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| <a name="math_toolkit.policy.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 is a Policy Anyway?</a> |
| </h4></div></div></div> |
| <p> |
| A policy is a compile-time mechanism for customising the behaviour of a |
| special function, or a statistical distribution. With Policies you can |
| control: |
| </p> |
| <div class="itemizedlist"><ul type="disc"> |
| <li> |
| What action to take when an error occurs. |
| </li> |
| <li> |
| What happens when you call a function that is mathematically undefined |
| (for example, if you ask for the mean of a Cauchy distribution). |
| </li> |
| <li> |
| What happens when you ask for a quantile of a discrete distribution. |
| </li> |
| <li> |
| Whether the library is allowed to internally promote <code class="computeroutput"><span class="keyword">float</span></code> 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> <span class="keyword">double</span></code> |
| in order to improve precision. |
| </li> |
| <li> |
| What precision to use when calculating the result. |
| </li> |
| </ul></div> |
| <p> |
| Some of these policies could arguably be runtime variables, but then we |
| couldn't use compile-time dispatch internally to select the best evaluation |
| method for the given policies. |
| </p> |
| <p> |
| For this reason a Policy is a <span class="emphasis"><em>type</em></span>: in fact it's an |
| 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"><></span></code>. This class is just a compile-time-container |
| of user-selected policies (sometimes called a type-list): |
| </p> |
| <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> |
| <span class="comment">// |
| </span><span class="comment">// Define a policy that sets ::errno on overflow, and does |
| </span><span class="comment">// not promote double to long double internally: |
| </span><span class="comment">// |
| </span><span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special"><</span><span class="identifier">domain_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>,</span> <span class="identifier">promote_double</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span> <span class="special">></span> <span class="identifier">mypolicy</span><span class="special">;</span> |
| </pre> |
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| Hubert Holin, Xiaogang Zhang, Bruno Lalande, Johan Råde, Gautam Sewani and |
| Thijs van den Berg<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
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