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<title>Arguments</title>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="phoenix.modules.core.arguments"></a><a class="link" href="arguments.html" title="Arguments">Arguments</a>
</h4></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">phoenix</span><span class="special">/</span><span class="identifier">core</span><span class="special">/</span><span class="identifier">argument</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
We use an instance of:
</p>
<pre class="programlisting"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special"><</span><span class="identifier">N</span><span class="special">>::</span><span class="identifier">type</span>
</pre>
<p>
to represent the Nth function argument. The argument placeholder acts as
an imaginary data-bin where a function argument will be placed.
</p>
<h6>
<a name="phoenix.modules.core.arguments.h0"></a>
<span class="phrase"><a name="phoenix.modules.core.arguments.predefined_arguments"></a></span><a class="link" href="arguments.html#phoenix.modules.core.arguments.predefined_arguments">Predefined
Arguments</a>
</h6>
<p>
There are a few predefined instances of <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special"><</span><span class="identifier">N</span><span class="special">>::</span><span class="identifier">type</span></code>
named <code class="computeroutput"><span class="identifier">arg1</span></code>..<code class="computeroutput"><span class="identifier">argN</span></code>, and its <a href="http://www.boost.org/libs/lambda/doc/index.html" target="_top">BLL</a>
counterpart <code class="computeroutput"><span class="identifier">_1</span></code>..<code class="computeroutput"><span class="identifier">_N</span></code>. (where N is a predefined maximum).
</p>
<p>
Here are some sample preset definitions of <code class="computeroutput"><span class="identifier">arg1</span></code>..<code class="computeroutput"><span class="identifier">argN</span></code>
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">placeholders</span>
<span class="special">{</span>
<span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special"><</span><span class="number">1</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">arg1</span> <span class="special">=</span> <span class="special">{};</span>
<span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special"><</span><span class="number">2</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">arg2</span> <span class="special">=</span> <span class="special">{};</span>
<span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special"><</span><span class="number">3</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">arg3</span> <span class="special">=</span> <span class="special">{};</span>
<span class="special">}</span>
</pre>
<p>
and its <a href="http://www.boost.org/libs/lambda/doc/index.html" target="_top">BLL</a>
<code class="computeroutput"><span class="identifier">_1</span></code>..<code class="computeroutput"><span class="identifier">_N</span></code>
style counterparts:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">placeholders</span>
<span class="special">{</span>
<span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special"><</span><span class="number">1</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{};</span>
<span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special"><</span><span class="number">2</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{};</span>
<span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special"><</span><span class="number">3</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_3</span> <span class="special">=</span> <span class="special">{};</span>
<span class="special">}</span>
</pre>
<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>
You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_ARG_LIMIT</span></code>,
the predefined maximum placeholder index. By default, <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_ARG_LIMIT</span></code>
is set to <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>
(See <a class="link" href="../../actor.html" title="Actor">Actor</a>).
</p></td></tr>
</table></div>
<h6>
<a name="phoenix.modules.core.arguments.h1"></a>
<span class="phrase"><a name="phoenix.modules.core.arguments.user_defined_arguments"></a></span><a class="link" href="arguments.html#phoenix.modules.core.arguments.user_defined_arguments">User Defined
Arguments</a>
</h6>
<p>
When appropriate, you can define your own <code class="computeroutput"><span class="identifier">argument</span></code>
names. For example:
</p>
<pre class="programlisting"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special"><</span><span class="number">1</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">x</span><span class="special">;</span> <span class="comment">// note one based index</span>
</pre>
<p>
<code class="computeroutput"><span class="identifier">x</span></code> may now be used as a
parameter to a lazy function:
</p>
<pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="number">6</span><span class="special">)</span>
</pre>
<p>
which is equivalent to:
</p>
<pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="number">6</span><span class="special">)</span>
</pre>
<h6>
<a name="phoenix.modules.core.arguments.h2"></a>
<span class="phrase"><a name="phoenix.modules.core.arguments.evaluating_an_argument"></a></span><a class="link" href="arguments.html#phoenix.modules.core.arguments.evaluating_an_argument">Evaluating
an Argument</a>
</h6>
<p>
An argument, when evaluated, selects the Nth argument from the those passed
in by the client.
</p>
<p>
For example:
</p>
<pre class="programlisting"><span class="keyword">char</span> <span class="identifier">c</span> <span class="special">=</span> <span class="char">'A'</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">123</span><span class="special">;</span>
<span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">s</span> <span class="special">=</span> <span class="string">"Hello World"</span><span class="special">;</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">arg1</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> <span class="comment">// Get the 1st argument: c</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">arg1</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">s</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> <span class="comment">// Get the 1st argument: i</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">arg2</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">s</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> <span class="comment">// Get the 2nd argument: s</span>
</pre>
<p>
will print out:
</p>
<pre class="programlisting"><span class="identifier">A</span>
<span class="number">123</span>
<span class="identifier">Hello</span> <span class="identifier">World</span>
</pre>
<h6>
<a name="phoenix.modules.core.arguments.h3"></a>
<span class="phrase"><a name="phoenix.modules.core.arguments.extra_arguments"></a></span><a class="link" href="arguments.html#phoenix.modules.core.arguments.extra_arguments">Extra
Arguments</a>
</h6>
<p>
In C and C++, a function can have extra arguments that are not at all used
by the function body itself. These extra arguments are simply ignored.
</p>
<p>
Phoenix also allows extra arguments to be passed. For example, recall our
original <code class="computeroutput"><span class="identifier">add</span></code> function:
</p>
<pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">arg2</span><span class="special">)</span>
</pre>
<p>
We know now that partially applying this function results to a function
that expects 2 arguments. However, the library is a bit more lenient and
allows the caller to supply more arguments than is actually required. Thus,
<code class="computeroutput"><span class="identifier">add</span></code> actually allows 2
<span class="emphasis"><em>or more</em></span> arguments. For instance, with:
</p>
<pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">arg2</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">z</span><span class="special">)</span>
</pre>
<p>
the third argument <code class="computeroutput"><span class="identifier">z</span></code> is
ignored. Taking this further, in-between arguments are also ignored. Example:
</p>
<pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">arg5</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">c</span><span class="special">,</span> <span class="identifier">d</span><span class="special">,</span> <span class="identifier">e</span><span class="special">)</span>
</pre>
<p>
Here, arguments b, c, and d are ignored. The function <code class="computeroutput"><span class="identifier">add</span></code>
takes in the first argument (<code class="computeroutput"><span class="identifier">arg1</span></code>)
and the fifth argument (<code class="computeroutput"><span class="identifier">arg5</span></code>).
</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>
There are a few reasons why enforcing strict arity is not desirable.
A case in point is the callback function. Typical callback functions
provide more information than is actually needed. Lambda functions are
often used as callbacks.
</p></td></tr>
</table></div>
</div>
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<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014 Joel de Guzman, Dan Marsden, Thomas Heller, John
Fletcher<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>)
</p>
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