知乎相关链接：http://www.zhihu.com/question/20102376
@property (nonatomic, copy) NSString *cString;
@property (nonatomic, strong) NSString *sString;
NSMutableString *mString = [[NSMutableString alloc] initWithString:@"123"];
self.cString = mString;
self.sString = mString;
[mString appendString:@"abc"];
NSLog(@"coypString：%p---%@", self.cString, self.cString);
NSLog(@"strongString：%p---%@", self.sString, self.sString);
NSLog(@"mString：%p---%@", mString, mString);
coypString：0xa000000003332313---123
strongString：0x7fae89576df0---123abc
mString：0x7fae89576df0---123abc
