title: How to copy an object in ARC
body: <p>Now it hasn't the copy property in ARC,so how should I do when I want copy an object?
  Then I write a test code:</p>
  
  <p>test.h</p>
  
  <pre><code>@property (strong, nonatomic) NSMutableString *str1;
  @property (strong, nonatomic) NSMutableString *str2;
  </code></pre>
  
  <p>test.m</p>
  
  <pre><code>self.str1 = [[NSMutableString alloc] initWithString:@"Hello"];
  self.str2 = [self.str1 copy];
  [self.str2 appendString:@"World"];
  NSLog(@"str1:%@,str2:%@",self.str1,self.str2);
  </code></pre>
  
  <p>When I run,it got crash:</p>
  
  <pre><code>Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to mutate immutable object with appendString:'
  </code></pre>
  
  <p>So,how should I do?</p>