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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"><</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><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><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><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><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> | |
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