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<h4>Nat!</h4>
<p>Senior Mull</p>
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<h2>De Re mulle-objc <div style="font-size:10px; display: inline-block;">0.50.0</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-mulle-objc/intro.html" title="De Re mulle-objc">De Re mulle-objc</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/modern.html" title="Project Setup">Project Setup</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/good.html" title="Good mulle-objc Code">Good mulle-objc Code</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/basics.html" title="A complete class">A complete class</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/classcluster.html" title="Advanced: Classcluster Classes">Advanced: Classcluster Classes</a>
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<h5> Or read it like a <a href="book.html"><b>book</b></a>.</h6>
</div> <!-- contents -->
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<div id="post-second" class="eight columns">
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<div>
<h2>De Re mulle-objc <div style="font-size:10px; display: inline-block;">0.50.0</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>Advanced: Classcluster Classes</h1>
<div>
<h2 id="untested-draft">UNTESTED DRAFT</h2>
<p>A classcluster is a fairly complicated setup, that is used to hide implementation details of various classes under one common name. <strong>mulle-objc</strong> simplifies the setup, but it is by no means simple.</p>
<p>As an example, we want to create a classcluster for a <a href="https://en.wikipedia.org/wiki/Bitset"><strong>BitSet</strong></a> class.
We assume, that in the majority of cases, the bitset will be empty. So as an optimization we want to provide a special
<strong>EmptyBitSet</strong> class, to conserve space. Otherwise</p>
<p>We also want to have a mutable variant <strong>MutableBitSet</strong>, that is a subclass of <strong>BitSet</strong> (and not the other way around,
like in Apple Foundation).</p>
<h2 id="the-base-class">The base class</h2>
<p>This is the “user” facing class. All other classes will be more or less hidden. This class defines the API.
It also adopts the mulle-objcClassCluster protocolclass.</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">BitSet</span> <span class="p">:</span> <span class="nc">NSObject</span> <span class="o"><</span> <span class="n">MulleObjCClassCluster</span><span class="o">></span>
<span class="k">-</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="nf">initWithBits</span><span class="p">:(</span><span class="n">NSUInteger</span> <span class="o">*</span><span class="p">)</span> <span class="n">bits</span>
<span class="n">count</span><span class="o">:</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">@end</span>
<span class="c1">// methods to be implemented by classcluster classes</span>
<span class="k">@interface</span> <span class="nc">BitSet</span><span class="p">(</span> <span class="nl">ClassCluster</span><span class="p">)</span>
<span class="k">-</span> <span class="p">(</span><span class="n">BOOL</span><span class="p">)</span> <span class="nf">boolAtIndex</span><span class="p">:(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">index</span><span class="p">;</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>The implementation now either produces an <strong>EmptyBitSet</strong> or a <strong>ConcreteBitSet</strong>, depending on all input
bits being clear or not:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#import "BitSet.h"
</span>
<span class="k">@implementation</span> <span class="nc">BitSet</span>
<span class="k">-</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="n">init</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="p">[[</span><span class="n">EmptyBitSet</span> <span class="nf">sharedInstance</span><span class="p">]</span> <span class="nf">retain</span><span class="p">]);</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="nf">initWithBits</span><span class="p">:(</span><span class="n">NSUInteger</span> <span class="o">*</span><span class="p">)</span> <span class="n">bits</span>
<span class="n">count</span><span class="o">:</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="n">NSUInteger</span> <span class="o">*</span><span class="n">p</span><span class="p">;</span>
<span class="n">NSUInteger</span> <span class="o">*</span><span class="n">sentinel</span><span class="p">;</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">bits</span><span class="p">;</span>
<span class="n">sentinel</span> <span class="o">=</span> <span class="o">&</span><span class="n">p</span><span class="p">[</span> <span class="nf">count</span><span class="p">];</span>
<span class="k">while</span><span class="p">(</span> <span class="n">p</span> <span class="o"><</span> <span class="n">sentinel</span><span class="p">)</span>
<span class="k">if</span><span class="p">(</span> <span class="o">*</span><span class="n">p</span><span class="o">++</span><span class="p">)</span>
<span class="k">return</span><span class="p">(</span> <span class="p">[</span><span class="n">ConcreteBitset</span> <span class="nf">newWithBits</span><span class="p">:</span><span class="n">bits</span>
