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   4 <title>Concept Expr</title>
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  25 <div class="refentry">
  26 <a name="Expr"></a><div class="titlepage"></div>
  27 <div class="refnamediv">
  28 <h2><span class="refentrytitle">Concept Expr</span></h2>
  29 <p>Expr</p>
  30 </div>
  31 <div class="refsect1">
  32 <a name="idp393577504"></a><h2>Description</h2>
  33 <p>
  34       An Expr represents a tagged node in an expression tree.
  35       The children of the Expr must themselves satisfy the
  36       Expr concept. The Expr has an arity representing the
  37       number of children. If the number of children is zero,
  38       the Expr also has a value. An Expr also has an associated
  39       <a class="link" href="Domain.html" title="Concept Domain">Domain</a>.
  40     </p>
  41 </div>
  42 <div class="refsect1">
  43 <a name="idp393579600"></a><h2>Associated types</h2>
  44 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  45 <li class="listitem">
  46 <p><span class="bold"><strong>proto_tag</strong></span></p>
  47 <pre class="literallayout">Expr::proto_tag</pre>
  48 <p>
  49       </p>
  50 <p>The tag type of the Expr.</p>
  51 <p>
  52     </p>
  53 </li>
  54 <li class="listitem">
  55 <p><span class="bold"><strong>proto_args</strong></span></p>
  56 <pre class="literallayout">Expr::proto_args</pre>
  57 <p>
  58       </p>
  59 <p>A typelist representing either the types of the child nodes, or,
  60         if the arity of the Expr is 0, of the value of the terminal.</p>
  61 <p>
  62     </p>
  63 </li>
  64 <li class="listitem">
  65 <p><span class="bold"><strong>proto_arity</strong></span></p>
  66 <pre class="literallayout">Expr::proto_arity</pre>
  67 <p>
  68       </p>
  69 <p>
  70         The arity (number of child nodes) of the Expr.
  71         <code class="computeroutput">proto_arity</code> is an MPL Integral Constant.
  72       </p>
  73 <p>
  74     </p>
  75 </li>
  76 <li class="listitem">
  77 <p><span class="bold"><strong>proto_grammar</strong></span></p>
  78 <pre class="literallayout">Expr::proto_grammar</pre>
  79 <p>
  80       </p>
  81 <p>
  82         A typedef for an instantiation of
  83         <code class="computeroutput"><a class="link" href="boost/proto/basic_expr.html" title="Struct template basic_expr">
  84           proto::basic_expr&lt;&gt;
  85         </a></code>
  86         that is equivalent to Expr. Expression types are equivalent if they have the
  87         same <code class="computeroutput">proto_tag</code>, <code class="computeroutput">proto_args</code>, and <code class="computeroutput">proto_arity</code>.
  88       </p>
  89 <p>
  90     </p>
  91 </li>
  92 <li class="listitem">
  93 <p><span class="bold"><strong>proto_base_expr</strong></span></p>
  94 <pre class="literallayout">Expr::proto_base_expr</pre>
  95 <p>
  96       </p>
  97 <p>
  98         A typedef for an instantiation of
  99         <code class="computeroutput"><a class="link" href="boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> or
 100         <code class="computeroutput"><a class="link" href="boost/proto/basic_expr.html" title="Struct template basic_expr">proto::basic_expr&lt;&gt;</a></code>
 101         that is equivalent to Expr. Expression types are equivalent if they have the
 102         same <code class="computeroutput">proto_tag</code>, <code class="computeroutput">proto_args</code>, and <code class="computeroutput">proto_arity</code>.
 103       </p>
 104 <p>
 105     </p>
 106 </li>
 107 <li class="listitem">
 108 <p><span class="bold"><strong>proto_derived_expr</strong></span></p>
 109 <pre class="literallayout">Expr::proto_derived_expr</pre>
 110 <p>
 111       </p>
 112 <p>
 113         A typedef for <code class="computeroutput">Expr</code>.
 114       </p>
 115 <p>
 116     </p>
 117 </li>
 118 <li class="listitem">
 119 <p><span class="bold"><strong>proto_domain</strong></span></p>
 120 <pre class="literallayout">Expr::proto_domain</pre>
 121 <p>
 122       </p>
 123 <p>
 124         The Domain of the Expr. <code class="computeroutput">proto_domain</code>
 125         models <a class="link" href="Domain.html" title="Concept Domain">Domain</a>.
