果，按照正常思维应该分别打印PSSub与PS,然而实际运行结果都打印PSSub
self是类的隐藏参数，指向当前调用方法的对象。而
super并不是如我们所想是指向当前对象父类的指针。其实super是一个编译器标识符，在运行时中与self相同，指向同一个消息接受者，只是self会优先在当前类的methodLists中查找方法，而super则是优先从父类中查找。clang一下
$ clang -rewrite-objc PS.m
运行时代码如下
static instancetype _I_PSSub_init(PSSub * self, SEL _cmd) {
self = ((PSSub *(*)(__rw_objc_super *, SEL))(void *)objc_msgSendSuper)((__rw_objc_super){(id)self, (id)class_getSuperclass(objc_getClass("PSSub"))}, sel_registerName("init"));
if (self) {
NSLog((NSString *)&__NSConstantStringImpl__var_folders_lf_6jrk2sf11bd4pjvkrxfm_z4h0000gn_T_PS_d6ceaa_mi_0, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)self, sel_registerName("class"))));
NSLog((NSString *)&__NSConstantStringImpl__var_folders_lf_6jrk2sf11bd4pjvkrxfm_z4h0000gn_T_PS_d6ceaa_mi_1, NSStringFromClass(((Class (*)(__rw_objc_super *, SEL))(void *)objc_msgSendSuper)((__rw_objc_super){(id)self, (id)class_getSuperclass(objc_getClass("PSSub"))}, sel_registerName("class"))));
}
return self;
}
删除无关类型方法代码后：
objc_msgSend((id)self, sel_registerName("class"));
objc_msgSendSuper((__rw_objc_super){(id)self, (id)class_getSuperclass(objc_getClass("PSSub"))}, sel_registerName("class"));
查看函数定义：
/**
* Sends a message with a simple return value to an instance of a class.
*
* @param self A pointer to the instance of the class that is to receive the message.
* @param op The selector of the method that handles the message.
* @param ...
* A variable argument list containing the arguments to the method.
*
* @return The return value of the method.
*
* @note When it encounters a method call, the compiler generates a call to one of the
* functions \c objc_msgSend, \c objc_msgSend_stret, \c objc_msgSendSuper, or \c objc_msgSendSuper_stret.
* Messages sent to an object’s superclass (using the \c super keyword) are sent using \c objc_msgSendSuper;
* other messages are sent using \c objc_msgSend. Methods that have data structures as return values
* are sent using \c objc_msgSendSuper_stret and \c objc_msgSend_stret.
*/
OBJC_EXPORT id objc_msgSend(id self, SEL op, ...)
__OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);
/**
* Sends a message with a simple return value to the superclass of an instance of a class.
*
* @param super A pointer to an \c objc_super data structure. Pass values identifying the
* context the message was sent to, including the instance of the class that is to receive the
* message and the superclass at which to start searching for the method implementation.
* @param op A pointer of type SEL. Pass the selector of the method that will handle the message.
* @param ...
* A variable argument list containing the arguments to the method.
*
* @return The return value of the method identified by \e op.
*
* @see objc_msgSend
*/
OBJC_EXPORT id objc_msgSendSuper(struct objc_super *super, SEL op, ...)
__OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);
可知objc_msgSend函数中的self参数是指指向接收消息的类的实例的指针，即消息接受者，而op参数则是指处理该消息的selector；objc_msgSendSuper函数中的参数super则是一个objc_super结构体，objc_super结构体定义如下:
/// Specifies the superclass of an instance.
struct objc_super {
/// Specifies an instance of a class.
__unsafe_unretained id receiver;
/// Specifies the particular superclass of the instance to message.
__unsafe_unretained Class super_class;
/* super_class is the first class to search */
};
其中receiver是指类的实例，super_class则是指该实例的父类。可以看到在编译后的C++代码中有个__rw_objc_super结构体：
struct __rw_objc_super {
struct objc_object *object;
struct objc_object *superClass;
__rw_objc_super(struct objc_object *o, struct objc_object *s) : object(o), superClass(s) {}
};
其实即为objc_super结构体。通过(__rw_objc_super){(id)self, (id)class_getSuperclass(objc_getClass("PSSub"))}该段代码可知：我们把self以及PSSub的父类通过结构体的构造方法构造了一个__rw_objc_super结构体，也就是objc_super。因此objc_super结构体中的receiver既是self。所以[self class]和[super class]指向的是同一个消息接受者，只是self会优先从当前类的实现中寻找方法处理消息，而super则是会优先从objc_super结构体中的super_class也就是父类的实现中查找。PS及PSSub中均未实现- (Class)class;方法，因此会逐级向上查找最终调用基类NSObject的- (Class)class;方法，通过官方开源的NSObject的- (Class)class;方法代码：
- (Class)class{
return object_getClass(self);
}
可知，消息接受者是self，而[self class]和[super class]指向的是同一个消息接受者，因此该段代码均打印PSSub。
