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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_typeerasure.multi"></a><a class="link" href="multi.html" title="Functions with Multiple Arguments">Functions with Multiple Arguments</a> |
| </h2></div></div></div> |
| <p> |
| (For the source of the examples in this section see <a href="../../../libs/type_erasure/example/multi.cpp" target="_top">multi.cpp</a>) |
| </p> |
| <p> |
| Operations can have more than one <code class="computeroutput">any</code> |
| argument. Let's use binary addition as an example. |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">any</span><span class="special"><</span> |
| <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span> |
| <span class="identifier">copy_constructible</span><span class="special"><>,</span> |
| <span class="identifier">typeid_</span><span class="special"><>,</span> |
| <span class="identifier">addable</span><span class="special"><>,</span> |
| <span class="identifier">ostreamable</span><span class="special"><></span> |
| <span class="special">></span> |
| <span class="special">></span> <span class="identifier">any_type</span><span class="special">;</span> |
| <span class="identifier">any_type</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span> |
| <span class="identifier">any_type</span> <span class="identifier">y</span><span class="special">(</span><span class="number">7</span><span class="special">);</span> |
| <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> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">z</span> <span class="special"><<</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> |
| </pre> |
| <p> |
| </p> |
| <p> |
| This is <span class="emphasis"><em>not</em></span> a multimethod. The underlying types of the |
| arguments of <code class="computeroutput"><span class="special">+</span></code> must be the same |
| or the behavior is undefined. This example is correct because the arguments |
| both hold <code class="computeroutput"><span class="keyword">int</span></code>'s. |
| </p> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| Adding <code class="computeroutput"><a class="link" href="../boost/type_erasure/relaxed.html" title="Struct relaxed">relaxed</a></code> |
| leads an exception rather than undefined behavior if the argument types are |
| wrong. |
| </p></td></tr> |
| </table></div> |
| <p> |
| <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"><></span></code> requires the types of the arguments |
| to be exactly the same. This doesn't cover all uses of addition though. For |
| example, pointer arithmetic takes a pointer and an integer and returns a pointer. |
| We can capture this kind of relationship among several types by identifying |
| 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 |
| <code class="computeroutput"><span class="identifier">_b</span></code> represent the integer. |
| </p> |
| <p> |
| </p> |
| <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> |
| |
| <span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span> |
| <span class="identifier">copy_constructible</span><span class="special"><</span><span class="identifier">_a</span><span class="special">>,</span> |
| <span class="identifier">copy_constructible</span><span class="special"><</span><span class="identifier">_b</span><span class="special">>,</span> |
| <span class="identifier">typeid_</span><span class="special"><</span><span class="identifier">_a</span><span class="special">>,</span> |
| <span class="identifier">addable</span><span class="special"><</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">></span> |
| <span class="special">></span> <span class="identifier">requirements</span><span class="special">;</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Our new concept, <code class="computeroutput"><span class="identifier">addable</span><span class="special"><</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">></span></code> |
| captures the rules of pointer addition: <code class="computeroutput"><span class="identifier">_a</span> |
| <span class="special">+</span> <span class="identifier">_b</span> <span class="special">-></span> <span class="identifier">_a</span></code>. |
| </p> |
| <p> |
| Also, we can no longer capture the variables independently. |
| </p> |
| <pre class="programlisting"><span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">></span> <span class="identifier">ptr</span><span class="special">(&</span><span class="identifier">array</span><span class="special">[</span><span class="number">0</span><span class="special">]);</span> <span class="comment">// illegal</span> |
| </pre> |
| <p> |
| This doesn't work because the library needs to know the type that _b binds |
| to when it captures the concept bindings. We need to specify the bindings of |
| both placeholders when we construct the <code class="computeroutput">any</code>. |
| </p> |
| <p> |
| </p> |
| <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"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">_a</span><span class="special">,</span> <span class="keyword">int</span><span class="special">*>,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">_b</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span> <span class="special">></span> <span class="identifier">types</span><span class="special">;</span> |
| <span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">></span> <span class="identifier">ptr</span><span class="special">(&</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"><</span><span class="identifier">types</span><span class="special">>());</span> |
| <span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_b</span><span class="special">></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"><</span><span class="identifier">types</span><span class="special">>());</span> |
| <span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">></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> |
| <span class="comment">// x now holds array + 2</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Now that the arguments of <code class="computeroutput"><span class="special">+</span></code> aren't |
| the same type, we require that both arguments agree that <code class="computeroutput"><span class="identifier">_a</span></code> |
| maps to <code class="computeroutput"><span class="keyword">int</span><span class="special">*</span></code> |
| and that <code class="computeroutput"><span class="identifier">_b</span></code> maps to <code class="computeroutput"><span class="keyword">int</span></code>. |
| </p> |
| <p> |
| We can also use <code class="computeroutput"><a class="link" href="../boost/type_erasure/tuple.html" title="Class template tuple">tuple</a></code> |
| 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> |
| is just a convenience class that combines the placeholder bindings it gets |
| from all its arguments. |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="identifier">tuple</span><span class="special"><</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">></span> <span class="identifier">t</span><span class="special">(&</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> |
| <span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">></span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">get</span><span class="special"><</span><span class="number">0</span><span class="special">>(</span><span class="identifier">t</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">get</span><span class="special"><</span><span class="number">1</span><span class="special">>(</span><span class="identifier">t</span><span class="special">));</span> |
| </pre> |
| <p> |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2011-2013 Steven Watanabe<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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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