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<h2>De Re NSObject <div style="font-size:10px; display: inline-block;">0.0.1</div> <a href="feed.xml"><img class="inline-img" src='images/feed-icon-28x28.png'></a></h2>
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<h1>NSObject: -retain / -release</h1>
<div>
<p>Let’s dive into the fascinating world of memory management in <a href="//mulle-objc.github.io">mulle-objc</a>, a
system that sets itself apart with its unique approach to object lifecycle
management. As you’ll soon discover, mulle-objc employs a highly efficient
Manual Memory Management (MMR) system, eschewing Automatic Reference Counting (
ARC) in favor of a more predictable and controllable approach.</p>
<p>Have you ever wondered how objects in a programming language know when they’re
no longer needed? In mulle-objc, this is handled through a sophisticated retain
counting mechanism. Unlike some contemporary systems that abstract away memory
management, mulle-objc gives you, the developer, direct control over an object’s
lifecycle.</p>
<p>At the heart of mulle-objc’s memory management are six core operations. It is
important to know these operations, but in a well designed system, you will
rarely use them directly. And there are only two methods you will encounter
overriding with your own implementations:</p>
<table>
<thead>
<tr>
<th>Operation</th>
<th>Override</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code class="language-plaintext highlighter-rouge">+alloc</code> / <code class="language-plaintext highlighter-rouge">+new</code></td>
<td>NO</td>
<td>Creates an object with an initial reference count of 1</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-retain</code></td>
<td>NO</td>
<td>Increments the reference count</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-release</code></td>
<td>NO</td>
<td>Decrements the reference count</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-retainCount</code></td>
<td>NO</td>
<td>Returns the current reference count</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-finalize</code></td>
<td>YES</td>
<td>Called from -release when the reference count reaches zero. Can also be triggered by <code class="language-plaintext highlighter-rouge">-mullePerformFinalize</code>. Allows the object to free resources and untie itself from other objects. Runs only once in an object’s lifetime.</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-dealloc</code></td>
<td>YES</td>
<td>Performs final cleanup when the reference count reaches zero</td>
</tr>
</tbody>
</table>
<p>Anyway, let’s break this down with an example:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Document</span> <span class="o">*</span><span class="n">document</span><span class="p">;</span>
<span class="n">document</span> <span class="o">=</span> <span class="p">[</span><span class="n">Document</span> <span class="nf">new</span><span class="p">];</span> <span class="c1">// count = 1</span>
<span class="p">[</span><span class="n">document</span> <span class="nf">retain</span><span class="p">];</span> <span class="c1">// count = 2</span>
<span class="p">[</span><span class="n">document</span> <span class="nf">release</span><span class="p">];</span> <span class="c1">// count = 1</span>
<span class="p">[</span><span class="n">document</span> <span class="nf">release</span><span class="p">];</span> <span class="c1">// count = 0, triggers finalize and then dealloc</span>
<span class="c1">// DO NOT MESSAGE document AGAIN</span>
</code></pre></div></div>
<p>In this sequence, we create a <code class="language-plaintext highlighter-rouge">Document</code> object, increase its retain count, then
decrease it twice.</p>
<p>When <code class="language-plaintext highlighter-rouge">-retainCount</code> reaches zero, the <code class="language-plaintext highlighter-rouge">-release</code> method automatically
calls <code class="language-plaintext highlighter-rouge">-finalize</code> and then <code class="language-plaintext highlighter-rouge">-dealloc</code> in sequence, ultimately leading to the
objects destruction.</p>
<p><img src="images/object-lifetime-2.svg" alt="Lifetime" /></p>
<blockquote>
<p><strong>LAW</strong>: Once the number of <code class="language-plaintext highlighter-rouge">-release</code> calls to an object exceeds the
number of <code class="language-plaintext highlighter-rouge">-retain</code> calls, the object becomes deallocated and invalid.</p>
</blockquote>
<h2 id="when-to-call--retain-and--release">When to call -retain and -release</h2>
<p>When you create another object or receive one as a method or function argument,
you may want to keep it around with your object. Effectively you
are keeping a <em>reference</em> to it. Now when you do that, you increase the
reference count of that object with <code class="language-plaintext highlighter-rouge">-retain</code>. Conversely, when you are done
with it, you use <code class="language-plaintext highlighter-rouge">-release</code> to relinquish ownership.</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@class</span> <span class="nc">Person</span><span class="p">;</span>
<span class="k">@interface</span> <span class="nc">Document</span> <span class="p">:</span> <span class="nc">NSObject</span>
<span class="p">{</span>
<span class="n">Person</span> <span class="o">*</span><span class="n">_author</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">Document</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="nf">setAuthor</span><span class="p">:(</span><span class="n">Person</span> <span class="o">*</span><span class="p">)</span> <span class="n">person</span>
<span class="p">{</span>
<span class="c1">// THIS CODE IS JUST FOR ILLUSTRATION PURPOSES</span>
<span class="c1">// DO NOT WRITE ACCESSORS THAT CALL -release INSTEAD OF -autorelease</span>
<span class="k">if</span><span class="p">(</span> <span class="n">_author</span> <span class="o">!=</span> <span class="n">person</span><span class="p">)</span>
<span class="p">{</span>
<span class="p">[</span><span class="n">_author</span> <span class="nf">release</span><span class="p">];</span> <span class="c1">// remove old author</span>
<span class="n">_author</span> <span class="o">=</span> <span class="p">[</span><span class="n">person</span> <span class="nf">retain</span><span class="p">];</span> <span class="c1">// keep new author</span>
<span class="p">}</span>
<span class="p">}</span>
</code></pre></div></div>
<p>In general though in mulle-objc you use a <a href="//mulle-kybernetik.com/de-re-property/">@property</a> to keep an object reference, which simplifies the proceedings.</p>
<h2 id="caveats">Caveats</h2>
<p>But what about the challenges of this system? While it offers great control, it
does require careful management from the developer. You must ensure that every
<code class="language-plaintext highlighter-rouge">-retain</code> is balanced by a <code class="language-plaintext highlighter-rouge">-release</code>, that <code class="language-plaintext highlighter-rouge">-dealloc</code> releases all retained
objects, and that you never access released objects.</p>
<p>These requirements can potentially lead to memory leaks or crashes, if not
handled correctly. The next chapter discusses how mulle-objc mitigates many of
these challenges through its autorelease pools.</p>
</div>
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