1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Arguments</title> 5<link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../index.html" title="Chapter 1. Phoenix 3.2.0"> 8<link rel="up" href="../core.html" title="Core"> 9<link rel="prev" href="references.html" title="References"> 10<link rel="next" href="nothing.html" title="Nothing"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="references.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../core.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="nothing.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="phoenix.modules.core.arguments"></a><a class="link" href="arguments.html" title="Arguments">Arguments</a> 28</h4></div></div></div> 29<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> 30</pre> 31<p> 32 We use an instance of: 33 </p> 34<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> 35</pre> 36<p> 37 to represent the Nth function argument. The argument placeholder acts as 38 an imaginary data-bin where a function argument will be placed. 39 </p> 40<h6> 41<a name="phoenix.modules.core.arguments.h0"></a> 42 <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 43 Arguments</a> 44 </h6> 45<p> 46 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> 47 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> 48 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). 49 </p> 50<p> 51 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> 52 </p> 53<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">placeholders</span> 54<span class="special">{</span> 55 <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> 56 <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> 57 <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> 58<span class="special">}</span> 59</pre> 60<p> 61 and its <a href="http://www.boost.org/libs/lambda/doc/index.html" target="_top">BLL</a> 62 <code class="computeroutput"><span class="identifier">_1</span></code>..<code class="computeroutput"><span class="identifier">_N</span></code> 63 style counterparts: 64 </p> 65<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">placeholders</span> 66<span class="special">{</span> 67 <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> 68 <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> 69 <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> 70<span class="special">}</span> 71</pre> 72<div class="note"><table border="0" summary="Note"> 73<tr> 74<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td> 75<th align="left">Note</th> 76</tr> 77<tr><td align="left" valign="top"><p> 78 You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_ARG_LIMIT</span></code>, 79 the predefined maximum placeholder index. By default, <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_ARG_LIMIT</span></code> 80 is set to <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code> 81 (See <a class="link" href="../../actor.html" title="Actor">Actor</a>). 82 </p></td></tr> 83</table></div> 84<h6> 85<a name="phoenix.modules.core.arguments.h1"></a> 86 <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 87 Arguments</a> 88 </h6> 89<p> 90 When appropriate, you can define your own <code class="computeroutput"><span class="identifier">argument</span></code> 91 names. For example: 92 </p> 93<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> 94</pre> 95<p> 96 <code class="computeroutput"><span class="identifier">x</span></code> may now be used as a 97 parameter to a lazy function: 98 </p> 99<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> 100</pre> 101<p> 102 which is equivalent to: 103 </p> 104<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> 105</pre> 106<h6> 107<a name="phoenix.modules.core.arguments.h2"></a> 108 <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 109 an Argument</a> 110 </h6> 111<p> 112 An argument, when evaluated, selects the Nth argument from the those passed 113 in by the client. 114 </p> 115<p> 116 For example: 117 </p> 118<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> 119<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">123</span><span class="special">;</span> 120<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> 121 122<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> 123<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> 124<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> 125</pre> 126<p> 127 will print out: 128 </p> 129<pre class="programlisting"><span class="identifier">A</span> 130<span class="number">123</span> 131<span class="identifier">Hello</span> <span class="identifier">World</span> 132</pre> 133<h6> 134<a name="phoenix.modules.core.arguments.h3"></a> 135 <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 136 Arguments</a> 137 </h6> 138<p> 139 In C and C++, a function can have extra arguments that are not at all used 140 by the function body itself. These extra arguments are simply ignored. 141 </p> 142<p> 143 Phoenix also allows extra arguments to be passed. For example, recall our 144 original <code class="computeroutput"><span class="identifier">add</span></code> function: 145 </p> 146<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> 147</pre> 148<p> 149 We know now that partially applying this function results to a function 150 that expects 2 arguments. However, the library is a bit more lenient and 151 allows the caller to supply more arguments than is actually required. Thus, 152 <code class="computeroutput"><span class="identifier">add</span></code> actually allows 2 153 <span class="emphasis"><em>or more</em></span> arguments. For instance, with: 154 </p> 155<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> 156</pre> 157<p> 158 the third argument <code class="computeroutput"><span class="identifier">z</span></code> is 159 ignored. Taking this further, in-between arguments are also ignored. Example: 160 </p> 161<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> 162</pre> 163<p> 164 Here, arguments b, c, and d are ignored. The function <code class="computeroutput"><span class="identifier">add</span></code> 165 takes in the first argument (<code class="computeroutput"><span class="identifier">arg1</span></code>) 166 and the fifth argument (<code class="computeroutput"><span class="identifier">arg5</span></code>). 167 </p> 168<div class="note"><table border="0" summary="Note"> 169<tr> 170<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td> 171<th align="left">Note</th> 172</tr> 173<tr><td align="left" valign="top"><p> 174 There are a few reasons why enforcing strict arity is not desirable. 175 A case in point is the callback function. Typical callback functions 176 provide more information than is actually needed. Lambda functions are 177 often used as callbacks. 178 </p></td></tr> 179</table></div> 180</div> 181<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 182<td align="left"></td> 183<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas 184 Heller, John Fletcher<p> 185 Distributed under the Boost Software License, Version 1.0. (See accompanying 186 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>) 187 </p> 188</div></td> 189</tr></table> 190<hr> 191<div class="spirit-nav"> 192<a accesskey="p" href="references.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../core.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="nothing.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 193</div> 194</body> 195</html> 196
