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| <div class="section" lang="en"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <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 the Policy Defaults</a> |
| </h4></div></div></div> |
| <p> |
| The default policies used by the library are changed by the usual configuration |
| macro method. |
| </p> |
| <p> |
| 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> |
| 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> |
| 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> |
| exception. |
| </p> |
| <div class="tip"><table border="0" summary="Tip"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../../../doc/src/images/tip.png"></td> |
| <th align="left">Tip</th> |
| </tr> |
| <tr><td align="left" valign="top"> |
| <p> |
| For Microsoft Visual Studio,you can add to the Project Property Page, |
| C/C++, Preprocessor, Preprocessor definitions like: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_MATH_ASSERT_UNDEFINED_POLICY</span><span class="special">=</span><span class="number">0</span> |
| <span class="identifier">BOOST_MATH_OVERFLOW_ERROR_POLICY</span><span class="special">=</span><span class="identifier">errno_on_error</span></pre> |
| <p> |
| </p> |
| <p> |
| This may be helpful to avoid complications with pre-compiled headers |
| that may mean that the equivalent definitions in source code: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_ASSERT_UNDEFINED_POLICY</span> <span class="keyword">false</span> |
| <span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_OVERFLOW_ERROR_POLICY</span> <span class="identifier">errno_on_error</span></pre> |
| <p> |
| </p> |
| <p> |
| <span class="bold"><strong>may be ignored</strong></span>. |
| </p> |
| <p> |
| The compiler command line shows: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="special">/</span><span class="identifier">D</span> <span class="string">"BOOST_MATH_ASSERT_UNDEFINED_POLICY=0"</span> |
| <span class="special">/</span><span class="identifier">D</span> <span class="string">"BOOST_MATH_OVERFLOW_ERROR_POLICY=errno_on_error"</span></pre> |
| <p> |
| </p> |
| </td></tr> |
| </table></div> |
| <p> |
| There is however a very important caveat to this: |
| </p> |
| <div class="important"><table border="0" summary="Important"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../../../../doc/src/images/important.png"></td> |
| <th align="left">Important</th> |
| </tr> |
| <tr><td align="left" valign="top"> |
| <p> |
| <span class="bold"><strong><span class="emphasis"><em>Default policies changed by setting configuration |
| macros must be changed uniformly in every translation unit in the program.</em></span></strong></span> |
| </p> |
| <p> |
| Failure to follow this rule may result in violations of the "One |
| Definition Rule (ODR)" and result in unpredictable program behaviour. |
| </p> |
| </td></tr> |
| </table></div> |
| <p> |
| That means there are only two safe ways to use these macros: |
| </p> |
| <div class="itemizedlist"><ul type="disc"> |
| <li> |
| Edit them in <a href="../../../../../../../../boost/math/tools/user.hpp" target="_top">boost/math/tools/user.hpp</a>, |
| so that the defaults are set on an installation-wide basis. Unfortunately |
| this may not be convenient if you are using a pre-installed Boost distribution |
| (on Linux for example). |
| </li> |
| <li> |
| Set the defines in your project's Makefile or build environment, so |
| that they are set uniformly across all translation units. |
| </li> |
| </ul></div> |
| <p> |
| What you should <span class="bold"><strong>not</strong></span> do is: |
| </p> |
| <div class="itemizedlist"><ul type="disc"><li> |
| Set the defines in the source file using <code class="computeroutput"><span class="preprocessor">#define</span></code> |
| as doing so almost certainly will break your program, unless you're |
| absolutely certain that the program is restricted to a single translation |
| unit. |
| </li></ul></div> |
| <p> |
| And, yes, you will find examples in our test programs where we break this |
| rule: but only because we know there will always be a single translation |
| unit only: <span class="emphasis"><em>don't say that you weren't warned!</em></span> |
| </p> |
| <p> |
| </p> |
| <p> |
| The following example demonstrates the effect of setting the macro BOOST_MATH_DOMAIN_ERROR_POLICY |
| when an invalid argument is encountered. For the purposes of this example, |
| we'll pass a negative degrees of freedom parameter to the student's t |
| distribution. |
| </p> |
| <p> |
| </p> |
| <p> |
| Since we know that this is a single file program we could just add: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| to the top of the source file to change the default policy to one that |
| simply returns a NaN when a domain error occurs. Alternatively we could |
| use: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">errno_on_error</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| To ensure the <code class="computeroutput"><span class="special">::</span><span class="identifier">errno</span></code> |
| is set when a domain error occurs as well as returning a NaN. |
| </p> |
| <p> |
| </p> |
| <p> |
| This is safe provided the program consists of a single translation unit |
| <span class="emphasis"><em>and</em></span> we place the define <span class="emphasis"><em>before</em></span> |
