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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>AAO: Always autoreleased objects</h1>
<div>
<p>In mulle-objc the <em>expectation</em> is that all objects are created autoreleased.</p>
<blockquote>
<p>RULE: Objects are passed autoreleased to functions and methods and objects
are also returned autoreleased from functions and methods.</p>
</blockquote>
<p>When you <code class="language-plaintext highlighter-rouge">-autorelease</code> an object, typically during creation, you are tieing
the lifetime of your object to the lifetime of the current autorelease pool.
This is an <strong>explicitly</strong> autoreleased object, as the autorelease
pool contains a pointer to it.</p>
<p>Your object is now guaranteed to live at least as long as the autorelease pool.
It will live longer, if it is directly or indirectly part of another autorelease
pool.</p>
<p>In a method, you know the incoming objects are autoreleased. So you don’t
<code class="language-plaintext highlighter-rouge">-release</code> or <code class="language-plaintext highlighter-rouge">-autorelease</code> them. If you want to return a new object,
you return an autoreleased object only.</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@implementation</span> <span class="nc">Document</span>
<span class="k">+</span> <span class="p">(</span><span class="n">Document</span> <span class="o">*</span><span class="p">)</span> <span class="nf">improvedDocument</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="p">{</span>
<span class="n">Document</span> <span class="o">*</span><span class="n">improved</span><span class="p">;</span>
<span class="k">if</span><span class="p">(</span> <span class="p">[</span><span class="n">document</span> <span class="nf">isPerfect</span><span class="p">])</span>
<span class="k">return</span><span class="p">(</span> <span class="n">document</span><span class="p">);</span> <span class="c1">// incoming is autoreleased, outgoing therefore too</span>
<span class="n">improved</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">// do something smart ...</span>
<span class="p">[</span><span class="n">improved</span> <span class="nf">setPerfect</span><span class="p">:</span><span class="nb">YES</span><span class="p">];</span>
<span class="k">return</span><span class="p">(</span> <span class="n">document</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">...</span>
</code></pre></div></div>
<p>To keep an object alive longer, another objects needs to <code class="language-plaintext highlighter-rouge">-retain</code> it,
to obtain ownership. As the owning object is oblidged to <code class="language-plaintext highlighter-rouge">-autorelease</code> it
before the death of the owner, it will need to keep a reference to this object
for as long as it needs to hold unto it.</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">Person</span> <span class="p">:</span> <span class="nc">NSObject</span>
<span class="k">@end</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="k">@public</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">author</span>
<span class="p">{</span>
<span class="p">[</span><span class="n">_author</span> <span class="nf">autorelease</span><span class="p">];</span>
<span class="n">_author</span> <span class="o">=</span> <span class="p">[</span><span class="n">author</span> <span class="nf">retain</span><span class="p">];</span>
<span class="p">}</span>
<span class="k">@end</span>
<span class="kt">int</span> <span class="nf">main</span><span class="p">(</span> <span class="kt">void</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">Document</span> <span class="o">*</span><span class="n">document</span><span class="p">;</span>
<span class="n">Person</span> <span class="o">*</span><span class="n">person</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">object</span><span class="p">];</span>
<span class="k">@autoreleasepool</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="nf">object</span><span class="p">];</span>
<span class="p">[</span><span class="n">document</span> <span class="nf">setPerson</span><span class="p">:</span><span class="n">person</span><span class="p">];</span>
<span class="c1">// #1</span>
<span class="p">}</span>
<span class="c1">// #2</span>
<span class="n">mulle_printf</span><span class="p">(</span> <span class="s">"%@</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">document</span><span class="o">-></span><span class="n">_author</span><span class="p">);</span>
<span class="p">[</span><span class="n">document</span> <span class="nf">setPerson</span><span class="p">:</span><span class="nb">nil</span><span class="p">];</span>
<span class="c1">// #3</span>
<span class="k">return</span><span class="p">(</span> <span class="mi">0</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div></div>
<p>The drawings show the fate of the <em>person</em> object:</p>
<table>
<tbody>
<tr>
<td><img src="images/pool-object-ownership-3.svg" alt="1" width="120" /> <code class="language-plaintext highlighter-rouge">#1</code> <strong>explicit</strong></td>
<td><img src="images/pool-object-ownership-7.svg" alt="2" width="120" /> <code class="language-plaintext highlighter-rouge">#2</code> <strong>implicit</strong></td>
<td><img src="images/pool-object-ownership-8.svg" alt="6" width="120" /> <code class="language-plaintext highlighter-rouge">#3</code> <strong>explicit</strong></td>
</tr>
<tr>
<td><em>person</em> is created in an inferior pool and referenced by <em>document</em></td>
<td>The inferior pool is gone, <em>person</em> is still alive through a <code class="language-plaintext highlighter-rouge">-retain</code> by <em>document</em></td>
<td><em>document</em> relinquishes ownership with <code class="language-plaintext highlighter-rouge">-autorelease</code> of <em>person</em></td>
</tr>
</tbody>
</table>
<p>Your retained object is now an <strong>implicitly</strong> autoreleased object. When the
owner cuts the reference, it will <code class="language-plaintext highlighter-rouge">-autorelease</code> your object, therefore turning
it back into an <strong>expliticly</strong> autoreleased object.</p>
<p>These basic relationships form the foundation of mulle-objc’s object ownership
convention.</p>
<blockquote>
<p>RULE: The only times that <code class="language-plaintext highlighter-rouge">-release</code> calls are made on an object</p>
<ol>
<li>if it is not part of any autorelease pool either explicitly or implicitly</li>
<li>during the death of an autorelease pool (including <code class="language-plaintext highlighter-rouge">-dealloc</code>)</li>
<li>in the scope of a function or method, if each <code class="language-plaintext highlighter-rouge">release</code> undoes a preceeding <code class="language-plaintext highlighter-rouge">-retain</code> in the same function and the object is no longer accessed or returned after the last <code class="language-plaintext highlighter-rouge">release</code>.</li>
</ol>
</blockquote>
<h3 id="not-all-objects-are-autoreleased-in-mulle-objc">Not all objects are autoreleased in mulle-objc</h3>
<p>The <code class="language-plaintext highlighter-rouge">NSThread</code> objects typically do not reside in autorelease pools. Very short
lived objects may never enter an autorelease pool. Static <code class="language-plaintext highlighter-rouge">NSString</code> objects
are never placed in an autorelease pool. Doing a <code class="language-plaintext highlighter-rouge">-autorelease</code> on a class or a
<a href="//www.mulle-kybernetik.com/weblog/2016/mulle_objc_boon_or_bane.html">TPS object</a>
does nothing and there can be more special cases.</p>
<p>Nevertheless as long as these objects are “static” or permanent, they count
as “implicitly” autoreleased, belonging to a virtual universe autoreleasepool.
If at any time they would lose their permanence, they would need to be placed
into a true autorelease pool.</p>
<blockquote>
<p>This was probably the most important chapter of this guide. Always autoreleased
objects are the philosophical foundation of <a href="//mulle-objc.github.io">mulle-objc</a>.</p>
</blockquote>
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var u="//www.mulle-kybernetik.com/matomo/";
_paq.push(['setTrackerUrl', u+'matomo.php']);
_paq.push(['setSiteId', '1']);
var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0];
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<noscript><p><img src="//www.mulle-kybernetik.com/matomo/matomo.php?idsite=1&rec=1" style="border:0;" alt="" /></p></noscript>
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</html>