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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: -finalize</h1>
<div>
<p>One of the most intriguing aspects of <a href="//mulle-objc.github.io">mulle-objc</a>’s memory management is the
<code class="language-plaintext highlighter-rouge">-finalize</code> method. This method serves a crucial role in resource management,
particularly for cleaning up external resources like file handles. It’s
guaranteed to run only once in an object’s lifetime, either through <code class="language-plaintext highlighter-rouge">-release</code>
or <code class="language-plaintext highlighter-rouge">-mullePerformFinalize</code>. Here’s how you might implement it:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@implementation</span> <span class="nc">Person</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">finalize</span>
<span class="p">{</span>
<span class="k">if</span><span class="p">(</span> <span class="n">_fileHandle</span> <span class="o">!=</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">close</span><span class="p">(</span> <span class="n">_fileHandle</span><span class="p">);</span>
<span class="n">_fileHandle</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">[</span><span class="n">super</span> <span class="nf">finalize</span><span class="p">];</span> <span class="c1">// release properties</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">dealloc</span>
<span class="p">{</span>
<span class="p">[</span><span class="n">_address</span> <span class="nf">release</span><span class="p">];</span>
<span class="p">[</span><span class="n">super</span> <span class="nf">dealloc</span><span class="p">];</span> <span class="c1">// always call super last</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>Notice how <code class="language-plaintext highlighter-rouge">-finalize</code> handles resource cleanup, while <code class="language-plaintext highlighter-rouge">-dealloc</code> focuses on
releasing retained objects. This separation of concerns allows for more robust
and predictable cleanup processes.</p>
<h2 id="-mulleperformfinalize-runs--finalize-on-demand">-mullePerformFinalize runs -finalize on demand</h2>
<p>You can use <code class="language-plaintext highlighter-rouge">-mullePerformFinalize</code> to finalize an object “manually”:</p>
<p><img src="images/object-finalize.svg" alt="" /></p>
<p>The finalization can happen in the “middle of the lifetime” of the object.
And it is guaranteed that <code class="language-plaintext highlighter-rouge">-finalize</code> is only called once.</p>
<blockquote>
<p>LAW: Never call <code class="language-plaintext highlighter-rouge">-finalize</code> directly, always use <code class="language-plaintext highlighter-rouge">-mullePerformFinalize</code></p>
</blockquote>
<p>A finalized object is still useable in the object hierarchy,
but it is not active anymore. An example, where this is useful, is a window
controller, where the window close button has been clicked. It may still
redraw, but it doesn’t react to any event actions any more.</p>
<h2 id="-finalize-runs-automatically-before--dealloc">-finalize runs automatically before -dealloc</h2>
<p>When the <code class="language-plaintext highlighter-rouge">-retainCount</code> is decremented to zero via <code class="language-plaintext highlighter-rouge">-release</code>, an object gets
the <code class="language-plaintext highlighter-rouge">-finalize</code> message, unless <code class="language-plaintext highlighter-rouge">-mullePerformFinalize</code> has been called on it
already:</p>
<p><img src="images/object-lifetime.svg" alt="" /></p>
<p>If the <code class="language-plaintext highlighter-rouge">-retainCount</code> remains unchanged throughout <code class="language-plaintext highlighter-rouge">-finalize</code>, then <code class="language-plaintext highlighter-rouge">-dealloc</code>
is called afterwards.</p>
<p>During <code class="language-plaintext highlighter-rouge">-finalize</code> all <a href="//www.mulle-kybernetik.com/de-re-property/">@properties</a> will be cleared. So <code class="language-plaintext highlighter-rouge">-finalize</code> is used to free resources and cut links to other objects. Objects that are cleared by <code class="language-plaintext highlighter-rouge">-finalize</code> will be released with <code class="language-plaintext highlighter-rouge">-autorelease</code> and not with <code class="language-plaintext highlighter-rouge">-release</code>. <code class="language-plaintext highlighter-rouge">-dealloc</code> will ideally at most contain <code class="language-plaintext highlighter-rouge">-release</code> calls and <code class="language-plaintext highlighter-rouge">[super dealloc]</code>. Anything else can be done in <code class="language-plaintext highlighter-rouge">-finalize</code>.</p>
<h2 id="important--finalize-considerations">Important -finalize considerations</h2>
<p><code class="language-plaintext highlighter-rouge">-finalize</code> can happen at any time, so the rules differ from those of <code class="language-plaintext highlighter-rouge">-dealloc</code>
which is guaranteed to run only single-threaded with a retainCount of 1. In
contrast to <code class="language-plaintext highlighter-rouge">-dealloc</code>:</p>
<ol>
<li>Use <code class="language-plaintext highlighter-rouge">-autorelease</code> to release instance variables and <code class="language-plaintext highlighter-rouge">nil</code> them</li>
<li>You can use using property setters for the release of instance variables.</li>
<li>Call <code class="language-plaintext highlighter-rouge">[super finalize]</code> when convenient, or omit if you handle all properties
yourself</li>
</ol>
<h2 id="you-can-write--finalize-dealloc-portably">You can write -finalize/-dealloc portably</h2>
<p>If you use <code class="language-plaintext highlighter-rouge">-finalize</code>, you will be incompatible with non-ARC Apple. This can be
remedied, by structuring your <code class="language-plaintext highlighter-rouge">-finalize/-dealloc</code> code like this:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">_finalize</span>
<span class="p">{</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">finalize</span>
<span class="p">{</span>
<span class="p">[</span><span class="n">self</span> <span class="nf">_finalize</span><span class="p">];</span>
<span class="p">[</span><span class="n">super</span> <span class="nf">finalize</span><span class="p">];</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">dealloc</span>
<span class="p">{</span>
<span class="cp">#ifndef __MULLE_OBJC__
</span> <span class="p">[</span><span class="n">self</span> <span class="nf">_finalize</span><span class="p">];</span>
<span class="cp">#endif
</span> <span class="p">[</span><span class="n">super</span> <span class="nf">dealloc</span><span class="p">];</span>
<span class="p">}</span>
</code></pre></div></div>
<h2 id="caveat">Caveat</h2>
<p><code class="language-plaintext highlighter-rouge">-finalize</code> is single-threaded, just like <code class="language-plaintext highlighter-rouge">-init</code> and <code class="language-plaintext highlighter-rouge">-dealloc</code> when called
during <code class="language-plaintext highlighter-rouge">release</code>. When you invoke <code class="language-plaintext highlighter-rouge">-mullePerformFinalize</code> it can only be
guaranteed, that no other thread will be executing <code class="language-plaintext highlighter-rouge">-finalize</code> (ever). But it
is <em>not guaranteed</em> that no other thread is accessing the object. That’s
different to <code class="language-plaintext highlighter-rouge">-dealloc</code>, where the guarantee is that no other thread will
ever call this object again.</p>
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