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<title>Functions with Multiple Arguments</title>
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<div class="section">
<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"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></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"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></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>
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<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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