1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Exceptions</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Phoenix 3.2.0"> 8<link rel="up" href="../lazy_list.html" title="Lazy List"> 9<link rel="prev" href="tutorial_with_examples/list_generation.html" title="List Generation"> 10<link rel="next" href="implementation_details.html" title="Implementation Details"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="tutorial_with_examples/list_generation.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../lazy_list.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="implementation_details.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="phoenix.lazy_list.exceptions"></a><a class="link" href="exceptions.html" title="Exceptions">Exceptions</a> 28</h3></div></div></div> 29<p> 30 Exceptions are used when there is a danger of a runaway or illegal operations 31 on an empty list. 32 </p> 33<p> 34 The key example is to take the length of a non-terminating list, e.g. 35 </p> 36<pre class="programlisting"><span class="identifier">length</span><span class="special">(</span><span class="identifier">enum_from</span><span class="special">(</span><span class="number">1</span><span class="special">))</span> 37</pre> 38<p> 39 This is protected using an exception: 40 </p> 41<pre class="programlisting"><span class="identifier">lazy_exception</span> 42</pre> 43<p> 44 Note that this is implemented such that defining 45 </p> 46<pre class="programlisting"><span class="identifier">BOOST_PHOENIX_NO_LAZY_EXCEPTIONS</span> 47</pre> 48<p> 49 will enable the user to turn off the exceptions at their own risk. 50 </p> 51<pre class="programlisting"><span class="identifier">BOOST_PHOENIX_FUNCTION_MAX_LAZY_LIST_LENGTH</span> 52</pre> 53<p> 54 is currently defined as 1000 and again the user can define their own value. 55 </p> 56<p> 57 In the length function this how it is implemented: 58 </p> 59<pre class="programlisting"> <span class="keyword">struct</span> <span class="identifier">Length</span> <span class="special">{</span> 60 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> 61 62 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">L</span><span class="special">></span> 63 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">L</span><span class="special">)></span> 64 <span class="special">{</span> 65 <span class="keyword">typedef</span> <span class="identifier">size_t</span> <span class="identifier">type</span><span class="special">;</span> 66 <span class="special">};</span> 67 68 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">L</span><span class="special">></span> 69 <span class="identifier">size_t</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">const</span> <span class="identifier">L</span><span class="special">&</span> <span class="identifier">ll</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">{</span> 70 <span class="keyword">typename</span> <span class="identifier">L</span><span class="special">::</span><span class="identifier">delay_result_type</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">delay</span><span class="special">(</span><span class="identifier">ll</span><span class="special">);</span> 71 <span class="identifier">size_t</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 72 <span class="keyword">while</span><span class="special">(</span> <span class="special">!</span><span class="identifier">null</span><span class="special">(</span><span class="identifier">l</span><span class="special">)()</span> <span class="special">)</span> <span class="special">{</span> 73 <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">tail</span><span class="special">(</span><span class="identifier">l</span><span class="special">);</span> 74 <span class="special">++</span><span class="identifier">x</span><span class="special">;</span> 75 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span> <span class="special">></span> <span class="identifier">BOOST_PHOENIX_FUNCTION_MAX_LAZY_LIST_LENGTH</span><span class="special">)</span> 76 <span class="keyword">break</span><span class="special">;</span> 77 <span class="special">}</span> 78<span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_PHOENIX_NO_LAZY_EXCEPTIONS</span> 79 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span> <span class="special">></span> <span class="identifier">BOOST_PHOENIX_FUNCTION_MAX_LAZY_LIST_LENGTH</span><span class="special">)</span> 80 <span class="keyword">throw</span> <span class="identifier">lazy_exception</span><span class="special">(</span><span class="string">"Your list is too long!!"</span><span class="special">);</span> 81<span class="preprocessor">#endif</span> 82 <span class="keyword">return</span> <span class="identifier">x</span><span class="special">;</span> 83 <span class="special">}</span> 84 <span class="special">};</span> 85</pre> 86</div> 87<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 88<td align="left"></td> 89<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas 90 Heller, John Fletcher<p> 91 Distributed under the Boost Software License, Version 1.0. (See accompanying 92 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>) 93 </p> 94</div></td> 95</tr></table> 96<hr> 97<div class="spirit-nav"> 98<a accesskey="p" href="tutorial_with_examples/list_generation.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../lazy_list.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="implementation_details.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 99</div> 100</body> 101</html> 102