<span class="nl">count:</span><span class="n">count</span><span class="p">]);</span>
<span class="k">return</span><span class="p">(</span> <span class="p">[[</span><span class="n">EmptyBitSet</span> <span class="nf">sharedInstance</span><span class="p">]</span> <span class="nf">retain</span><span class="p">]);</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<h4 id="some-notes-on-the-implementation">Some notes on the implementation.</h4>
<p>As we are implementing a classcluster, we know that the <code class="language-plaintext highlighter-rouge">-initWithBits:count:</code>
method is operating on a special kind of object, the placeholder. A placeholder
is a constant instance of <strong>BitSet</strong> in our case. A constant instance ignores
all <code class="language-plaintext highlighter-rouge">-retain</code>/<code class="language-plaintext highlighter-rouge">-release</code> calls, so we do not need to <code class="language-plaintext highlighter-rouge">-[self release]</code> in
<code class="language-plaintext highlighter-rouge">-initWithBits:count:</code> to avoid leaks.</p>
<p>We are using a shared instance for <strong>EmptyBitSet</strong> to reduce the footprint of
the application. Only one instance of this immutable bitset will be generated.</p>
<div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note:</b> </div>
<h2 id="the-concrete-classes">The concrete classes</h2>
<p>The <strong>EmptyBitSet</strong> is very simple, as the instance creation is done with +
<code class="language-plaintext highlighter-rouge">+sharedInstance</code> provided by <code class="language-plaintext highlighter-rouge">MulleObjCSingleton</code> already:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#import "BitSet.h"
</span>
<span class="k">@interface</span> <span class="nc">EmptyBitSet</span> <span class="p">:</span> <span class="nc">BitSet</span> <span class="o"><</span><span class="n">MulleObjCSingleton</span><span class="o">></span>
<span class="k">-</span> <span class="p">(</span><span class="n">BOOL</span><span class="p">)</span> <span class="nf">boolAtIndex</span><span class="p">:(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">index</span><span class="p">;</span>
<span class="k">@end</span>
</code></pre></div></div>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#import "EmptyBitSet.h"
</span>
<span class="k">@implementation</span> <span class="nc">EmptyBitSet</span>
<span class="k">-</span> <span class="p">(</span><span class="n">BOOL</span><span class="p">)</span> <span class="nf">boolAtIndex</span><span class="p">:(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">index</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="nb">NO</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#import "BitSet.h"
</span>
<span class="k">@interface</span> <span class="nc">ConcreteBitSet</span> <span class="p">:</span> <span class="nc">BitSet</span>
<span class="p">{</span>
<span class="n">NSUInteger</span> <span class="o">*</span><span class="n">_bits</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">BOOL</span><span class="p">)</span> <span class="nf">boolAtIndex</span><span class="p">:(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">index</span><span class="p">;</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>In the case of <strong>ConcreteBitSet</strong> we need to be aware that we are subclassing
the placeholder class <strong>BitSet</strong>. So <code class="language-plaintext highlighter-rouge">+alloc</code> will just produce the same
placeholder object that ‘self’ already is. We therefore need to implement the
instance creation and deallocation with primitive runtime functions ourselves:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#import "ConcreteBitSet.h"
</span>
<span class="k">@implementation</span> <span class="nc">ConcreteBitSet</span>
<span class="k">-</span> <span class="p">(</span><span class="n">id</span><span class="p">)</span> <span class="nf">newWithBits</span><span class="p">:(</span><span class="n">NSUInteger</span> <span class="o">*</span><span class="p">)</span> <span class="n">bits</span>
<span class="n">count</span><span class="o">:</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="n">ConcreteBitSet</span> <span class="o">*</span><span class="n">set</span><span class="p">;</span>
<span class="kt">size_t</span> <span class="n">size</span><span class="p">;</span>
<span class="n">set</span> <span class="o">=</span> <span class="n">NSAllocateObject</span><span class="p">(</span> <span class="n">self</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">);</span>
<span class="n">size</span> <span class="o">=</span> <span class="k">sizeof</span><span class="p">(</span> <span class="n">NSUInteger</span><span class="p">)</span> <span class="o">*</span> <span class="n">count</span><span class="p">;</span>
<span class="n">set</span><span class="o">-></span><span class="n">_bits</span> <span class="o">=</span> <span class="n">mulle</span><span class="o">-</span><span class="n">objcObjectAllocateNonZeroedMemory</span><span class="p">(</span> <span class="n">set</span><span class="p">,</span> <span class="n">size</span><span class="p">);</span>
