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4 | <title>The Input Policy</title> | |
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12 | <td width="21"> <h1></h1></td> | |
13 | <td width="885"> <font face="Verdana, Arial, Helvetica, sans-serif"><b><font size="6">The | |
14 | Input Policy</font></b></font></td> | |
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27 | ||
28 | <blockquote> | |
29 | <p><a href="class_reference_inptpolcy.html#introduction">Introduction</a><br> | |
30 | <a href="class_reference_inptpolcy.html#header_synopsis">Header 'wave/cpp_iteration_context.hpp' | |
31 | synopsis</a><br> | |
32 | <a href="class_reference_inptpolcy.html#template_parameters">Template parameters</a><br> | |
33 | <a href="class_reference_inptpolcy.html#member_functions">Member functions</a></p> | |
34 | </blockquote> | |
35 | <h2><b><a name="introduction"></a>Introduction</b></h2> | |
36 | <p>The input policy type may be specified as a template parameter to the <tt>wave::context</tt> | |
37 | object and is used for customizing the way, how an included file is to be represented | |
38 | by a pair of iterators pointing to the beginning and the end of the resulting | |
39 | input sequence. If this template parameter is not given while instantiating | |
40 | the context object, it defaults to the <tt>iteration_context_policies::load_file_to_string</tt> | |
41 | type. </p> | |
42 | <h2><b><a name="header_synopsis"></a>Header <a href="http://svn.boost.org/trac/boost/browser/trunk/boost/wave/util/iteration_context.hpp">wave/iteration_context.hpp</a> | |
43 | synopsis</b></h2> | |
44 | <p>The following code listing does not show the required interface only, but for | |
45 | brevity reasons the whole implementation of an input policy, which loads the | |
46 | given file into a string variable and exposes the begin() and end() iterators | |
47 | of this string to the <tt>Wave</tt> library.</p> | |
48 | <pre><span class="keyword">namespace</span> boost { | |
49 | <span class="keyword">namespace</span> wave { | |
50 | <span class="keyword">namespace</span> iteration_context_policies { | |
51 | ||
52 | <span class="keyword">struct</span> load_file_to_string { | |
53 | ||
54 | <span class="keyword">template</span> <<span class="keyword">typename</span> IterContext> | |
55 | <span class="keyword">class</span> inner { | |
56 | ||
57 | <span class="keyword">public</span>: | |
58 | <span class="comment">// expose the begin and end iterators for the</span> | |
59 | <span class="comment">// included file</span> | |
60 | <span class="keyword">template</span> <typename Position> | |
61 | <span class="keyword">static</span> | |
62 | <span class="keyword">void</span> <a href="class_reference_inptpolcy.html#init_iterators">init_iterators</a>(IterContext&iter_ctx, | |
63 | Position const &act_pos) | |
64 | { | |
65 | <span class="keyword">typedef typename</span> IterContext::iterator_type iterator_type; | |
66 | ||
67 | <span class="keyword">std::ifstream</span> instream(iter_ctx.filename.c_str()); | |
68 | if (!instream.is_open()) { | |
69 | CPP_THROW(preprocess_exception, bad_include_file, | |
70 | iter_ctx.filename, act_pos); | |
71 | } | |
72 | ||
73 | iter_ctx.instring = <span class="keyword">std::string</span>( | |
74 | <span class="keyword">std::istreambuf_iterator</span><char><char>(instream.rdbuf()), | |
75 | <span class="keyword">std::istreambuf_iterator</span><char><char>()); | |
76 | ||
77 | iter_ctx.first = iterator_type(iter_ctx.instring.begin(), | |
78 | iter_ctx.instring.end(), | |
79 | PositionT(iter_ctx.filename)); | |
80 | iter_ctx.last = iterator_type(); | |
81 | } | |
82 | ||
83 | <span class="keyword">private</span>: | |
84 | <span class="keyword">std::string</span> instring; | |
85 | }; | |
86 | }; | |
87 | ||
88 | } <span class="comment">// namespace iteration_context_policies</span> | |
89 | } <span class="comment">// namespace wave </span> | |
90 | } <span class="comment">// namespace boost </span> </pre> | |
91 | <p>As you can see, an <tt>input_policy</tt> for the <tt>wave::context</tt> object | |
92 | should implement one function only, the init_iterators function. The policy | |
93 | shown is implemented with the help of an embedded class to avoid the need for | |
94 | template template parameters, which aren't implemented by all systems today. | |
95 | This embedded class should have the name <tt>inner</tt>.</p> | |
96 | <h3><a name="template_parameters"></a>Template Parameters</h3> | |
97 | <p>The <tt>inner</tt> class is instantiated with one template parameter, the iteration | |
98 | context type, from which the policy is a part of. The iterator type <tt>iterator_type</tt> | |
99 | which is used to access the underlying input stream has to be derived through | |
100 | a typedef as shown. The iterator pair to initialize (which is accessible as | |
101 | <tt>iter_ctx.first</tt> and <tt>iter_ctx.last</tt>) has to initialized from | |
102 | an abritrary iterator type, representing the actual input stream.</p> | |
103 | <h3><a name="member_functions"></a>Member Functions</h3> | |
104 | <p><a name="init_iterators"></a><b>init_iterators</b></p> | |
105 | <pre> <span class="keyword">template</span> <<span class="keyword">typename</span> Position> | |
106 | <span class="keyword">static void</span> init_iterators( | |
107 | IterContext iter_ctx, | |
108 | Position const &act_pos);</pre> | |
109 | ||
110 | <p>directive was found in the input token stream. The main rationale for this | |
111 | function is to initialize the pair of iterators <tt>iter_ctx.first</tt> and | |
112 | <tt>iter_ctx.last</tt>, which are to be used to access the input stream corresponding | |
113 | to the include file to be inserted from inside the preprocessing engine.</p> | |
114 | </blockquote> | |
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121 | </tr> | |
122 | </table> | |
123 | <hr size="1"> | |
124 | <p class="copyright">Copyright © 2003-2011 Hartmut Kaiser<br> | |
125 | <br> | |
126 | <font size="2">Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) </font> </p> | |
127 | <span class="updated"></span> | |
128 | <p class="copyright"><span class="updated">Last updated: | |
129 | <!-- #BeginDate format:fcAm1m -->Sunday, October 12, 2008 20:14<!-- #EndDate --> | |
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