總結(jié)下我的懂得：
strong weak是ARC惹人的，strong相當(dāng)于retain，然則對于有的類型，例如NSString，則應(yīng)用strong相當(dāng)于應(yīng)用copy
如許直接用strong就可以直接處理retain和strong的情況了。
weak相當(dāng)于assign，然則比后者多一點：對象被干掉落時會將weak引用設(shè)為nil，而對nil發(fā)送消息都不會導(dǎo)致崩潰
    NSString *strongStr;
不然weak引用為野指針，會出現(xiàn)問題
@interface AppDelegate : UIResponder <UIApplicationDelegate>
{
    NSString *copStr;
}
@property (retain, nonatomic) NSString *retainStr;
@property (strong, nonatomic) NSString *strongStr;
@property (copy, nonatomic) NSString *copStr;
@end
    NSMutableString *newStr = [NSMutableString stringWithFormat:@"abcd"];
    self.retainStr = newStr;
    self.strongStr = newStr;
    self.copStr = newStr;
    
    [newStr appendString:@"123"];
    
    NSLog(@"strong string:%@", self.strongStr);
    NSLog(@"retain sting:%@", self.retainStr);
    NSLog(@"copy string:%@", self.copStr);
輸出結(jié)不雅：
2014-05-02 13:31:17.660 TestMultipleThreadDownload[552:60b] retain sting:abcd123
這里說strong 有copy的功能是缺點的哦。實驗例子
2014-05-02 13:31:17.662 TestMultipleThreadDownload[552:60b] strong string:abcd123
    NSString *retainStr;
2014-05-02 13:31:17.663 TestMultipleThreadDownload[552:60b] copy string:abcd