strong:適用于OC對(duì)象，作用和非ARC中的retain作用相同，它修飾的成員變量為強(qiáng)指針類型
weak:適用于OC對(duì)象，作用和非ARC中的assign作用相同，修飾的成員變量為弱指針類型
assign:適用于非OC對(duì)象類型

在OC對(duì)象循環(huán)引用的時(shí)候一端為strong類型，另一段為weak類型

示例代碼如下：

/****************************** Teacher.h文件 ***********************************/#import <Foundation/Foundation.h>@class Student;@interface Teacher : NSObject@PRoperty (nonatomic,strong) Student *student;@property (nonatomic,strong) NSString *teacherName;@end/****************************** Teacher.m文件 ***********************************/#import "Teacher.h"#import "Student.h"@implementation Teacher- (void)dealloc{    NSLog(@"叫%@的Teacher對(duì)象被銷(xiāo)毀了",_teacherName);}@end/****************************** Student.h文件 ***********************************/#import <Foundation/Foundation.h>@class Teacher;@interface Student : NSObject@property (nonatomic,weak) Teacher *teahcher;@property (nonatomic,strong) NSStirng *studentName;@end/****************************** Student.m文件 ***********************************/#import "Student.h"#import "Teacher.h"@implementation Student- (void)dealloc{    NSLog(@"叫%@的Student對(duì)象被銷(xiāo)毀了",_stuName);}@end/****************************** main.m文件 ***********************************/#import <Foundation/Foundation.h>#import "Teacher.h"#import "Student.h"int main(int argc, const char * argv[]){        Teacher *teacher = [[Teacher alloc] init];    teacher.teacherName  = @"張老師";        Student *student = [[Student alloc] init];    student.stuName = @"李同學(xué)";    　　// Student類對(duì)象中的Teacher屬性為弱引用    student.teahcher = teacher;        // Teacher類對(duì)象中的Student屬性為強(qiáng)引用    teacher.student = student;    return 0;}