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<h1>NSObject: -autorelease</h1>
<div>
<p>Welcome to the world of <a href="//mulle-objc.github.io">mulle-objc</a>’s memory management, where <strong>NSAutoreleasePool</strong> reigns supreme.</p>
<p>You learned about the retain counting and the lifetime of objects in a previous
chapter, and now comes the chapter that will try to sell you, that you don’t
need to worry about retain counting. As now, the <code class="language-plaintext highlighter-rouge">NSAutoreleasePool</code> makes its
appearance on the scene.</p>
<h2 id="the-nsautoreleasepool-class">The NSAutoreleasePool class</h2>
<p>Basically a <code class="language-plaintext highlighter-rouge">NSAutoreleasePool</code> just manages an array of pointers to objects.
You can add objects to the pool by using the <code class="language-plaintext highlighter-rouge">-autorelease</code> convenience
method on <code class="language-plaintext highlighter-rouge">NSObject</code>. This method locates the current pool for the running
thread and adds the object to the array.</p>
<p>The autorelease pool has one operational method <code class="language-plaintext highlighter-rouge">-drain</code>, which iterates
through this array, sending a <code class="language-plaintext highlighter-rouge">-release</code> message to each object. You usually
call <code class="language-plaintext highlighter-rouge">-drain</code> only indirectly, once a NSAutoreleasePool expires.</p>
<p>Creating an pool in your code is never necessary, but it can be
a useful optimization, when you create lots of short lived objects. But you
can if you want with the <code class="language-plaintext highlighter-rouge">@autoreleasepool</code> directive. More on this later.</p>
<h3 id="thread-specific-pool-stacks">Thread-Specific Pool Stacks</h3>
<p>Each thread in mulle-objc maintains its own stack of autorelease pools. This
design ensures thread safety without the need for expensive locking mechanisms.</p>
<p><img src="images/thread_2-pool_1-obj_0.svg" alt="Pools And Threads" height="300" /></p>
<p>As shown above, Thread #1 and Thread #2 each manage their own pools
independently, preventing any cross-thread interference.
Each thread under mulle-objc control, even the main thread creates an
autorelease pool when it starts up.</p>
<blockquote>
<p>RULE: An object can only be messaged by a thread, if the object resides
in the autoreleasepool stack of that thread.</p>
</blockquote>
<p>This ensures that autoreleased objects always have a pool to be added to, even
in the absence of explicit <code class="language-plaintext highlighter-rouge">@autoreleasepool</code> directives.</p>
<h2 id="nested-pool-management">Nested Pool Management</h2>
<table>
<tbody>
<tr>
<td><img src="images/thread_1-pool_2-obj_0.svg" alt="Single Nested Pool" width="250" /></td>
<td><img src="images/thread_1-pool_2-obj_1.svg" alt="One Object Added" width="250" /></td>
</tr>
<tr>
<td>Pools can be nested within each other, creating a hierarchical structure for fine-grained memory management</td>
<td>Objects are always added to the innermost pool in the current thread’s stack.</td>
</tr>
</tbody>
</table>
<p>The next code snippet and the accompanying drwawing visualize how nested
autorelease pools collect and dispose of objects during program execution.</p>
<blockquote>
<p>NOTE: The <code class="language-plaintext highlighter-rouge">+object</code> method creates an instance and then calls <code class="language-plaintext highlighter-rouge">-autorelease</code>
on it.</p>
</blockquote>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@autoreleasepool</span>
<span class="p">{</span> <span class="c1">// pool 1 starts</span>
<span class="n">Document</span> <span class="o">*</span><span class="n">a</span><span class="p">;</span>
<span class="n">Document</span> <span class="o">*</span><span class="n">b</span><span class="p">;</span>
<span class="n">Document</span> <span class="o">*</span><span class="n">c</span><span class="p">;</span>
<span class="n">a</span> <span class="o">=</span> <span class="p">[</span><span class="n">Document</span> <span class="nf">object</span><span class="p">];</span>
<span class="n">b</span> <span class="o">=</span> <span class="p">[</span><span class="n">Document</span> <span class="nf">object</span><span class="p">];</span> <span class="c1">// #1#</span>
<span class="k">@autoreleasepool</span>
<span class="p">{</span> <span class="c1">// pool 2 starts</span>
<span class="n">c</span> <span class="o">=</span> <span class="p">[</span><span class="n">Document</span> <span class="nf">object</span><span class="p">];</span> <span class="c1">// #2</span>
<span class="c1">// c added to pool 2</span>
<span class="p">}</span> <span class="c1">// pool 2 drains, c released // #3</span>
<span class="c1">// #4</span>
<span class="n">mulle_printf</span><span class="p">(</span> <span class="s">"a %@ and b %@ are still alive</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">a</span><span class="p">);</span>
<span class="c1">// this would crash likely</span>
<span class="c1">// mulle_printf( "c %@ is dead\n", c);</span>
<span class="p">}</span> <span class="c1">// pool 1 drains</span>
</code></pre></div></div>
<p>When an autorelease pool is drained, it releases all the objects it contains.
