Class, Method，SEL, IMP

Class：

定义在 objc.h:

/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;

objc_class定义在runtime.h：

    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;

#if !__OBJC2__
    Class _Nullable super_class                              OBJC2_UNAVAILABLE;
    const char * _Nonnull name                               OBJC2_UNAVAILABLE;
    long version                                             OBJC2_UNAVAILABLE;
    long info                                                OBJC2_UNAVAILABLE;
    long instance_size                                       OBJC2_UNAVAILABLE;
    struct objc_ivar_list * _Nullable ivars                  OBJC2_UNAVAILABLE;
    struct objc_method_list * _Nullable * _Nullable methodLists                    OBJC2_UNAVAILABLE;
    struct objc_cache * _Nonnull cache                       OBJC2_UNAVAILABLE;
    struct objc_protocol_list * _Nullable protocols          OBJC2_UNAVAILABLE;
#endif
} OBJC2_UNAVAILABLE;

Method

Method 定义在 runtime.h:

struct objc_method {
    SEL _Nonnull method_name                                 OBJC2_UNAVAILABLE;
    char * _Nullable method_types                            OBJC2_UNAVAILABLE;
    IMP _Nonnull method_imp                                  OBJC2_UNAVAILABLE;
}                                                            OBJC2_UNAVAILABLE;

Method方法包括了 SEL 也就是方法名称，types，标示方法的参数类型，IMP标示该方法具体实现的函数指针

SEL

SEL 定义在objc.h:

typedef struct objc_selector *SEL;

表示的方法的签名

IMP

IMP定义在 objc.h

typedef void (*IMP)(void /* id, SEL, ... */ ); 

id:

/// A pointer to an instance of a class.
typedef struct objc_object *id;

/// Represents an instance of a class.
struct objc_object {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;
};

id 是一个指向 objc_object的结构体指针，objc_object里只有一个 isa

摘自：https://blog.csdn.net/kesalin/article/details/6689226