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  25 <div class="section">
  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="math_toolkit.pol_tutorial.changing_policy_defaults"></a><a class="link" href="changing_policy_defaults.html" title="Changing the Policy Defaults">Changing
  28       the Policy Defaults</a>
  29 </h3></div></div></div>
  30 <p>
  31         The default policies used by the library are changed by the usual configuration
  32         macro method.
  33       </p>
  34 <p>
  35         For example, passing <code class="computeroutput"><span class="special">-</span><span class="identifier">DBOOST_MATH_DOMAIN_ERROR_POLICY</span><span class="special">=</span><span class="identifier">errno_on_error</span></code>
  36         to your compiler will cause domain errors to set <code class="computeroutput"><span class="special">::</span><span class="identifier">errno</span></code> and return a <a href="http://en.wikipedia.org/wiki/NaN" target="_top">NaN</a>
  37         rather than the usual default behaviour of throwing a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">domain_error</span></code>
  38         exception.
  39       </p>
  40 <div class="tip"><table border="0" summary="Tip">
  41 <tr>
  42 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
  43 <th align="left">Tip</th>
  44 </tr>
  45 <tr><td align="left" valign="top">
  46 <p>
  47           For Microsoft Visual Studio,you can add to the Project Property Page, C/C++,
  48           Preprocessor, Preprocessor definitions like:
  49         </p>
  50 <pre class="programlisting"><span class="identifier">BOOST_MATH_ASSERT_UNDEFINED_POLICY</span><span class="special">=</span><span class="number">0</span>
  51 <span class="identifier">BOOST_MATH_OVERFLOW_ERROR_POLICY</span><span class="special">=</span><span class="identifier">errno_on_error</span></pre>
  52 <p>
  53           This may be helpful to avoid complications with pre-compiled headers that
  54           may mean that the equivalent definitions in source code:
  55         </p>
  56 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_ASSERT_UNDEFINED_POLICY</span> <span class="keyword">false</span>
  57 <span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_OVERFLOW_ERROR_POLICY</span> <span class="identifier">errno_on_error</span></pre>
  58 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
  59               may be ignored*.
  60             </li></ul></div>
  61 <p>
  62           The compiler command line shows:
  63         </p>
  64 <pre class="programlisting"><span class="special">/</span><span class="identifier">D</span> <span class="string">"BOOST_MATH_ASSERT_UNDEFINED_POLICY=0"</span>
  65 <span class="special">/</span><span class="identifier">D</span> <span class="string">"BOOST_MATH_OVERFLOW_ERROR_POLICY=errno_on_error"</span></pre>
  66 </td></tr>
  67 </table></div>
  68 <p>
  69         There is however a very important caveat to this:
  70       </p>
  71 <div class="important"><table border="0" summary="Important">
  72 <tr>
  73 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../../doc/src/images/important.png"></td>
  74 <th align="left">Important</th>
  75 </tr>
  76 <tr><td align="left" valign="top">
  77 <p>
  78           <span class="bold"><strong><span class="emphasis"><em>Default policies changed by setting configuration
  79           macros must be changed uniformly in every translation unit in the program.</em></span></strong></span>
  80         </p>
  81 <p>
  82           Failure to follow this rule may result in violations of the "One Definition
  83           Rule (ODR)" and result in unpredictable program behaviour.
  84         </p>
  85 </td></tr>
  86 </table></div>
  87 <p>
  88         That means there are only two safe ways to use these macros:
  89       </p>
  90 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  91 <li class="listitem">
  92             Edit them in <a href="../../../../../../boost/math/tools/user.hpp" target="_top">boost/math/tools/user.hpp</a>,
  93             so that the defaults are set on an installation-wide basis. Unfortunately
  94             this may not be convenient if you are using a pre-installed Boost distribution
  95             (on Linux for example).
  96           </li>
  97 <li class="listitem">
  98             Set the defines in your project's Makefile or build environment, so that
  99             they are set uniformly across all translation units.
 100           </li>
 101 </ul></div>
 102 <p>
 103         What you should <span class="bold"><strong>not</strong></span> do is:
 104       </p>
 105 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 106             Set the defines in the source file using <code class="computeroutput"><span class="preprocessor">#define</span></code>
 107             as doing so almost certainly will break your program, unless you're absolutely
 108             certain that the program is restricted to a single translation unit.
 109           </li></ul></div>
 110 <p>
 111         And, yes, you will find examples in our test programs where we break this
 112         rule: but only because we know there will always be a single translation
 113         unit only: <span class="emphasis"><em>don't say that you weren't warned!</em></span>
 114       </p>
 115 <p>
 116         The following example demonstrates the effect of setting the macro BOOST_MATH_DOMAIN_ERROR_POLICY
 117         when an invalid argument is encountered. For the purposes of this example,
 118         we'll pass a negative degrees of freedom parameter to the student's t distribution.
 119       </p>
 120 <p>
 121         Since we know that this is a single file program we could just add:
 122       </p>
 123 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
 124 </pre>
 125 <p>
 126         to the top of the source file to change the default policy to one that simply
 127         returns a NaN when a domain error occurs. Alternatively we could use:
 128       </p>
 129 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">errno_on_error</span>
 130 </pre>
 131 <p>
 132         To ensure the <code class="computeroutput"><span class="special">::</span><span class="identifier">errno</span></code>
 133         is set when a domain error occurs as well as returning a NaN.
