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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.lazy_list.exceptions"></a><a class="link" href="exceptions.html" title="Exceptions">Exceptions</a> |
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| <p> |
| Exceptions are used when there is a danger of a runaway or illegal operations |
| on an empty list. |
| </p> |
| <p> |
| The key example is to take the length of a non-terminating list, e.g. |
| </p> |
| <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> |
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| <p> |
| This is protected using an exception: |
| </p> |
| <pre class="programlisting"><span class="identifier">lazy_exception</span> |
| </pre> |
| <p> |
| Note that this is implemented such that defining |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_PHOENIX_NO_LAZY_EXCEPTIONS</span> |
| </pre> |
| <p> |
| will enable the user to turn off the exceptions at their own risk. |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_PHOENIX_FUNCTION_MAX_LAZY_LIST_LENGTH</span> |
| </pre> |
| <p> |
| is currently defined as 1000 and again the user can define their own value. |
| </p> |
| <p> |
| In the length function this how it is implemented: |
| </p> |
| <pre class="programlisting"> <span class="keyword">struct</span> <span class="identifier">Length</span> <span class="special">{</span> |
| <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> |
| |
| <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> |
| <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> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">size_t</span> <span class="identifier">type</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">L</span><span class="special">></span> |
| <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> |
| <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> |
| <span class="identifier">size_t</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> |
| <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> |
| <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> |
| <span class="special">++</span><span class="identifier">x</span><span class="special">;</span> |
| <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> |
| <span class="keyword">break</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_PHOENIX_NO_LAZY_EXCEPTIONS</span> |
| <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> |
| <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> |
| <span class="preprocessor">#endif</span> |
| <span class="keyword">return</span> <span class="identifier">x</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| </pre> |
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| Heller, John Fletcher<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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>) |
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