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    <p>The other answerers are all correct, but a friend asked me to explain this to him and what it really boils down to is how Ruby handles variables, so I thought I would share some simple pictures / explanations I wrote for him (apologies for the length and probably some oversimplification):</p> <hr> <h3>Q1: What happens when you assign a new variable <code>str</code> to a value of <code>'foo'</code>?</h3> <pre><code>str = 'foo' str.object_id # =&gt; 2000 </code></pre> <p><img src="https://i.stack.imgur.com/GzWSD.png" alt="enter image description here"></p> <h3>A: A label called <code>str</code> is created that points at the object <code>'foo'</code>, which for the state of this Ruby interpreter happens to be at memory location <code>2000</code>.</h3> <hr> <h3>Q2: What happens when you assign the existing variable <code>str</code> to a new object using <code>=</code>?</h3> <pre><code>str = 'bar'.tap{|b| puts "bar: #{b.object_id}"} # bar: 2002 str.object_id # =&gt; 2002 </code></pre> <p><img src="https://i.stack.imgur.com/5UABQ.png" alt="enter image description here"></p> <h3>A: The label <code>str</code> now points to a different object.</h3> <hr> <h3>Q3: What happens when you assign a new variable <code>=</code> to <code>str</code>?</h3> <pre><code>str2 = str str2.object_id # =&gt; 2002 </code></pre> <p><img src="https://i.stack.imgur.com/BVXvI.png" alt="enter image description here"></p> <h3>A: A new label called <code>str2</code> is created that points at the <em>same object</em> as <code>str</code>.</h3> <hr> <h3>Q4: What happens if the object referenced by <code>str</code> and <code>str2</code> gets changed?</h3> <pre><code>str2.replace 'baz' str2 # =&gt; 'baz' str # =&gt; 'baz' str.object_id # =&gt; 2002 str2.object_id # =&gt; 2002 </code></pre> <p><img src="https://i.stack.imgur.com/ZMFEd.png" alt="enter image description here"></p> <h3>A: Both labels still point at the same object, but that object itself has mutated (its contents have changed to be something else).</h3> <hr> <h3>How does this relate to the original question?</h3> <p>It's basically the same as what happens in Q3/Q4; the method gets its own private copy of the variable / label (<code>str2</code>) that gets passed in to it (<code>str</code>). It can't change which object the label <code>str</code> <em>points to</em>, but it can change the <em>contents of</em> the object that they both reference to contain else:</p> <pre><code>str = 'foo' def mutate(str2) puts "str2: #{str2.object_id}" str2.replace 'bar' str2 = 'baz' puts "str2: #{str2.object_id}" end str.object_id # =&gt; 2004 mutate(str) # str2: 2004, str2: 2006 str # =&gt; "bar" str.object_id # =&gt; 2004 </code></pre>
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    1. POIs Ruby pass by reference or by value?
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    1. COGreat reply, love the pictures! Definitely helped me grasp how ruby handles value/reference in a couple minutes. Granted, I knew when I wrote Ruby 4 yrs ago and completely forgot. :)
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    2. CO@Jason Glad you found it useful!
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    3. COWish I could favourite an answer! Very nice, respect.
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