1 2 3<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 4 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 5<html xmlns="http://www.w3.org/1999/xhtml"> 6 <head> 7 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 8 9 <title>Technicalities - Boost.GIL documentation</title> 10 <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> 11 <link rel="stylesheet" href="../_static/style.css" type="text/css" /> 12 <script type="text/javascript"> 13 var DOCUMENTATION_OPTIONS = { 14 URL_ROOT: '../', 15 VERSION: '', 16 COLLAPSE_MODINDEX: false, 17 FILE_SUFFIX: '.html' 18 }; 19 </script> 20 <script type="text/javascript" src="../_static/jquery.js"></script> 21 <script type="text/javascript" src="../_static/underscore.js"></script> 22 <script type="text/javascript" src="../_static/doctools.js"></script> 23 <link rel="index" title="Index" href="../genindex.html" /> 24 <link rel="search" title="Search" href="../search.html" /> 25 <link rel="top" title="Boost.GIL documentation" href="../index.html" /> 26 <link rel="up" title="Design Guide" href="index.html" /> 27 <link rel="next" title="Extending" href="extending.html" /> 28 <link rel="prev" title="Examples" href="examples.html" /> 29 </head> 30 <body> 31 <div class="header"> 32 <table border="0" cellpadding="7" cellspacing="0" width="100%" summary= 33 "header"> 34 <tr> 35 <td valign="top" width="300"> 36 <h3><a href="../index.html"><img 37 alt="C++ Boost" src="../_static/gil.png" border="0"></a></h3> 38 </td> 39 40 <td > 41 <h1 align="center"><a href="../index.html"></a></h1> 42 </td> 43 <td> 44 <div id="searchbox" style="display: none"> 45 <form class="search" action="../search.html" method="get"> 46 <input type="text" name="q" size="18" /> 47 <input type="submit" value="Search" /> 48 <input type="hidden" name="check_keywords" value="yes" /> 49 <input type="hidden" name="area" value="default" /> 50 </form> 51 </div> 52 <script type="text/javascript">$('#searchbox').show(0);</script> 53 </td> 54 </tr> 55 </table> 56 </div> 57 <hr/> 58 <div class="content"> 59 <div class="navbar" style="text-align:right;"> 60 61 62 <a class="prev" title="Examples" href="examples.html"><img src="../_static/prev.png" alt="prev"/></a> 63 <a class="up" title="Design Guide" href="index.html"><img src="../_static/up.png" alt="up"/></a> 64 <a class="next" title="Extending" href="extending.html"><img src="../_static/next.png" alt="next"/></a> 65 66 </div> 67 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 78reference to a given object. Examples of these are GIL’s reference to 79a planar pixel (<code class="docutils literal"><span class="pre">planar_pixel_reference</span></code>) and GIL’s sub-byte channel 80references. Writing a reference proxy class can be tricky. One problem 81is that the proxy reference is constructed as a temporary object and 82returned 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"><</span><span class="n">T</span><span class="o">></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 91function that takes a mutable pixel:</p> 92<div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">Pixel</span><span class="o">></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">&</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 100reference. 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"><</span><span class="k">typename</span> <span class="n">T</span><span class="o">></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">&</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">&</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">-></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"><</span><span class="n">rgb_planar_pixel_iterator</span><span class="o">></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"><</span><span class="n">T</span><span class="o">></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 125your reference class. The default <code class="docutils literal"><span class="pre">std::swap</span></code> will not work 126correctly. You must use a real value type as the temporary. A further 127complication is that in some implementations of the STL the <code class="docutils literal"><span class="pre">swap</span></code> 128function is incorrectly called qualified, as <code class="docutils literal"><span class="pre">std::swap</span></code>. The only 129way for these STL algorithms to use your overload is if you define it 130in 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"><</span><span class="k">typename</span> <span class="n">T</span><span class="o">></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"><</span><span class="n">T</span><span class="o">>&</span> <span class="n">x</span><span class="p">,</span> <span class="n">my_reference_proxy</span><span class="o"><</span><span class="n">T</span><span class="o">>&</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"><</span><span class="n">T</span><span class="o">></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 144references are always shallow and assignment operators are deep.</p> 145<p>We are grateful to Dave Abrahams, Sean Parent and Alex Stepanov for 146suggesting the above solution.</p> 147</div> 148</div> 149 150 151 <div class="navbar" style="text-align:right;"> 152 153 154 <a class="prev" title="Examples" href="examples.html"><img src="../_static/prev.png" alt="prev"/></a> 155 <a class="up" title="Design Guide" href="index.html"><img src="../_static/up.png" alt="up"/></a> 156 <a class="next" title="Extending" href="extending.html"><img src="../_static/next.png" alt="next"/></a> 157 158 </div> 159 </div> 160 <div class="footer" role="contentinfo"> 161 Last updated on 2020-08-11 15:08:48. 162 Created using <a href="http://sphinx-doc.org/">Sphinx</a> 1.5.6. 163 </div> 164 </body> 165</html>