 134       </p>
 135 <p>
 136         This is safe provided the program consists of a single translation unit
 137         <span class="emphasis"><em>and</em></span> we place the define <span class="emphasis"><em>before</em></span>
 138         any #includes. Note that should we add the define after the includes then
 139         it will have no effect! A warning such as:
 140       </p>
 141 <pre class="programlisting">warning C4005: 'BOOST_MATH_OVERFLOW_ERROR_POLICY' : macro redefinition</pre>
 142 <p>
 143         is a certain sign that it will <span class="emphasis"><em>not</em></span> have the desired
 144         effect.
 145       </p>
 146 <p>
 147         We'll begin our sample program with the needed includes:
 148       </p>
 149 <pre class="programlisting">   <span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
 150
 151 <span class="comment">// Boost</span>
 152 <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">students_t</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 153    <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">students_t</span><span class="special">;</span>  <span class="comment">// Probability of students_t(df, t).</span>
 154
 155 <span class="comment">// std</span>
 156 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
 157    <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">;</span>
 158    <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 159
 160 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">stdexcept</span><span class="special">&gt;</span>
 161
 162
 163 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cstddef</span><span class="special">&gt;</span>
 164    <span class="comment">// using ::errno</span>
 165 </pre>
 166 <p>
 167         Next we'll define the program's main() to call the student's t distribution
 168         with an invalid degrees of freedom parameter, the program is set up to handle
 169         either an exception or a NaN:
 170       </p>
 171 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 172 <span class="special">{</span>
 173    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Example error handling using Student's t function. "</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 174    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"BOOST_MATH_DOMAIN_ERROR_POLICY is set to: "</span>
 175       <span class="special">&lt;&lt;</span> <span class="identifier">BOOST_STRINGIZE</span><span class="special">(</span><span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 176
 177    <span class="keyword">double</span> <span class="identifier">degrees_of_freedom</span> <span class="special">=</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span> <span class="comment">// A bad argument!</span>
 178    <span class="keyword">double</span> <span class="identifier">t</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
 179
 180    <span class="keyword">try</span>
 181    <span class="special">{</span>
 182       <span class="identifier">errno</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="comment">// Clear/reset.</span>
 183       <span class="identifier">students_t</span> <span class="identifier">dist</span><span class="special">(</span><span class="identifier">degrees_of_freedom</span><span class="special">);</span> <span class="comment">// exception is thrown here if enabled.</span>
 184       <span class="keyword">double</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">cdf</span><span class="special">(</span><span class="identifier">dist</span><span class="special">,</span> <span class="identifier">t</span><span class="special">);</span>
 185       <span class="comment">// Test for error reported by other means:</span>
 186       <span class="keyword">if</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">isnan</span><span class="special">)(</span><span class="identifier">p</span><span class="special">))</span>
 187       <span class="special">{</span>
 188          <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"cdf returned a NaN!"</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 189          <span class="keyword">if</span> <span class="special">(</span><span class="identifier">errno</span> <span class="special">!=</span> <span class="number">0</span><span class="special">)</span>
 190          <span class="special">{</span> <span class="comment">// So errno has been set.</span>
 191            <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"errno is set to: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">errno</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 192          <span class="special">}</span>
 193       <span class="special">}</span>
 194       <span class="keyword">else</span>
 195          <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Probability of Student's t is "</span> <span class="special">&lt;&lt;</span> <span class="identifier">p</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 196    <span class="special">}</span>
 197    <span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span><span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span>
 198    <span class="special">{</span>
 199       <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span>
 200          <span class="string">"\n"</span><span class="string">"Message from thrown exception was:\n   "</span> <span class="special">&lt;&lt;</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 201    <span class="special">}</span>
 202    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 203 <span class="special">}</span> <span class="comment">// int main()</span>
 204 </pre>
 205 <p>
 206         Here's what the program output looks like with a default build (one that
 207         <span class="bold"><strong>does throw exceptions</strong></span>):
 208       </p>
 209 <pre class="programlisting">Example error handling using Student's t function.
 210 BOOST_MATH_DOMAIN_ERROR_POLICY is set to: throw_on_error
 211
 212 Message from thrown exception was:
 213    Error in function boost::math::students_t_distribution&lt;double&gt;::students_t_distribution:
 214    Degrees of freedom argument is -1, but must be &gt; 0 !
 215 </pre>
 216 <p>
 217         Alternatively let's build with:
 218       </p>
 219 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
 220 </pre>
 221 <p>
 222         Now the program output is:
 223       </p>
 224 <pre class="programlisting">Example error handling using Student's t function.
 225 BOOST_MATH_DOMAIN_ERROR_POLICY is set to: ignore_error
 226 cdf returned a NaN!
 227 </pre>
 228 <p>
 229         And finally let's build with:
 230       </p>
 231 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">errno_on_error</span>
 232 </pre>
 233 <p>
 234         Which gives the output show errno:
 235       </p>
 236 <pre class="programlisting">Example error handling using Student's t function.
 237 BOOST_MATH_DOMAIN_ERROR_POLICY is set to: errno_on_error
 238 cdf returned a NaN!
 239 errno is set to: 33
 240 </pre>
 241 </div>
 242 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 243 <td align="left"></td>
 244 <td align="right"><div class="copyright-footer">Copyright &#169; 2006-2019 Nikhar
 245       Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos,
 246       Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan
 247       R&#229;de, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg,
 248       Daryle Walker and Xiaogang Zhang<p>
 249         Distributed under the Boost Software License, Version 1.0. (See accompanying
 250         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>)
 251       </p>
 252 </div></td>
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