ceph/src/boost/libs/math/doc/html/math_toolkit/pol_tutorial/what_is_a_policy.html

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  27 <a name="math_toolkit.pol_tutorial.what_is_a_policy"></a><a class="link" href="what_is_a_policy.html" title="So Just What is a Policy Anyway?">So Just What
  28       is a Policy Anyway?</a>
  29 </h3></div></div></div>
  30 <p>
  31         A policy is a compile-time mechanism for customising the behaviour of a special
  32         function, or a statistical distribution. With Policies you can control:
  33       </p>
  34 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  35 <li class="listitem">
  36             What action to take when an error occurs.
  37           </li>
  38 <li class="listitem">
  39             What happens when you call a function that is mathematically undefined
  40             (for example, if you ask for the mean of a Cauchy distribution).
  41           </li>
  42 <li class="listitem">
  43             What happens when you ask for a quantile of a discrete distribution.
  44           </li>
  45 <li class="listitem">
  46             Whether the library is allowed to internally promote <code class="computeroutput"><span class="keyword">float</span></code>
  47             to <code class="computeroutput"><span class="keyword">double</span></code> and <code class="computeroutput"><span class="keyword">double</span></code> to <code class="computeroutput"><span class="keyword">long</span>
  48             <span class="keyword">double</span></code> in order to improve precision.
  49           </li>
  50 <li class="listitem">
  51             What precision to use when calculating the result.
  52           </li>
  53 </ul></div>
  54 <p>
  55         Some of these policies could arguably be runtime variables, but then we couldn't
  56         use compile-time dispatch internally to select the best evaluation method
  57         for the given policies.
  58       </p>
  59 <p>
  60         For this reason a Policy is a <span class="emphasis"><em>type</em></span>: in fact it's an
  61         instance of the class template <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">policy</span><span class="special">&lt;&gt;</span></code>. This class is just a compile-time-container
  62         of user-selected policies (sometimes called a type-list):
  63       </p>
  64 <pre class="programlisting"><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">;</span>
  65 <span class="comment">//</span>
  66 <span class="comment">// Define a policy that sets ::errno on overflow, and does</span>
  67 <span class="comment">// not promote double to long double internally:</span>
  68 <span class="comment">//</span>
  69 <span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special">&lt;</span><span class="identifier">domain_error</span><span class="special">&lt;</span><span class="identifier">errno_on_error</span><span class="special">&gt;,</span> <span class="identifier">promote_double</span><span class="special">&lt;</span><span class="keyword">false</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">mypolicy</span><span class="special">;</span>
  70 </pre>
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  74 <td align="right"><div class="copyright-footer">Copyright &#169; 2006-2010, 2012-2014 Nikhar Agrawal,
  75       Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert
  76       Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan R&#229;de, Gautam Sewani,
  77       Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p>
  78         Distributed under the Boost Software License, Version 1.0. (See accompanying
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