IMP本质上是一个通用的函数指针

IMP：通用的函数指针

/// 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

#if !OBJC_OLD_DISPATCH_PROTOTYPES

OBJC_EXPORT void objc_msgSend(void /* id self, SEL op, ... */ )

    __OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);

OBJC_EXPORT void objc_msgSendSuper(void /* struct objc_super *super, SEL op, ... */ )

    __OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);

#else