This happens automatically at the end of an @autoreleasepool scope.</p>
<table>
<tbody>
<tr>
<td><img src="images/nested-pools-2.svg" alt="1" width="120" /> <code class="language-plaintext highlighter-rouge">#1</code></td>
<td><img src="images/nested-pools-3.svg" alt="2" width="120" /> <code class="language-plaintext highlighter-rouge">#2</code></td>
<td><img src="images/nested-pools-4.svg" alt="3" width="120" /> <code class="language-plaintext highlighter-rouge">#3</code></td>
<td><img src="images/nested-pools-2.svg" alt="4" width="120" /> <code class="language-plaintext highlighter-rouge">#4</code></td>
</tr>
</tbody>
</table>
<h3 id="creating-autoreleased-and-zeroed-c-memory">Creating autoreleased and zeroed C memory</h3>
<p>An autoreleasepool can also facilitate C coding. You can use
<code class="language-plaintext highlighter-rouge">MulleObjCCallocAutoreleased</code> to create an autoreleased and zeroed
block of memory:</p>
<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">p</span> <span class="o">=</span> <span class="n">MulleObjCCallocAutoreleased</span><span class="p">(</span> <span class="mi">1</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">(</span> <span class="k">struct</span> <span class="n">whatever</span><span class="p">));</span>
</code></pre></div></div>
<p>This is like a <code class="language-plaintext highlighter-rouge">calloc</code> you don’t have to free. Remember though, you can’
retain an allocation like this.</p>
<h3 id="caveat">Caveat</h3>
<p>While mulle-objc’s autorelease system offers great convenience, it’s important
to be aware of potential pitfalls. For instance, creating and keeping
to many autoreleased objects and never getting to a pool drain will lead to
increased memory usage.</p>
<p>This can happen for example in long-running operations, like a database import
from JSON for example. The remedy is to use <code class="language-plaintext highlighter-rouge">@autoreleasepool</code> blocks or
to manually drain pools once in a while.</p>
<h3 id="discussion-on-nsautoreleasepool-vs-garbage-collection">Discussion on NSAutoreleasePool vs Garbage Collection</h3>
<p>NSAutoreleasePool offers several advantages over garbage collection,
particularly in terms of performance and predictability.</p>
<ul>
<li><strong>Deterministic Memory Management</strong>: With NSAutoreleasePool, developers have
control over when memory is released, allowing for more predictable memory
usage patterns. In contrast, garbage collection can introduce latency as it
runs in the background, potentially leading to unpredictable pauses in
application performance.</li>
<li><strong>Lower Overhead</strong>: NSAutoreleasePool has a lower runtime overhead compared to
garbage collection. It does not require the additional resources needed to
track object references and manage cycles, which can be particularly
beneficial in resource-constrained environments.</li>
<li><strong>Fine-Grained Control</strong>: Developers can create and drain autorelease pools at
specific points in their code, allowing for fine-tuned memory management
strategies. This is especially useful in applications with varying memory
demands, where developers can optimize memory usage based on the current
workload.</li>
<li><strong>Control over Resource</strong>: Developers can release resources (like file
handles) when they want with <code class="language-plaintext highlighter-rouge">-mullePerformFinalize</code>, compared to garbage
collection which is unpredictable.</li>
</ul>
</div>
</article>
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