<span class="n">set</span><span class="o">-></span><span class="n">_count</span> <span class="o">=</span> <span class="n">count</span><span class="p">;</span>
<span class="n">memcpy</span><span class="p">(</span> <span class="n">set</span><span class="o">-></span><span class="n">_bits</span><span class="p">,</span> <span class="n">bits</span><span class="p">,</span> <span class="n">size</span><span class="p">);</span>
<span class="k">return</span><span class="p">(</span> <span class="n">set</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="n">MulleObjCObjectDeallocateMemory</span><span class="p">(</span> <span class="n">self</span><span class="o">-></span><span class="n">_bits</span><span class="p">);</span>
<span class="n">NSDeallocateObject</span><span class="p">(</span> <span class="n">self</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="n">BOOL</span><span class="p">)</span> <span class="nf">boolAtIndex</span><span class="p">:(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">index</span>
<span class="p">{</span>
<span class="n">NSUInteger</span> <span class="n">i</span><span class="p">;</span>
<span class="n">NSUInteger</span> <span class="n">bit</span><span class="p">;</span>
<span class="n">i</span> <span class="o">=</span> <span class="n">index</span> <span class="o">/</span> <span class="p">(</span><span class="k">sizeof</span><span class="p">(</span> <span class="n">NSUInteger</span><span class="p">)</span> <span class="o">*</span> <span class="n">CHAR_BIT</span><span class="p">);</span>
<span class="n">bit</span> <span class="o">=</span> <span class="mi">1</span> <span class="o"><<</span> <span class="p">(</span><span class="n">index</span> <span class="o">&</span> <span class="p">(</span><span class="k">sizeof</span><span class="p">(</span> <span class="n">NSUInteger</span><span class="p">)</span> <span class="o">*</span> <span class="n">CHAR_BIT</span> <span class="o">-</span> <span class="mi">1</span><span class="p">));</span>
<span class="k">if</span><span class="p">(</span> <span class="n">i</span> <span class="o">>=</span> <span class="n">_count</span><span class="p">)</span>
<span class="k">return</span><span class="p">(</span> <span class="nb">NO</span><span class="p">);</span>
<span class="k">return</span><span class="p">(</span> <span class="n">_bits</span><span class="p">[</span> <span class="nf">i</span><span class="p">]</span> <span class="o">&</span> <span class="n">bit</span> <span class="p">?</span> <span class="nb">YES</span> <span class="p">:</span> <span class="nb">NO</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<h2 id="adding-a-class-to-a-classcluster">Adding a class to a classcluster</h2>
<p>You chance upon the <a href="https://github.com/CountOnes/hamming_weight">CountOnes</a>
project and its AVX2 implementation and would like to support it with
a class <code class="language-plaintext highlighter-rouge">ConcreteAVX2Bitset</code> for larger bitsets, when AVX2 is available.</p>
<p>You could then expand the current init method in <strong>BitSet</strong>:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">-</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="nf">initWithBits</span><span class="p">:(</span><span class="n">NSUInteger</span> <span class="o">*</span><span class="p">)</span> <span class="n">bits</span>
<span class="n">count</span><span class="o">:</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="n">NSUInteger</span> <span class="o">*</span><span class="n">p</span><span class="p">;</span>
<span class="n">NSUInteger</span> <span class="o">*</span><span class="n">sentinel</span><span class="p">;</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">bits</span><span class="p">;</span>
<span class="n">sentinel</span> <span class="o">=</span> <span class="o">&</span><span class="n">p</span><span class="p">[</span> <span class="nf">count</span><span class="p">];</span>
<span class="k">while</span><span class="p">(</span> <span class="n">p</span> <span class="o"><</span> <span class="n">sentinel</span><span class="p">)</span>
<span class="k">if</span><span class="p">(</span> <span class="o">*</span><span class="n">p</span><span class="o">++</span><span class="p">)</span>
<span class="k">if</span><span class="p">(</span> <span class="n">count</span> <span class="o">>=</span> <span class="mi">16</span><span class="p">)</span>
<span class="k">return</span><span class="p">(</span> <span class="p">[</span><span class="n">ConcreteAVX2Bitset</span> <span class="nf">newWithBits</span><span class="p">:</span><span class="n">bits</span>
<span class="nl">count:</span><span class="n">count</span><span class="p">]);</span>
<span class="k">else</span>
<span class="k">return</span><span class="p">(</span> <span class="p">[</span><span class="n">ConcreteBitset</span> <span class="nf">newWithBits</span><span class="p">:</span><span class="n">bits</span>
<span class="nl">count:</span><span class="n">count</span><span class="p">]);</span>
<span class="k">return</span><span class="p">(</span> <span class="p">[[</span><span class="n">EmptyBitSet</span> <span class="nf">sharedInstance</span><span class="p">]</span> <span class="nf">retain</span><span class="p">]);</span>
