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24 | </div> | |
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 | <p> | |
59 | <span class="bold"><strong>may be ignored</strong></span>. | |
60 | </p> | |
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"><</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> | |
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"><</span><span class="identifier">iostream</span><span class="special">></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"><</span><span class="identifier">stdexcept</span><span class="special">></span> | |
161 | ||
162 | ||
163 | <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstddef</span><span class="special">></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"><<</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> | |
174 | <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"BOOST_MATH_DOMAIN_ERROR_POLICY is set to: "</span> | |
175 | <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> | |
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"><<</span> <span class="string">"cdf returned a NaN!"</span> <span class="special"><<</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"><<</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> | |
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"><<</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> | |
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">&</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"><<</span> | |
200 | <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> | |
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<double>::students_t_distribution: | |
214 | Degrees of freedom argument is -1, but must be > 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 © 2006-2010, 2012-2014 Nikhar Agrawal, | |
245 | Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert | |
246 | Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani, | |
247 | Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p> | |
248 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
249 | 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>) | |
250 | </p> | |
251 | </div></td> | |
252 | </tr></table> | |
253 | <hr> | |
254 | <div class="spirit-nav"> | |
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