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  25 <div class="section math_toolkit_policy_pol_tutorial_changing_policy_defaults">
  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="math_toolkit.policy.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 </h4></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,
  48             C/C++, Preprocessor, Preprocessor definitions like:
  49           </p>
  50 <p>
  51 </p>
  52 <pre class="programlisting"><span class="identifier">BOOST_MATH_ASSERT_UNDEFINED_POLICY</span><span class="special">=</span><span class="number">0</span>
  53 <span class="identifier">BOOST_MATH_OVERFLOW_ERROR_POLICY</span><span class="special">=</span><span class="identifier">errno_on_error</span></pre>
  54 <p>
  55           </p>
  56 <p>
  57             This may be helpful to avoid complications with pre-compiled headers
  58             that may mean that the equivalent definitions in source code:
  59           </p>
  60 <p>
  61 </p>
  62 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_ASSERT_UNDEFINED_POLICY</span> <span class="keyword">false</span>
  63 <span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_OVERFLOW_ERROR_POLICY</span> <span class="identifier">errno_on_error</span></pre>
  64 <p>
  65           </p>
  66 <p>
  67             <span class="bold"><strong>may be ignored</strong></span>.
  68           </p>
  69 <p>
  70             The compiler command line shows:
  71           </p>
  72 <p>
  73 </p>
  74 <pre class="programlisting"><span class="special">/</span><span class="identifier">D</span> <span class="string">"BOOST_MATH_ASSERT_UNDEFINED_POLICY=0"</span>
  75 <span class="special">/</span><span class="identifier">D</span> <span class="string">"BOOST_MATH_OVERFLOW_ERROR_POLICY=errno_on_error"</span></pre>
  76 <p>
  77           </p>
  78 </td></tr>
  79 </table></div>
  80 <p>
  81           There is however a very important caveat to this:
  82         </p>
  83 <div class="important"><table border="0" summary="Important">
  84 <tr>
  85 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../../../../doc/src/images/important.png"></td>
  86 <th align="left">Important</th>
  87 </tr>
  88 <tr><td align="left" valign="top">
  89 <p>
  90             <span class="bold"><strong><span class="emphasis"><em>Default policies changed by setting configuration
  91             macros must be changed uniformly in every translation unit in the program.</em></span></strong></span>
  92           </p>
  93 <p>
  94             Failure to follow this rule may result in violations of the "One
  95             Definition Rule (ODR)" and result in unpredictable program behaviour.
  96           </p>
  97 </td></tr>
  98 </table></div>
  99 <p>
 100           That means there are only two safe ways to use these macros:
 101         </p>
 102 <div class="itemizedlist"><ul class="itemizedlist" type="disc">
 103 <li class="listitem">
 104               Edit them in <a href="../../../../../../../../boost/math/tools/user.hpp" target="_top">boost/math/tools/user.hpp</a>,
 105               so that the defaults are set on an installation-wide basis. Unfortunately
 106               this may not be convenient if you are using a pre-installed Boost distribution
 107               (on Linux for example).
 108             </li>
 109 <li class="listitem">
 110               Set the defines in your project's Makefile or build environment, so
 111               that they are set uniformly across all translation units.
 112             </li>
 113 </ul></div>
 114 <p>
 115           What you should <span class="bold"><strong>not</strong></span> do is:
 116         </p>
 117 <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
 118               Set the defines in the source file using <code class="computeroutput"><span class="preprocessor">#define</span></code>
 119               as doing so almost certainly will break your program, unless you're
 120               absolutely certain that the program is restricted to a single translation
 121               unit.
 122             </li></ul></div>
 123 <p>
 124           And, yes, you will find examples in our test programs where we break this
 125           rule: but only because we know there will always be a single translation
 126           unit only: <span class="emphasis"><em>don't say that you weren't warned!</em></span>
 127         </p>
 128 <p>
 129           The following example demonstrates the effect of setting the macro BOOST_MATH_DOMAIN_ERROR_POLICY
 130           when an invalid argument is encountered. For the purposes of this example,
 131           we'll pass a negative degrees of freedom parameter to the student's t distribution.
 132         </p>
 133 <p>
 134           Since we know that this is a single file program we could just add:
 135         </p>
 136 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
 137 </pre>
 138 <p>
 139           to the top of the source file to change the default policy to one that
 140           simply returns a NaN when a domain error occurs. Alternatively we could
 141           use:
 142         </p>
 143 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">errno_on_error</span>
 144 </pre>
 145 <p>
 146           To ensure the <code class="computeroutput"><span class="special">::</span><span class="identifier">errno</span></code>
 147           is set when a domain error occurs as well as returning a NaN.
