<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Changing the Policy Defaults</title>
<link rel="stylesheet" href="../../../../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.74.0">
<link rel="home" href="../../../index.html" title="Math Toolkit">
<link rel="up" href="../pol_tutorial.html" title="Policy Tutorial">
<link rel="prev" href="policy_usage.html" title="So How are Policies Used Anyway?">
<link rel="next" href="ad_hoc_dist_policies.html" title="Setting Policies for Distributions on an Ad Hoc Basis">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../../boost.png"></td>
<td align="center"><a href="../../../../../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="policy_usage.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../pol_tutorial.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="ad_hoc_dist_policies.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<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 NaN 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>
may be ignored.
</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="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="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="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
does throw exceptions):
</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!
errno is set to: 0
</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:
</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 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>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="policy_usage.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../pol_tutorial.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="ad_hoc_dist_policies.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
