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Wave: a Standard conformant C++ preprocessor library

, 10 Jan 2004
Describes a free and fully Standard conformant C++ preprocessor library
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>The Input Policy</title>
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  <tr> 
    <td width="21"> <h1></h1></td>
    <td width="885"> <font face="Verdana, Arial, Helvetica, sans-serif"><b><font size="6">The 
      Input Policy</font></b></font></td>
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<blockquote> 
  <p><a href="class_reference_inputpolicy.html#introduction">Introduction</a><br>
    <a href="class_reference_inputpolicy.html#header_synopsis">Header 'wave/cpp_iteration_context.hpp' 
    synopsis</a><br>
    <a href="class_reference_inputpolicy.html#template_parameters">Template parameters</a><br>
    <a href="class_reference_inputpolicy.html#member_functions">Member functions</a></p>
</blockquote>
<h2><b><a name="introduction"></a>Introduction</b></h2>
<p>The input policy type may be specified as a template parameter to the <tt>wave::context</tt> 
  object and is used for customizing the way, how an included file is to be represented 
  by a pair of iterators pointing to the beginning and the end of the resulting 
  input sequence. If this template parameter is not given while instantiating 
  the context object, it defaults to the <tt>iteration_context_policies::load_file_to_string</tt> 
  type. </p>
<h2><b><a name="header_synopsis"></a>Header <a href="http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/spirit/spirit/wave/wave/cpp_iteration_context.hpp?rev=HEAD&content-type=text/vnd.viewcvs-markup">wave/cpp_iteration_context.hpp</a> 
  synopsis</b></h2>
<p>The following code listing does not show the required interface only, but for 
  brevity reasons the whole implementation of an input policy, which loads the 
  given file into a string variable and exposes the begin() and end() iterators 
  of this string to the <tt>Wave</tt> library.</p>
<pre><span class="keyword">namespace</span> wave {
<span class="keyword">namespace</span> iteration_context_policies {

    <span class="keyword">struct</span> load_file_to_string {
    
        <span class="keyword">template</span> &lt;<span class="keyword">typename</span> IterContextT&gt;
        <span class="keyword">class</span> inner {
        
        <span class="keyword">public</span>:
            <span class="comment">// expose the begin and end iterators for the</span>
            <span class="comment">// included file</span>
            <span class="keyword">template</span> &lt;typename PositionT&gt;
            <span class="keyword">static</span> 
            <span class="keyword">void</span> <a href="class_reference_inputpolicy.html#init_iterators">init_iterators</a>(IterContextT &iter_ctx, 
                PositionT const &act_pos)
            {
                <span class="keyword">typedef typename</span> IterContextT::iterator_t iterator_t;
                
                <span class="keyword">std::ifstream</span> instream(iter_ctx.filename.c_str());
                if (!instream.is_open()) {
                    CPP_THROW(preprocess_exception, bad_include_file, 
                        iter_ctx.filename, act_pos);
                }
                
                iter_ctx.instring = <span class="keyword">std::string</span>(
                    <span class="keyword">std::istreambuf_iterator</span><char>(instream.rdbuf()),
                    <span class="keyword">std::istreambuf_iterator</span><char>());

                iter_ctx.first = iterator_t(iter_ctx.instring.begin(), 
                    iter_ctx.instring.end(), 
                    PositionT(iter_ctx.filename));
                iter_ctx.last = iterator_t();
            }

        <span class="keyword">private</span>:
            <span class="keyword">std::string</span> instring;
        };
    };

}   <span class="comment">// namespace iteration_context_policies</span><br>}   <span class="comment">// namespace wave </span>  </pre>
<p>As you can see, an <tt>input_policy</tt> for the <tt>wave::context</tt> object 
  should implement one function only, the init_iterators function. The policy 
  shown is implemented with the help of an embedded class to avoid the need for 
  template template parameters, which aren't implemented by all systems today. 
  This embedded class should have the name <tt>inner</tt>.</p>
<h3><a name="template_parameters"></a>Template Parameters</h3>
<p>The <tt>inner</tt> class is instantiated with one template parameter, the iteration 
  context type, from which the policy is a part of. The iterator type <tt>iterator_t</tt> 
  which is used to access the underlying input stream has to be derived through 
  a typedef as shown. The iterator pair to initialize (which is accessible as 
  <tt>iter_ctx.first</tt> and <tt>iter_ctx.last</tt>) has to initialized from 
  an abritrary iterator type, representing the actual input stream.</p>
<h3><a name="member_functions"></a>Member Functions</h3>
<p><a name="init_iterators"></a><b>init_iterators</b></p>
<pre>    <span class="keyword">template</span> &lt;<span class="keyword">typename</span> PositionT&gt;
    <span class="keyword">static void</span> init_iterators(
        IterContextT iter_ctx, 
        PositionT const &act_pos);</pre>
<blockquote> 
  <p>The <tt>init_iterators</tt> function is called, when an <tt>#include</tt> 
    directive was found in the input token stream. The main rationale for this 
    function is to initialize the pair of iterators <tt>iter_ctx.first</tt> and 
    <tt>iter_ctx.last</tt>, which are to be used to access the input stream corresponding 
    to the include file to be inserted from inside the preprocessing engine.<br>
  </p>
</blockquote>
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<p class="copyright">Copyright &copy; 2003-2004 Hartmut Kaiser<br>
  <br>
  <font size="2">Use, modification and distribution is subject to 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>
<span class="updated">Last updated: 
  <!-- #BeginDate format:fcAm1m -->Monday, January 5, 2004  14:57<!-- #EndDate -->
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About the Author

Hartmut Kaiser

United States United States
Actively involved in Boost and the development of the Spirit parser construction framework.

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