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  25 <div class="section math_toolkit_policy_pol_ref_assert_undefined">
  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="math_toolkit.policy.pol_ref.assert_undefined"></a><a class="link" href="assert_undefined.html" title="Mathematically Undefined Function Policies">Mathematically
  28         Undefined Function Policies</a>
  29 </h4></div></div></div>
  30 <p>
  31           There are some functions that are generic (they are present for all the
  32           statistical distributions supported) but which may be mathematically undefined
  33           for certain distributions, but defined for others.
  34         </p>
  35 <p>
  36           For example, the Cauchy distribution does not have a meaningful mean, so
  37           what should
  38         </p>
  39 <pre class="programlisting"><span class="identifier">mean</span><span class="special">(</span><span class="identifier">cauchy</span><span class="special">&lt;&gt;());</span>
  40 </pre>
  41 <p>
  42           return, and should such an expression even compile at all?
  43         </p>
  44 <p>
  45           The default behaviour is for all such functions to not compile at all -
  46           in fact they will raise a <a href="http://www.boost.org/libs/static_assert/index.html" target="_top">static
  47           assertion</a> - but by changing the policy we can have them return
  48           the result of a domain error instead (which may well throw an exception,
  49           depending on the error handling policy).
  50         </p>
  51 <p>
  52           This behaviour is controlled by the <code class="computeroutput"><span class="identifier">assert_undefined</span><span class="special">&lt;&gt;</span></code> policy:
  53         </p>
  54 <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>
  55
  56 <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">bool</span> <span class="identifier">b</span><span class="special">&gt;</span>
  57 <span class="keyword">class</span> <span class="identifier">assert_undefined</span><span class="special">;</span>
  58
  59 <span class="special">}}}</span> <span class="comment">//namespaces</span>
  60 </pre>
  61 <p>
  62           For example:
  63         </p>
  64 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
  65
  66 <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>
  67 <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>
  68
  69 <span class="comment">// This will not compile, cauchy has no mean!</span>
  70 <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>
  71
  72 <span class="comment">// This will compile, but raises a domain error!</span>
  73 <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">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">policy</span><span class="special">&lt;</span><span class="identifier">assert_undefined</span><span class="special">&lt;</span><span class="keyword">false</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;());</span>
  74 </pre>
  75 <p>
  76           <code class="computeroutput"><span class="identifier">policy</span><span class="special">&lt;</span><span class="identifier">assert_undefined</span><span class="special">&lt;</span><span class="keyword">false</span><span class="special">&gt;</span></code>
  77           behaviour can also be obtained by defining the macro
  78         </p>
  79 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_ASSERT_UNDEFINED_POLICY</span> <span class="keyword">false</span>
  80 </pre>
  81 <p>
  82           at the head of the file - see <a class="link" href="policy_defaults.html" title="Using Macros to Change the Policy Defaults">Using
  83           Macros to Change the Policy Defaults</a>.
  84         </p>
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  89       Lalande, Johan R&#229;de, Gautam Sewani, Thijs van den Berg and Benjamin Sobotta<p>
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