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1 | <html> |
| 2 | <head> | |
| 3 | <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> | |
| 4 | <title>Exceptions</title> | |
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| 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> | |
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| 99 | </div> | |
| 100 | </body> | |
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