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| <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> |
| <a name="phoenix.modules.core.arguments.predefined_arguments"></a><h6> |
| <a name="id808492"></a> |
| <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> |
| <a name="phoenix.modules.core.arguments.user_defined_arguments"></a><h6> |
| <a name="id809017"></a> |
| <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> |
| <a name="phoenix.modules.core.arguments.evaluating_an_argument"></a><h6> |
| <a name="id809168"></a> |
| <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> |
| <a name="phoenix.modules.core.arguments.extra_arguments"></a><h6> |
| <a name="id809466"></a> |
| <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> |
| <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 © 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<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> |
| </div></td> |
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