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<!doctype html>
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<title>@property definition – De Re @property</title>
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<h4>Nat!</h4>
<p>Senior Mull</p>
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<h2>De Re @property <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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<h6>© 2025 by Nat!, Mulle kybernetiK. All rights reserved.</h6>
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<h4>Contents</h4>
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<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-property/intro.html" title="Intro">Intro</a>
</article>
<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-property/declaration.html" title="@property declaration">@property declaration</a>
</article>
<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-property/attributes.html" title="@property( attributes)">@property( attributes)</a>
</article>
<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-property/definition.html" title="@property definition">@property definition</a>
</article>
<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-property/access.html" title="@property access">@property access</a>
</article>
<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-property/runtime.html" title="@property signature">@property signature</a>
</article>
<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-property/class-inheritance.html" title="@property inheritance by class">@property inheritance by class</a>
</article>
<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-property/protocol-inheritance.html" title="@property inheritance by protocol">@property inheritance by protocol</a>
</article>
<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-property/protocolclass-inheritance.html" title="@property inheritance by protocolclass">@property inheritance by protocolclass</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-property/redeclaration.html" title="@property redeclaration">@property redeclaration</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-property/synthesize-directive.html" title="@synthesize">@synthesize</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-property/dynamic-directive.html" title="@dynamic">@dynamic</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-property/outro.html" title="Outro">Outro</a>
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<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-property/assign.html" title="Reference: assign">Reference: assign</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-property/observable.html" title="Reference: observable">Reference: observable</a>
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</article>
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<li class="no-bullet"><a class="internal-link" href="/de-re-property/serializable.html" title="Reference: serializable">Reference: serializable</a>
</article>
</ul>
<h5> Or read it like a <a href="book.html"><b>book</b></a>.</h6>
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<div id="post-second" class="eight columns">
<div class="title">
<div>
<h2>De Re @property <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>
</div>
<h6>© 2025 by Nat!, Mulle kybernetiK. All rights reserved.</h6>
<p>
</div> <!-- title -->
<p>
<div id="main" role="main">
<article>
<h1>@property definition</h1>
<div>
<p>You don’t need to write any code in the <code class="language-plaintext highlighter-rouge">@implementation</code> of your class. But you of course do
need to define the <code class="language-plaintext highlighter-rouge">@implementation</code> somewhere:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">MyClass</span> <span class="p">:</span> <span class="nc">NSObject</span>
<span class="k">@property</span><span class="p">(</span> <span class="n">assign</span><span class="p">)</span> <span class="n">NSUInteger</span> <span class="n">count</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">MyClass</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>The compiler will take note of <code class="language-plaintext highlighter-rouge">@property( assign) NSUInteger count;</code> in the <code class="language-plaintext highlighter-rouge">@interface</code> section
and already add an instance variable of the same name (prefixed with <code class="language-plaintext highlighter-rouge">_</code>)
to the list of instance variables of the class.</p>
<p>The compiler will then declare two accessor methods <code class="language-plaintext highlighter-rouge">- (NSUInteger) count</code> for
retrieving the instance variable value and <code class="language-plaintext highlighter-rouge">- (void) setCount:(NSUInteger)value</code> for setting it.</p>
<p>Once the compiler sees the corresponding <code class="language-plaintext highlighter-rouge">@implementation</code>, it will actually generate code
for these two accessor methods.</p>
<p>In essence a <code class="language-plaintext highlighter-rouge">@property</code> is the combination of</p>
<ol>
<li>A protected “Instance Variable” (ivar):
<ul>
<li>The actual memory location storing the value</li>
<li>The ivar name is the property name, prefixed with an underscore (<code class="language-plaintext highlighter-rouge">_</code>)</li>
<li>Direct access possible within the accessor implementation, but <code class="language-plaintext highlighter-rouge">@protected</code> from the outside</li>
</ul>
</li>
<li>A “Getter Method”:
<ul>
<li>Returns the value of the ivar</li>
<li>Usually named exactly like the property</li>
</ul>
</li>
<li>A “Setter Method”:
<ul>
<li>Sets the value of the ivar</li>
<li>Its name is the property name capitalized and prefixed with set</li>
</ul>
</li>
</ol>
<p>So writing:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">MyClass</span> <span class="p">:</span> <span class="nc">NSObject</span>
<span class="k">@property</span><span class="p">(</span> <span class="n">assign</span><span class="p">)</span> <span class="n">NSUInteger</span> <span class="n">count</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">MyClass</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>is almost equivalent to:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">MyClass</span> <span class="p">:</span> <span class="nc">NSObject</span>
<span class="p">{</span>
<span class="n">NSUInteger</span> <span class="n">_count</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">count</span><span class="p">;</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="nf">setCount</span><span class="p">:(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">value</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">MyClass</span>
<span class="k">-</span> <span class="p">(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">count</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="n">_count</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="nf">setCount</span><span class="p">:(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">value</span>
<span class="p">{</span>
<span class="n">_count</span> <span class="o">=</span> <span class="n">value</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>Why only <em>almost equivalent</em> ? That is, because the compiler will also generate additional runtime
information for the property. At runtime you can query for the property name, and
can easily get the associated setter and getter methods, which is a major optimization
for key value coding (KVC). You can also query the properties declared attributes at runtime, which
are encoded in the <a href="runtime.html">signature</a>.</p>
</div>
</article>
</div>
<p>
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