| any #includes. Note that should we add the define after the includes |
| then it will have no effect! A warning such as: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting">warning C4005: 'BOOST_MATH_OVERFLOW_ERROR_POLICY' : macro redefinition</pre> |
| <p> |
| </p> |
| <p> |
| is a certain sign that it will <span class="emphasis"><em>not</em></span> have the desired |
| effect. |
| </p> |
| <p> |
| </p> |
| <p> |
| We'll begin our sample program with the needed includes: |
| </p> |
| <p> |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="comment">// Boost |
| </span><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">students_t</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <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> |
| <span class="comment">// std |
| </span><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> |
| <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">;</span> |
| <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> |
| |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">stdexcept</span><span class="special">></span> |
| <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span><span class="special">;</span></pre> |
| <p> |
| </p> |
| <p> |
| </p> |
| <p> |
| Next we'll define the program's main() to call the student's t distribution |
| with an invalid degrees of freedom parameter, the program is set up to |
| handle either an exception or a NaN: |
| </p> |
| <p> |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Example error handling using Student's t function. "</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> |
| <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"BOOST_MATH_DOMAIN_ERROR_POLICY is set to: "</span> |
| <span class="special"><<</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"><<</span> <span class="identifier">endl</span><span class="special">;</span> |
| |
| <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> <span class="keyword">double</span> <span class="identifier">t</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span> |
| |
| <span class="keyword">try</span> |
| <span class="special">{</span> |
| <span class="identifier">errno</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="comment">// Clear/reset. |
| </span> <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> <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> |
| <span class="comment">// Test for error reported by other means: |
| </span> <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> |
| <span class="special">{</span> |
| <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"cdf returned a NaN!"</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> |
| <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> |
| <span class="special">{</span> <span class="comment">// So errno has been set. |
| </span> <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"errno is set to: "</span> <span class="special"><<</span> <span class="identifier">errno</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">}</span> |
| <span class="keyword">else</span> |
| <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Probability of Student's t is "</span> <span class="special"><<</span> <span class="identifier">p</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> |
| <span class="special">}</span> |
| <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">&</span> <span class="identifier">e</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> |
| <span class="string">"\n"</span><span class="string">"Message from thrown exception was:\n "</span> <span class="special"><<</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> |
| <span class="special">}</span> <span class="comment">// int main()</span></pre> |
| <p> |
| </p> |
| <p> |
| </p> |
| <p> |
| Here's what the program output looks like with a default build (one that |
| <span class="bold"><strong>does throw exceptions</strong></span>): |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting">Example error handling using Student's t function. |
| BOOST_MATH_DOMAIN_ERROR_POLICY is set to: throw_on_error |
| |
| Message from thrown exception was: |
| Error in function boost::math::students_t_distribution<double>::students_t_distribution: |
| Degrees of freedom argument is -1, but must be > 0 ! |
| </pre> |
| <p> |
| </p> |
| <p> |
| Alternatively let's build with: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Now the program output is: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting">Example error handling using Student's t function. |
| BOOST_MATH_DOMAIN_ERROR_POLICY is set to: ignore_error |
| cdf returned a NaN! |
| </pre> |
| <p> |
| </p> |
| <p> |
| And finally let's build with: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">errno_on_error</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Which gives the output show errno: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting">Example error handling using Student's t function. |
| BOOST_MATH_DOMAIN_ERROR_POLICY is set to: errno_on_error |
| cdf returned a NaN! |
| errno is set to: 33 |
| </pre> |
| <p> |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2006 , 2007, 2008, 2009, 2010 John Maddock, Paul A. Bristow, |
| 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 |
| 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>) |
| </p> |
| </div></td> |
| </tr></table> |
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