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inheritance.html
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<h1>Inheritance</h1>
<div>
<h2 id="syntax-topic">Syntax topic</h2>
<div class="language-objective-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">SomeClass</span> <span class="p">:</span> <span class="nc">SomeSuperClass</span>
<span class="k">@end</span>
</code></pre></div></div>
<h2 id="inheritance">Inheritance</h2>
<p>In terms of a C <code class="language-plaintext highlighter-rouge">struct</code>, inheritance is akin to nesting a struct within another
struct:</p>
<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">struct</span> <span class="n">a</span>
<span class="p">{</span>
<span class="kt">int</span> <span class="n">v</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">struct</span> <span class="n">b</span>
<span class="p">{</span>
<span class="k">struct</span> <span class="n">a</span> <span class="n">a</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">w</span><span class="p">;</span>
<span class="p">};</span>
</code></pre></div></div>
<p>The Objective-C syntax <code class="language-plaintext highlighter-rouge">@interface <class> : <otherClass></code> is used
to define the inheritance:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">A</span>
<span class="p">{</span>
<span class="kt">int</span> <span class="n">u</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">v</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getU</span><span class="p">;</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getV</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">A</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getU</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="n">u</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getV</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="n">v</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">B</span> <span class="p">:</span> <span class="nc">A</span>
<span class="p">{</span>
<span class="kt">int</span> <span class="n">w</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getW</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">B</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getV</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="p">[</span><span class="n">super</span> <span class="nf">getU</span><span class="p">]</span> <span class="o">+</span> <span class="p">[</span><span class="n">super</span> <span class="nf">getV</span><span class="p">]);</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getW</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="n">w</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>The runtime will now look like this:</p>
<p><img src="images/inheritance.svg" alt="Inheritance" />There is a uni-directional relationship <strong>super</strong> between the class B and its <strong>superclass</strong> A..</p>
<p>The instance variables and methods of class A are <em>inherited</em> by B.As you can see, the contents of the ivarlists and methodlists of A are not copied down to B.Nevertheless, all methods and instance variables of A are now available in B.</p>
<h3 id="super">super</h3>
<p>With the special <code class="language-plaintext highlighter-rouge">[super method]</code> syntax, a method of a superclass can be
invoked, even if there is a method with the same name in the current class.</p>
<h3 id="special-case-the-root-class">Special case: the root class</h3>
<p>A root class is a class that does not have a superclass. There is a special
treatment for ‘+’ class methods on these root classes.</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">RootClass</span>
<span class="k">+</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">classMethod</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">instanceMethod</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">RootClass</span>
<span class="k">+</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">classMethod</span>
<span class="p">{</span>
<span class="n">printf</span><span class="p">(</span> <span class="s">"%s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">__FUNCTION__</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">instanceMethod</span>
<span class="p">{</span>
<span class="n">printf</span><span class="p">(</span> <span class="s">"%s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">__FUNCTION__</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>When you run</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">[</span><span class="n">cls</span> <span class="nf">classMethod</span><span class="p">];</span>
<span class="p">[</span><span class="n">cls</span> <span class="nf">instanceMethod</span><span class="p">];</span>
<span class="p">[</span><span class="n">instance</span> <span class="nf">instanceMethod</span><span class="p">];</span>
<span class="p">[</span><span class="n">instance</span> <span class="nf">classMethod</span><span class="p">];</span>
</code></pre></div></div>
<p>the output will be:</p>
<div class="language-console highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="go">+[RootClass classMethod]
-[RootClass instanceMethod]
-[RootClass instanceMethod]
mulle_objc_universe 0x5B74C1D11080 fatal: unknown method c3074566 "-classMethod" in class 4b9217f9 "RootClass"
</span></code></pre></div></div>
<p>So what is going on here?</p>
<p><img src="images/RootClass.svg" alt="Root class" /></p>
<p>If you follow the <code class="language-plaintext highlighter-rouge">isa/meta</code> path, you will see that the metaclass of the
metaclass as actually the infraclass again. So a rootclass class methods
are augmented with the infraclass methods.</p>
<p>This is something to be aware of. So root classes usually have
a class method implementation for every instance method to avoid surprises.
Another option is to put a empty root class on top.</p>
<hr />
<h2 id="api">API</h2>
<table>
<thead>
<tr>
<th>Function/Method Name</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code class="language-plaintext highlighter-rouge">-superclass</code></td>
<td>Get receiver’s superclass</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">+superclass</code></td>
<td>Get class’s superclass</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">+isSubclassOfClass:</code></td>
<td>Check if class inherits from given class</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-isKindOfClass:</code></td>
<td>Check if receiver inherits from or is instance of class</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-isMemberOfClass:</code></td>
<td>Check if receiver is direct instance of class</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">[super ...]</code></td>
<td>Language feature to call superclass implementation</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCClassIsSubclassOfClass</code></td>
<td>C function to check class inheritance relationship</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">NSObjectIsKindOfClass</code></td>
<td>C function for instance/class relationship check</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCInstanceIsMemberOfClass</code></td>
<td>C function for direct instance/class check</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCSuperIMP</code></td>
<td>Get superclass implementation of current method</td>
</tr>
</tbody>
</table>
</div>
</article>
</div>
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