 148         </p>
 149 <p>
 150           This is safe provided the program consists of a single translation unit
 151           <span class="emphasis"><em>and</em></span> we place the define <span class="emphasis"><em>before</em></span>
 152           any #includes. Note that should we add the define after the includes then
 153           it will have no effect! A warning such as:
 154         </p>
 155 <pre class="programlisting">warning C4005: 'BOOST_MATH_OVERFLOW_ERROR_POLICY' : macro redefinition</pre>
 156 <p>
 157           is a certain sign that it will <span class="emphasis"><em>not</em></span> have the desired
 158           effect.
 159         </p>
 160 <p>
 161           We'll begin our sample program with the needed includes:
 162         </p>
 163 <p>
 164 </p>
 165 <pre class="programlisting"><span class="comment">// Boost</span>
 166 <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>
 167    <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>
 168
 169 <span class="comment">// std</span>
 170 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
 171    <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">;</span>
 172    <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 173
 174 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">stdexcept</span><span class="special">&gt;</span>
 175    <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span><span class="special">;</span>
 176
 177 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cstddef</span><span class="special">&gt;</span>
 178    <span class="comment">// using ::errno</span>
 179 </pre>
 180 <p>
 181         </p>
 182 <p>
 183           Next we'll define the program's main() to call the student's t distribution
 184           with an invalid degrees of freedom parameter, the program is set up to
 185           handle either an exception or a NaN:
 186         </p>
 187 <p>
 188 </p>
 189 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 190 <span class="special">{</span>
 191    <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>
 192    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"BOOST_MATH_DOMAIN_ERROR_POLICY is set to: "</span>
 193       <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>
 194
 195    <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>
 196    <span class="keyword">double</span> <span class="identifier">t</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
 197
 198    <span class="keyword">try</span>
 199    <span class="special">{</span>
 200       <span class="identifier">errno</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="comment">// Clear/reset.</span>
 201       <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>
 202       <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>
 203       <span class="comment">// Test for error reported by other means:</span>
 204       <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>
 205       <span class="special">{</span>
 206          <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>
 207          <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>
 208          <span class="special">{</span> <span class="comment">// So errno has been set.</span>
 209            <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>
 210          <span class="special">}</span>
 211       <span class="special">}</span>
 212       <span class="keyword">else</span>
 213          <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>
 214    <span class="special">}</span>
 215    <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>
 216    <span class="special">{</span>
 217       <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span>
 218          <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>
 219    <span class="special">}</span>
 220    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 221 <span class="special">}</span> <span class="comment">// int main()</span>
 222 </pre>
 223 <p>
 224         </p>
 225 <p>
 226           Here's what the program output looks like with a default build (one that
 227           <span class="bold"><strong>does throw exceptions</strong></span>):
 228         </p>
 229 <pre class="programlisting">Example error handling using Student's t function.
 230 BOOST_MATH_DOMAIN_ERROR_POLICY is set to: throw_on_error
 231
 232 Message from thrown exception was:
 233    Error in function boost::math::students_t_distribution&lt;double&gt;::students_t_distribution:
 234    Degrees of freedom argument is -1, but must be &gt; 0 !
 235 </pre>
 236 <p>
 237           Alternatively let's build with:
 238         </p>
 239 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
 240 </pre>
 241 <p>
 242           Now the program output is:
 243         </p>
 244 <pre class="programlisting">Example error handling using Student's t function.
 245 BOOST_MATH_DOMAIN_ERROR_POLICY is set to: ignore_error
 246 cdf returned a NaN!
 247 </pre>
 248 <p>
 249           And finally let's build with:
 250         </p>
 251 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">errno_on_error</span>
 252 </pre>
 253 <p>
 254           Which gives the output show errno:
 255         </p>
 256 <pre class="programlisting">Example error handling using Student's t function.
 257 BOOST_MATH_DOMAIN_ERROR_POLICY is set to: errno_on_error
 258 cdf returned a NaN!
 259 errno is set to: 33
 260 </pre>
 261 </div>
 262 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 263 <td align="left"></td>
 264 <td align="right"><div class="copyright-footer">Copyright &#169; 2006-2010 John Maddock, Paul A. Bristow, Hubert Holin, Xiaogang Zhang, Bruno
 265       Lalande, Johan R&#229;de, Gautam Sewani, Thijs van den Berg and Benjamin Sobotta<p>
 266         Distributed under the Boost Software License, Version 1.0. (See accompanying
 267         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>)
 268       </p>
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