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   5 <title>Functions with Multiple Arguments</title>
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  27 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  28 <a name="boost_typeerasure.multi"></a><a class="link" href="multi.html" title="Functions with Multiple Arguments">Functions with Multiple Arguments</a>
  29 </h2></div></div></div>
  30 <p>
  31       (For the source of the examples in this section see <a href="../../../libs/type_erasure/example/multi.cpp" target="_top">multi.cpp</a>)
  32     </p>
  33 <p>
  34       Operations can have more than one <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
  35       argument. Let's use binary addition as an example.
  36     </p>
  37 <p>
  38 </p>
  39 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">any</span><span class="special">&lt;</span>
  40     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
  41         <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
  42         <span class="identifier">typeid_</span><span class="special">&lt;&gt;,</span>
  43         <span class="identifier">addable</span><span class="special">&lt;&gt;,</span>
  44         <span class="identifier">ostreamable</span><span class="special">&lt;&gt;</span>
  45     <span class="special">&gt;</span>
  46 <span class="special">&gt;</span> <span class="identifier">any_type</span><span class="special">;</span>
  47 <span class="identifier">any_type</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
  48 <span class="identifier">any_type</span> <span class="identifier">y</span><span class="special">(</span><span class="number">7</span><span class="special">);</span>
  49 <span class="identifier">any_type</span> <span class="identifier">z</span><span class="special">(</span><span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">);</span>
  50 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">z</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 17</span>
  51 </pre>
  52 <p>
  53     </p>
  54 <p>
  55       This is <span class="emphasis"><em>not</em></span> a multimethod. The underlying types of the
  56       arguments of <code class="computeroutput"><span class="special">+</span></code> must be the same
  57       or the behavior is undefined. This example is correct because the arguments
  58       both hold <code class="computeroutput"><span class="keyword">int</span></code>'s.
  59     </p>
  60 <div class="note"><table border="0" summary="Note">
  61 <tr>
  62 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
  63 <th align="left">Note</th>
  64 </tr>
  65 <tr><td align="left" valign="top"><p>
  66         Adding <code class="computeroutput"><a class="link" href="../boost/type_erasure/relaxed.html" title="Struct relaxed">relaxed</a></code>
  67         leads an exception rather than undefined behavior if the argument types are
  68         wrong.
  69       </p></td></tr>
  70 </table></div>
  71 <p>
  72       <code class="computeroutput"><a class="link" href="../boost/type_erasure/addable.html" title="Struct template addable">addable</a></code><code class="computeroutput"><span class="special">&lt;&gt;</span></code> requires the types of the arguments
  73       to be exactly the same. This doesn't cover all uses of addition though. For
  74       example, pointer arithmetic takes a pointer and an integer and returns a pointer.
  75       We can capture this kind of relationship among several types by identifying
  76       each type involved with a placeholder. We'll let the placeholder <code class="computeroutput"><span class="identifier">_a</span></code> represent the pointer and the placeholder
  77       <code class="computeroutput"><span class="identifier">_b</span></code> represent the integer.
  78     </p>
  79 <p>
  80 </p>
  81 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">array</span><span class="special">[</span><span class="number">5</span><span class="special">];</span>
  82
  83 <span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
  84     <span class="identifier">copy_constructible</span><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">&gt;,</span>
  85     <span class="identifier">copy_constructible</span><span class="special">&lt;</span><span class="identifier">_b</span><span class="special">&gt;,</span>
  86     <span class="identifier">typeid_</span><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">&gt;,</span>
  87     <span class="identifier">addable</span><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">,</span> <span class="identifier">_b</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">&gt;</span>
  88 <span class="special">&gt;</span> <span class="identifier">requirements</span><span class="special">;</span>
  89 </pre>
  90 <p>
  91     </p>
  92 <p>
  93       Our new concept, <code class="computeroutput"><span class="identifier">addable</span><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">,</span> <span class="identifier">_b</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">&gt;</span></code>
  94       captures the rules of pointer addition: <code class="computeroutput"><span class="identifier">_a</span>
  95       <span class="special">+</span> <span class="identifier">_b</span> <span class="special">-&gt;</span> <span class="identifier">_a</span></code>.
  96     </p>
  97 <p>
  98       Also, we can no longer capture the variables independently.
  99 </p>
 100 <pre class="programlisting"><span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">&gt;</span> <span class="identifier">ptr</span><span class="special">(&amp;</span><span class="identifier">array</span><span class="special">[</span><span class="number">0</span><span class="special">]);</span> <span class="comment">// illegal</span>
 101 </pre>
 102 <p>
 103       This doesn't work because the library needs to know the type that _b binds
 104       to when it captures the concept bindings. We need to specify the bindings of
 105       both placeholders when we construct the <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>.
 106     </p>
 107 <p>
 108 </p>
 109 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">,</span> <span class="keyword">int</span><span class="special">*&gt;,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">_b</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">types</span><span class="special">;</span>
 110 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">&gt;</span> <span class="identifier">ptr</span><span class="special">(&amp;</span><span class="identifier">array</span><span class="special">[</span><span class="number">0</span><span class="special">],</span> <span class="identifier">make_binding</span><span class="special">&lt;</span><span class="identifier">types</span><span class="special">&gt;());</span>
 111 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_b</span><span class="special">&gt;</span> <span class="identifier">idx</span><span class="special">(</span><span class="number">2</span><span class="special">,</span> <span class="identifier">make_binding</span><span class="special">&lt;</span><span class="identifier">types</span><span class="special">&gt;());</span>
 112 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">ptr</span> <span class="special">+</span> <span class="identifier">idx</span><span class="special">);</span>
 113 <span class="comment">// x now holds array + 2</span>
 114 </pre>
 115 <p>
 116     </p>
 117 <p>
 118       Now that the arguments of <code class="computeroutput"><span class="special">+</span></code> aren't
 119       the same type, we require that both arguments agree that <code class="computeroutput"><span class="identifier">_a</span></code>
 120       maps to <code class="computeroutput"><span class="keyword">int</span><span class="special">*</span></code>
 121       and that <code class="computeroutput"><span class="identifier">_b</span></code> maps to <code class="computeroutput"><span class="keyword">int</span></code>.
 122     </p>
 123 <p>
 124       We can also use <code class="computeroutput"><a class="link" href="../boost/type_erasure/tuple.html" title="Class template tuple">tuple</a></code>
 125       to avoid having to write out the map out explicitly. <code class="computeroutput"><a class="link" href="../boost/type_erasure/tuple.html" title="Class template tuple">tuple</a></code>
 126       is just a convenience class that combines the placeholder bindings it gets
 127       from all its arguments.
 128     </p>
 129 <p>
 130 </p>
 131 <pre class="programlisting"><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">,</span> <span class="identifier">_b</span><span class="special">&gt;</span> <span class="identifier">t</span><span class="special">(&amp;</span><span class="identifier">array</span><span class="special">[</span><span class="number">0</span><span class="special">],</span> <span class="number">2</span><span class="special">);</span>
 132 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">get</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span><span class="identifier">t</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">get</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;(</span><span class="identifier">t</span><span class="special">));</span>
 133 </pre>
 134 <p>
 135     </p>
 136 </div>
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 138 <td align="left"></td>
 139 <td align="right"><div class="copyright-footer">Copyright &#169; 2011-2013 Steven Watanabe<p>
 140         Distributed under the Boost Software License, Version 1.0. (See accompanying
 141         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>)
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