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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="math_toolkit.policy.pol_ref.assert_undefined"></a><a class="link" href="assert_undefined.html" title="Mathematically Undefined Function Policies"> Mathematically |
| Undefined Function Policies</a> |
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| <p> |
| There are some functions that are generic (they are present for all the |
| statistical distributions supported) but which may be mathematically undefined |
| for certain distributions, but defined for others. |
| </p> |
| <p> |
| For example, the Cauchy distribution does not have a meaningful mean, so |
| what should |
| </p> |
| <pre class="programlisting"><span class="identifier">mean</span><span class="special">(</span><span class="identifier">cauchy</span><span class="special"><>());</span> |
| </pre> |
| <p> |
| return, and should such an expression even compile at all? |
| </p> |
| <p> |
| The default behaviour is for all such functions to not compile at all - |
| in fact they will raise a <a href="http://www.boost.org/libs/static_assert/index.html" target="_top">static |
| assertion</a> - but by changing the policy we can have them return |
| the result of a domain error instead (which may well throw an exception, |
| depending on the error handling policy). |
| </p> |
| <p> |
| This behaviour is controlled by the <code class="computeroutput"><span class="identifier">assert_undefined</span><span class="special"><></span></code> policy: |
| </p> |
| <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">math</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">policies</span> <span class="special">{</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">bool</span> <span class="identifier">b</span><span class="special">></span> |
| <span class="keyword">class</span> <span class="identifier">assert_undefined</span><span class="special">;</span> |
| |
| <span class="special">}}}</span> <span class="comment">//namespaces |
| </span></pre> |
| <p> |
| For example: |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">distributions</span><span class="special">/</span><span class="identifier">cauchy</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| |
| <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="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="comment">// This will not compile, cauchy has no mean! |
| </span><span class="keyword">double</span> <span class="identifier">m1</span> <span class="special">=</span> <span class="identifier">mean</span><span class="special">(</span><span class="identifier">cauchy</span><span class="special">());</span> |
| |
| <span class="comment">// This will compile, but raises a domain error! |
| </span><span class="keyword">double</span> <span class="identifier">m2</span> <span class="special">=</span> <span class="identifier">mean</span><span class="special">(</span><span class="identifier">cauchy_distribution</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">policy</span><span class="special"><</span><span class="identifier">assert_undefined</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span> <span class="special">></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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