| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>So Just What is a Policy Anyway?</title> |
| <link rel="stylesheet" href="../../math.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.77.1"> |
| <link rel="home" href="../../index.html" title="Math Toolkit 2.2.0"> |
| <link rel="up" href="../pol_tutorial.html" title="Policy Tutorial"> |
| <link rel="prev" href="../pol_tutorial.html" title="Policy Tutorial"> |
| <link rel="next" href="policy_tut_defaults.html" title="Policies Have Sensible Defaults"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> |
| <td align="center"><a href="../../../../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../pol_tutorial.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../pol_tutorial.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="policy_tut_defaults.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <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 |
| is a Policy Anyway?</a> |
| </h3></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 class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| What action to take when an error occurs. |
| </li> |
| <li class="listitem"> |
| 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 class="listitem"> |
| What happens when you ask for a quantile of a discrete distribution. |
| </li> |
| <li class="listitem"> |
| 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 class="listitem"> |
| 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> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2006-2010, 2012-2014 Nikhar Agrawal, |
| Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert |
| Holin, Bruno Lalande, John Maddock, Johan Råde, Gautam Sewani, Benjamin Sobotta, |
| Thijs van den Berg, Daryle Walker and Xiaogang Zhang<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>) |
| </p> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../pol_tutorial.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../pol_tutorial.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="policy_tut_defaults.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