<span class="p">}</span>
</code></pre></div></div>
<p>Or you could add a new <code class="language-plaintext highlighter-rouge">-initWithAVX2Bits:count:</code> method. The advantage of using another <code class="language-plaintext highlighter-rouge">-init</code>
methods is, that you can add it to your classcluster using a category:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@implementation</span> <span class="nc">BitSet</span><span class="p">(</span> <span class="nl">ConcreteAVX2Bitset</span><span class="p">)</span>
<span class="k">-</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="nf">initWithAVX2Bits</span><span class="p">:(</span><span class="n">NSUInteger</span> <span class="o">*</span><span class="p">)</span> <span class="n">bits</span>
<span class="n">count</span><span class="o">:</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="p">[</span><span class="n">ConcreteAVX2Bitset</span> <span class="nf">newWithBits</span><span class="p">:</span><span class="n">bits</span>
<span class="nl">count:</span><span class="n">count</span><span class="p">]);</span>
<span class="p">}</span>
</code></pre></div></div>
<h2 id="subclassing-a-classcluster">Subclassing a classcluster</h2>
<p>The main obstacle to subclassing a classcluster is the reimplementation of the
instance allocation methods. You should reimplement the following methods in
your subclass:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">+</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="n">alloc</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="n">NSAllocateObject</span><span class="p">(</span> <span class="n">self</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">));</span>
<span class="p">}</span>
<span class="k">+</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="nf">allocWithZone</span><span class="p">:(</span><span class="n">NSZone</span> <span class="o">*</span><span class="p">)</span> <span class="n">zone</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="n">NSAllocateObject</span><span class="p">(</span> <span class="n">self</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">));</span>
<span class="p">}</span>
<span class="k">+</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="n">new</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="p">[</span><span class="n">NSAllocateObject</span><span class="p">(</span> <span class="n">self</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">)</span> <span class="nf">init</span><span class="p">]);</span>
<span class="p">}</span>
</code></pre></div></div>
<p>Now your subclass and its subclass will create instances of the proper class.
But you will also need to override the init functions of the classcluster. In
the case of <strong>BitSet</strong> there is only one <code class="language-plaintext highlighter-rouge">-initWithBits:count:</code>, which makes
this easy.</p>
<h2 id="classcluster-on-top-of-classcluster">Classcluster on top of classcluster</h2>
<p>It’s entirely possible to create a classcluster on top of another classcluster,
as we will show with <strong>MutableBitSet</strong>.</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#import "BitSet.h"
</span>
<span class="k">@interface</span> <span class="nc">MutableBitSet</span> <span class="p">:</span> <span class="nc">BitSet</span> <span class="o"><</span><span class="n">MulleObjCClassCluster</span><span class="o">></span>
<span class="k">-</span> <span class="p">(</span><span class="n">BOOL</span><span class="p">)</span> <span class="nf">setBool</span><span class="p">:(</span><span class="n">BOOL</span><span class="p">)</span> <span class="n">flag</span>
<span class="n">atIndex</span><span class="o">:</span><span class="p">(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">index</span><span class="p">;</span>
<span class="k">-</span> <span class="p">(</span><span class="n">BOOL</span><span class="p">)</span> <span class="nf">boolAtIndex</span><span class="p">:(</span><span class="n">NSUInteger</span><span class="p">)</span> <span class="n">index</span><span class="p">;</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>The main thing we will have to override is the init method, as we will have to
use different classes. Because I am extremely lazy, I will restrict the code to
just one class:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#import "MutableBitSet.h"
</span>
<span class="k">@implementation</span> <span class="nc">MutableBitSet</span>
<span class="k">-</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="nf">initWithBits</span><span class="p">:(</span><span class="n">NSUInteger</span> <span class="o">*</span><span class="p">)</span> <span class="n">bits</span>
<span class="n">count</span><span class="o">:</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="p">[</span><span class="n">ConcreteMutableBitset</span> <span class="nf">newWithBits</span><span class="p">:</span><span class="n">bits</span>
<span class="nl">count:</span><span class="n">count</span><span class="p">]);</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
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