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<section id="extending-the-class">
<span id="extend-support"></span><h1>Extending the class<a class="headerlink" href="#extending-the-class" title="Link to this heading"></a></h1>
<nav class="contents local" id="contents">
<ul class="simple">
<li><p><a class="reference internal" href="#user-defined-axes" id="id1">User defined Axes</a></p></li>
</ul>
</nav>
<section id="user-defined-axes">
<h2><a class="toc-backref" href="#id1" role="doc-backlink">User defined Axes</a><a class="headerlink" href="#user-defined-axes" title="Link to this heading"></a></h2>
<p>In case you need a histogram with an axis of an arbitrary type that is not identified by
the C++ Standard Library, you need to provide an overload for the hashing function that is
used in the histogram class.</p>
<p>GIL’s histogram class uses <code class="docutils literal notranslate"><span class="pre">boost::hash_combine</span></code> in a sub routine to generate a hash from
the key.</p>
<p>So we need to provide an overload of <code class="docutils literal notranslate"><span class="pre">boost::hash_combine</span></code> for the purpose.</p>
<p>For example, let’s consider you need a histogram with an axis over class Test.</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="c1">// File : ./test.hpp</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;cstddef&gt;</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;functional&gt;</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Test</span>
<span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="p">{</span><span class="mi">0</span><span class="p">};</span>
<span class="w"> </span><span class="n">Test</span><span class="p">()</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">default</span><span class="p">;</span>
<span class="w"> </span><span class="n">Test</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">c</span><span class="p">)</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">a</span><span class="p">(</span><span class="n">c</span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="k">operator</span><span class="o">==</span><span class="p">(</span><span class="n">Test</span><span class="w"> </span><span class="k">const</span><span class="o">&amp;</span><span class="w"> </span><span class="n">other</span><span class="p">)</span><span class="w"> </span><span class="k">const</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">(</span><span class="n">a</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">other</span><span class="p">.</span><span class="n">a</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="k">namespace</span><span class="w"> </span><span class="nn">boost</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="kt">size_t</span><span class="w"> </span><span class="nf">hash_value</span><span class="p">(</span><span class="n">Test</span><span class="w"> </span><span class="k">const</span><span class="o">&amp;</span><span class="w"> </span><span class="n">t</span><span class="p">)</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// Replace with your hashing code</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">hash</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="w"> </span><span class="n">hasher</span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">hasher</span><span class="p">(</span><span class="n">t</span><span class="p">.</span><span class="n">a</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Now let’s get to the usage example.</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;test.hpp&gt;</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;boost/gil.hpp&gt;</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;iostream&gt;</span>
<span class="c1">// Mind the order of include, i.e. test.hpp before boost/gil.hpp</span>
<span class="k">using</span><span class="w"> </span><span class="k">namespace</span><span class="w"> </span><span class="nn">boost</span><span class="o">::</span><span class="nn">gil</span><span class="p">;</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">boost</span><span class="o">::</span><span class="n">gil</span><span class="o">::</span><span class="n">histogram</span><span class="o">&lt;</span><span class="n">Test</span><span class="o">&gt;</span><span class="w"> </span><span class="n">h</span><span class="p">;</span>
<span class="w"> </span><span class="n">Test</span><span class="w"> </span><span class="n">t</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span>
<span class="w"> </span><span class="n">h</span><span class="p">(</span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="o">&lt;&lt;</span><span class="n">h</span><span class="p">(</span><span class="n">t</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
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