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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">&lt;</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">&gt;</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">&lt;</span><span class="identifier">N</span><span class="special">&gt;::</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><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">&lt;</span><span class="identifier">N</span><span class="special">&gt;::</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">&lt;</span><span class="number">1</span><span class="special">&gt;::</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">&lt;</span><span class="number">2</span><span class="special">&gt;::</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">&lt;</span><span class="number">3</span><span class="special">&gt;::</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">&lt;</span><span class="number">1</span><span class="special">&gt;::</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">&lt;</span><span class="number">2</span><span class="special">&gt;::</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">&lt;</span><span class="number">3</span><span class="special">&gt;::</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">
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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><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">&lt;</span><span class="number">1</span><span class="special">&gt;::</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><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">&lt;&lt;</span> <span class="identifier">arg1</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">&lt;&lt;</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">&lt;&lt;</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">&lt;&lt;</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">&lt;&lt;</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">&lt;&lt;</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><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 &#169; 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>)
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