 126       </p>
 127 <p>
 128     </p>
 129 </li>
 130 <li class="listitem">
 131 <p><span class="bold"><strong>proto_childN</strong></span></p>
 132 <pre class="literallayout">Expr::proto_childN</pre>
 133 <p>
 134       </p>
 135 <p>The type of the Nth child of Expr. Requires
 136         <code class="computeroutput">0 == N::value || N::value &lt; proto_arity::value</code></p>
 137 <p>
 138     </p>
 139 </li>
 140 </ul></div>
 141 </div>
 142 <div class="refsect1">
 143 <a name="idp393609776"></a><h2>Notation</h2>
 144 <div class="variablelist"><dl class="variablelist">
 145 <dt><span class="term">Expr</span></dt>
 146 <dd>A type playing the role of expession-type in the <a class="link" href="Expr.html" title="Concept Expr">Expr</a> concept.</dd>
 147 <dt><span class="term">Tag</span></dt>
 148 <dd>A type playing the role of tag-type in the <a class="link" href="Expr.html" title="Concept Expr">Expr</a> concept.</dd>
 149 <dt><span class="term">Domain</span></dt>
 150 <dd>A type playing the role of domain-type in the <a class="link" href="Expr.html" title="Concept Expr">Expr</a> concept.</dd>
 151 <dt><span class="term">N</span></dt>
 152 <dd>A type playing the role of mpl-integral-constant-type in the <a class="link" href="Expr.html" title="Concept Expr">Expr</a> concept.</dd>
 153 <dt><span class="term"><code class="varname">e</code></span></dt>
 154 <dd>Object of type Expr</dd>
 155 </dl></div>
 156 </div>
 157 <div class="refsect1">
 158 <a name="idp393617984"></a><h2>Valid expressions</h2>
 159 <div class="informaltable"><table class="table">
 160 <colgroup>
 161 <col>
 162 <col>
 163 <col>
 164 <col>
 165 </colgroup>
 166 <thead><tr>
 167 <th>Name</th>
 168 <th>Expression</th>
 169 <th>Type</th>
 170 <th>Semantics</th>
 171 </tr></thead>
 172 <tbody>
 173 <tr>
 174 <td><p>Get N-th Child</p></td>
 175 <td><p>boost::proto::child&lt; N &gt;(e)</p></td>
 176 <td><p><span class="type">proto_childN</span></p></td>
 177 <td><p>Extracts the Nth child from this Expr.
 178       Requires <code class="computeroutput">N::value &lt; proto_arity::value</code>.</p></td>
 179 </tr>
 180 <tr>
 181 <td><p>Get Terminal Value</p></td>
 182 <td><p>boost::proto::value(e)</p></td>
 183 <td><p><span class="type">proto_child0</span></p></td>
 184 <td><p>
 185       Extracts the value from a terminal Expr.
 186       Requires <code class="computeroutput">0 == proto_arity::value</code>.
 187     </p></td>
 188 </tr>
 189 <tr>
 190 <td><p>Get Base</p></td>
 191 <td><p>e.proto_base()</p></td>
 192 <td><p><span class="type">proto_base_expr</span></p></td>
 193 <td><p>
 194         Returns an object of type
 195         <code class="computeroutput"><a class="link" href="boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> or
 196         <code class="computeroutput"><a class="link" href="boost/proto/basic_expr.html" title="Struct template basic_expr">proto::basic_expr&lt;&gt;</a></code>
 197         that is equivalent to <code class="computeroutput">e</code>.
 198     </p></td>
 199 </tr>
 200 </tbody>
 201 </table></div>
 202 </div>
 203 <div class="refsect1">
 204 <a name="idp393631008"></a><h2>Models</h2>
 205 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><span class="simplelist"><span class="type">boost::proto::literal&lt; int &gt;</span></span></li></ul></div>
 206 </div>
 207 </div>
 208 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 209 <td align="left"></td>
 210 <td align="right"><div class="copyright-footer">Copyright &#169; 2008 Eric Niebler<p>
 211         Distributed under the Boost Software License, Version 1.0. (See accompanying
 212         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
 213       </p>
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