@interface User : NSObject @property (nonatomic,retain) NSString* tRetain;
@property (nonatomic,assign) NSString* tAssign;
@property (nonatomic,copy) NSString* tcopy; @end
类User有个属性tRetain, 只是测试就用NSString类型了（此类型一般用copy, 因为可能是个NSMutableString,不希望在赋值后被其他地方修改内容）。
User* user = [[User alloc]init]; NSString* testRetain = [NSString stringWithFormat:@"retain"];
NSLog(@"testRetain.retainCount=%lu",(unsigned long)testRetain.retainCount); //1 user.tRetain = testRetain;
NSLog(@"testRetain.retainCount=%lu",(unsigned long)testRetain.retainCount); //2
NSLog(@"user.tRetain.retainCount=%lu",(unsigned long)user.tRetain.retainCount); //2 NSString* testRetain2 = @"retain2";
NSLog(@"testRetain2.retainCount=%lu",(unsigned long)testRetain2.retainCount); //4294967295 自动释放对象 返回max unsigned long user.tRetain = testRetain2;
NSLog(@"testRetain.retainCount=%lu",(unsigned long)testRetain.retainCount); //1
NSLog(@"testRetain2.retainCount=%lu",(unsigned long)testRetain2.retainCount); //4294967295
NSLog(@"user.RetainValue.retainCount=%lu",(unsigned long)user.tRetain.retainCount); //4294967295 NSString* testRetain3 = [NSString stringWithFormat: @"retain3"];
NSLog(@"testRetain3.retainCount=%lu",(unsigned long)testRetain3.retainCount); //1 [testRetain3 retain];
NSLog(@"testRetain3.retainCount=%lu",(unsigned long)testRetain3.retainCount); //2 NSString* testRetain4 = [NSString stringWithString:testRetain3];
NSLog(@"testRetain3.retainCount=%lu",(unsigned long)testRetain3.retainCount); //3
NSLog(@"testRetain4.retainCount=%lu",(unsigned long)testRetain4.retainCount); //3
strong是ARC后引入的关键字， 在ARC环境中等同于Retain。
NSSring* str = [NSString stringWithString:字符串]； 此方法相当于上文对一个retain属性赋值。 若后面的字符串参数的计数为4294967295，则str的计数也是。 若字符串参数可计数， 例如1， 则执行后计数加1.