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copied!<p><code>str1</code> and <code>str2</code> are <em>pointers</em> that reference the same area of memory. Your memory layout looks roughly like this: <img src="https://i.stack.imgur.com/xjmA4.png" alt="alt text"></p> <p>If you change where <code>str1</code> points, e.g., by doing this</p> <pre><code>str1 = @"new string"; </code></pre> <p>then <code>str2</code> will still reference "matrix", but <code>str1</code> will reference "new string": <img src="https://i.stack.imgur.com/MAOI3.png" alt="alt text"></p> <p>Let's say, though, that <code>str1</code> and <code>str2</code> actually pointed to an instance of an <code>NSMutableString</code>, and you did this instead:</p> <pre><code>[str2 setString:@"new string"]; </code></pre> <p>Note, then, that <code>str1</code> and <code>str2</code> would still point the same object, so by modifying <code>str2</code>, <code>str1</code> would also change to "new string".</p> <h2>Shallow copy vs. deep copy</h2> <p>A <em>shallow copy</em> is a copy of an object in which its instance variables still point to the same memory location as the original object's ivars. A <em>deep copy</em> is a copy in which copies of the instance variables are also made.</p> <p>Let's say you have a class, <code>MyClass</code>, that has two instance variables, each of type <code>NSString</code>. Here's a diagram of what the memory layout would roughly look like after a shallow and a deep copy:</p> <p><img src="https://i.stack.imgur.com/ke0RZ.png" alt="alt text"></p>

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