ios - Two class access each other by importing their head file(via #import ) causes error -

two class access each other importing head file(via #import ) causes error?

i have been suggested use @class classname in header file if needing access other classes . i've got principle of why should this. there small sample below, triggers errors like" unknown type name ‘xxxclass’”. mean “#import” doesn’t copy code here ? if not so, why unable detect ‘xxxclass’

i think not cycle retain problem, principle of error ?

============================  person   ===============================  person.h   #import <foundation/foundation.h>   #import "dog.h"  //@class dog;  @interface person : nsobject  @property (nonatomic, strong) dog *dog;   //error  :unknown type name ‘dog'  @end    person.m  #import "person.h"  @implementation person  - (void)dealloc{ nslog(@"person--dealloc");}  @end   ============================  dog   ===============================  dog.h  #import <foundation/foundation.h>  #import "person.h"  @interface dog : nsobject  @property (nonatomic, weak) person *person;  //error:unknown type name‘person'  @end    dog.m  #import "dog.h"  @implementation dog   - (void)dealloc{ nslog(@"dog--dealloc");}  @end   ============================  main   ===============================  main.m  #import <foundation/foundation.h>  #import "person.h"  #import "dog.h"  int main()  {       person *p = [[person alloc] init];    dog *d = [[dog alloc] init];     p.dog = d;    d.person = p;     return 0;   } 

basically, #import makes compile entire text of specified file 1 statement written. when have circular #import statements compiler becomes confused , throws error. why should use @class break cycle.