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  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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  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> &lt;<span class="keyword">typename</span> IterContext&gt;
  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> &lt;typename Position&gt;
  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>&lt;char&gt;(instream.rdbuf()),
  75                     <span class="keyword">std::istreambuf_iterator</span><char>&lt;char&gt;());
  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> &lt;<span class="keyword">typename</span> Position&gt;
 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>
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 124 <p class="copyright">Copyright &copy; 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>
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