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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="math_toolkit.pol_tutorial.policy_tut_defaults"></a><a class="link" href="policy_tut_defaults.html" title="Policies Have Sensible Defaults">Policies
  28       Have Sensible Defaults</a>
  29 </h3></div></div></div>
  30 <p>
  31         Most of the time you can just ignore the policy framework.
  32       </p>
  33 <p>
  34         <span class="emphasis"><em>*The defaults for the various policies are as follows, if these
  35         work OK for you then you can stop reading now!</em></span>
  36       </p>
  37 <div class="variablelist">
  38 <p class="title"><b></b></p>
  39 <dl class="variablelist">
  40 <dt><span class="term">Domain Error</span></dt>
  41 <dd><p>
  42               Throws a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">domain_error</span></code> exception.
  43             </p></dd>
  44 <dt><span class="term">Pole Error</span></dt>
  45 <dd><p>
  46               Occurs when a function is evaluated at a pole: throws a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">domain_error</span></code> exception.
  47             </p></dd>
  48 <dt><span class="term">Overflow Error</span></dt>
  49 <dd><p>
  50               Throws a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">overflow_error</span></code> exception.
  51             </p></dd>
  52 <dt><span class="term">Underflow</span></dt>
  53 <dd><p>
  54               Ignores the underflow, and returns zero.
  55             </p></dd>
  56 <dt><span class="term">Denormalised Result</span></dt>
  57 <dd><p>
  58               Ignores the fact that the result is denormalised, and returns it.
  59             </p></dd>
  60 <dt><span class="term">Rounding Error</span></dt>
  61 <dd><p>
  62               Throws a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">rounding_error</span></code> exception.
  63             </p></dd>
  64 <dt><span class="term">Internal Evaluation Error</span></dt>
  65 <dd><p>
  66               Throws a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">evaluation_error</span></code> exception.
  67             </p></dd>
  68 <dt><span class="term">Indeterminate Result Error</span></dt>
  69 <dd><p>
  70               Returns a result that depends on the function where the error occurred.
  71             </p></dd>
  72 <dt><span class="term">Promotion of float to double</span></dt>
  73 <dd><p>
  74               Does occur by default - gives full float precision results.
  75             </p></dd>
  76 <dt><span class="term">Promotion of double to long double</span></dt>
  77 <dd><p>
  78               Does occur by default if long double offers more precision than double.
  79             </p></dd>
  80 <dt><span class="term">Precision of Approximation Used</span></dt>
  81 <dd><p>
  82               By default uses an approximation that will result in the lowest level
  83               of error for the type of the result.
  84             </p></dd>
  85 <dt><span class="term">Behaviour of Discrete Quantiles</span></dt>
  86 <dd>
  87 <p>
  88               The quantile function will by default return an integer result that
  89               has been <span class="emphasis"><em>rounded outwards</em></span>. That is to say lower
  90               quantiles (where the probability is less than 0.5) are rounded downward,
  91               and upper quantiles (where the probability is greater than 0.5) are
  92               rounded upwards. This behaviour ensures that if an X% quantile is requested,
  93               then <span class="emphasis"><em>at least</em></span> the requested coverage will be present
  94               in the central region, and <span class="emphasis"><em>no more than</em></span> the requested
  95               coverage will be present in the tails.
  96             </p>
  97 <p>
  98               This behaviour can be changed so that the quantile functions are rounded
  99               differently, or even return a real-valued result using <a class="link" href="../pol_overview.html" title="Policy Overview">Policies</a>.
 100               It is strongly recommended that you read the tutorial <a class="link" href="understand_dis_quant.html" title="Understanding Quantiles of Discrete Distributions">Understanding
 101               Quantiles of Discrete Distributions</a> before using the quantile
 102               function on a discrete distribution. The <a class="link" href="../pol_ref/discrete_quant_ref.html" title="Discrete Quantile Policies">reference
 103               docs</a> describe how to change the rounding policy for these distributions.
 104             </p>
 105 </dd>
 106 </dl>
 107 </div>
 108 <p>
 109         What's more, if you define your own policy type, then it automatically inherits
 110         the defaults for any policies not explicitly set, so given:
 111       </p>
 112 <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>
 113 <span class="comment">//</span>
 114 <span class="comment">// Define a policy that sets ::errno on overflow, and does</span>
 115 <span class="comment">// not promote double to long double internally:</span>
 116 <span class="comment">//</span>
 117 <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>
 118 </pre>
 119 <p>
 120         then <code class="computeroutput"><span class="identifier">mypolicy</span></code> defines a policy
 121         where only the overflow error handling and <code class="computeroutput"><span class="keyword">double</span></code>-promotion
 122         policies differ from the defaults.
 123       </p>
 124 </div>
 125 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 126 <td align="left"></td>
 127 <td align="right"><div class="copyright-footer">Copyright &#169; 2006-2010, 2012-2014 Nikhar Agrawal,
 128       Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert
 129       Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan R&#229;de, Gautam Sewani,
 130       Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p>
 131         Distributed under the Boost Software License, Version 1.0. (See accompanying
 132         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>)
 133       </p>
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