1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<html> 3<head> 4<title>The Input Policy</title> 5<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> 6<link href="theme/style.css" rel="stylesheet" type="text/css"> 7</head> 8 9<body> 10<table width="100%" border="0" cellspacing="2" background="theme/bkd2.gif"> 11 <tr> 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> 15 <td width="96"><a href="http://www.boost.org"><img src="theme/wave.gif" width="93" height="68" align="right" border="0"></a></td> 16 </tr> 17</table> 18<br> 19<table border="0"> 20 <tr> 21 <td width="10"></td> 22 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 23 <td width="30"><a href="class_reference_context.html"><img src="theme/l_arr.gif" width="20" height="19" border="0"></a></td> 24 <td width="30"><a href="class_reference_ctxpolicy.html"><img src="theme/r_arr.gif" border="0"></a></td> 25 </tr> 26</table> 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 arbitrary 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> 115<table border="0"> 116 <tr> 117 <td width="10"></td> 118 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 119 <td width="30"><a href="class_reference_context.html"><img src="theme/l_arr.gif" width="20" height="19" border="0"></a></td> 120 <td width="30"><a href="class_reference_ctxpolicy.html"><img src="theme/r_arr.gif" border="0"></a></td> 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 --> 130 </span></p> 131</body> 132</html> 133
