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  68   <div class="section" id="technicalities">
  69 <h1>Technicalities</h1>
  70 <div class="contents local topic" id="contents">
  71 <ul class="simple">
  72 <li><a class="reference internal" href="#creating-a-reference-proxy" id="id1">Creating a reference proxy</a></li>
  73 </ul>
  74 </div>
  75 <div class="section" id="creating-a-reference-proxy">
  76 <h2><a class="toc-backref" href="#id1">Creating a reference proxy</a></h2>
  77 <p>Sometimes it is necessary to create a proxy class that represents a
  78 reference to a given object. Examples of these are GIL&#8217;s reference to
  79 a planar pixel (<code class="docutils literal"><span class="pre">planar_pixel_reference</span></code>) and GIL&#8217;s sub-byte channel
  80 references. Writing a reference proxy class can be tricky. One problem
  81 is that the proxy reference is constructed as a temporary object and
  82 returned by value upon dereferencing the iterator:</p>
  83 <div class="highlight-cpp"><div class="highlight"><pre><span class="k">struct</span> <span class="n">rgb_planar_pixel_iterator</span>
  84 <span class="p">{</span>
  85  <span class="k">typedef</span> <span class="n">my_reference_proxy</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="n">reference</span><span class="p">;</span>
  86  <span class="n">reference</span> <span class="k">operator</span><span class="o">*</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="n">reference</span><span class="p">(</span><span class="n">red</span><span class="p">,</span><span class="n">green</span><span class="p">,</span><span class="n">blue</span><span class="p">);</span> <span class="p">}</span>
  87 <span class="p">};</span>
  88 </pre></div>
  89 </div>
  90 <p>The problem arises when an iterator is dereferenced directly into a
  91 function that takes a mutable pixel:</p>
  92 <div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Pixel</span><span class="o">&gt;</span>    <span class="c1">// Models MutablePixelConcept</span>
  93 <span class="kt">void</span> <span class="n">invert_pixel</span><span class="p">(</span><span class="n">Pixel</span><span class="o">&amp;</span> <span class="n">p</span><span class="p">);</span>
  94
  95 <span class="n">rgb_planar_pixel_iterator</span> <span class="n">myIt</span><span class="p">;</span>
  96 <span class="n">invert_pixel</span><span class="p">(</span><span class="o">*</span><span class="n">myIt</span><span class="p">);</span>        <span class="c1">// compile error!</span>
  97 </pre></div>
  98 </div>
  99 <p>C++ does not allow for matching a temporary object against a non-constant
 100 reference. The solution is to:</p>
 101 <ul class="simple">
 102 <li>Use const qualifier on all members of the reference proxy object:</li>
 103 </ul>
 104 <div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span>
 105 <span class="k">struct</span> <span class="n">my_reference_proxy</span>
 106 <span class="p">{</span>
 107   <span class="k">const</span> <span class="n">my_reference_proxy</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">my_reference_proxy</span><span class="o">&amp;</span> <span class="n">p</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
 108   <span class="k">const</span> <span class="n">my_reference_proxy</span><span class="o">*</span> <span class="k">operator</span><span class="o">-&gt;</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="k">this</span><span class="p">;</span> <span class="p">}</span>
 109   <span class="p">...</span>
 110 <span class="p">};</span>
 111 </pre></div>
 112 </div>
 113 <ul class="simple">
 114 <li>Use different classes to denote mutable and constant reference
 115 (maybe based on the constness of the template parameter)</li>
 116 <li>Define the reference type of your iterator with const qualifier:</li>
 117 </ul>
 118 <div class="highlight-cpp"><div class="highlight"><pre><span class="k">struct</span> <span class="n">iterator_traits</span><span class="o">&lt;</span><span class="n">rgb_planar_pixel_iterator</span><span class="o">&gt;</span>
 119 <span class="p">{</span>
 120   <span class="k">typedef</span> <span class="k">const</span> <span class="n">my_reference_proxy</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="n">reference</span><span class="p">;</span>
 121 <span class="p">};</span>
 122 </pre></div>
 123 </div>
 124 <p>A second important issue is providing an overload for <code class="docutils literal"><span class="pre">swap</span></code> for
 125 your reference class. The default <code class="docutils literal"><span class="pre">std::swap</span></code> will not work
 126 correctly. You must use a real value type as the temporary. A further
 127 complication is that in some implementations of the STL the <code class="docutils literal"><span class="pre">swap</span></code>
 128 function is incorrectly called qualified, as <code class="docutils literal"><span class="pre">std::swap</span></code>. The only
 129 way for these STL algorithms to use your overload is if you define it
 130 in the <code class="docutils literal"><span class="pre">std</span></code> namespace:</p>
 131 <div class="highlight-cpp"><div class="highlight"><pre><span class="k">namespace</span> <span class="n">std</span>
 132 <span class="p">{</span>
 133  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span>
 134  <span class="kt">void</span> <span class="n">swap</span><span class="p">(</span><span class="n">my_reference_proxy</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="n">my_reference_proxy</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;&amp;</span> <span class="n">y</span><span class="p">)</span>
 135  <span class="p">{</span>
 136     <span class="n">my_value</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="n">tmp</span><span class="o">=</span><span class="n">x</span><span class="p">;</span>
 137     <span class="n">x</span><span class="o">=</span><span class="n">y</span><span class="p">;</span>
 138     <span class="n">y</span><span class="o">=</span><span class="n">tmp</span><span class="p">;</span>
 139  <span class="p">}</span>
 140 <span class="p">}</span>
 141 </pre></div>
 142 </div>
 143 <p>Lastly, remember that constructors and copy-constructors of proxy
 144 references are always shallow and assignment operators are deep.</p>
 145 <p>We are grateful to Dave Abrahams, Sean Parent and Alex Stepanov for
 146 suggesting the above solution.</p>
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