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<title>Setting Polices at Namespace Scope</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="math_toolkit.pol_ref.namespace_pol"></a><a class="link" href="namespace_pol.html" title="Setting Polices at Namespace Scope">Setting Polices at
Namespace Scope</a>
</h3></div></div></div>
<p>
Sometimes what you really want to do is bring all the special functions,
or all the distributions into a specific namespace-scope, along with a specific
policy to use with them. There are two macros defined to assist with that:
</p>
<pre class="programlisting"><span class="identifier">BOOST_MATH_DECLARE_SPECIAL_FUNCTIONS</span><span class="special">(</span><span class="identifier">Policy</span><span class="special">)</span>
</pre>
<p>
and:
</p>
<pre class="programlisting"><span class="identifier">BOOST_MATH_DECLARE_DISTRIBUTIONS</span><span class="special">(</span><span class="identifier">Type</span><span class="special">,</span> <span class="identifier">Policy</span><span class="special">)</span>
</pre>
<p>
You can use either of these macros after including any special function or
distribution header. For example:
</p>
<pre class="programlisting"><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">special_functions</span><span class="special">/</span><span class="identifier">gamma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="comment">//using boost::math::tgamma;</span>
<span class="comment">// Need not declare using boost::math::tgamma here,</span>
<span class="comment">// because will define tgamma in myspace using macro below.</span>
<span class="keyword">namespace</span> <span class="identifier">myspace</span>
<span class="special">{</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">;</span>
<span class="comment">// Define a policy that does not throw on overflow:</span>
<span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special">&lt;</span><span class="identifier">overflow_error</span><span class="special">&lt;</span><span class="identifier">errno_on_error</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">my_policy</span><span class="special">;</span>
<span class="comment">// Define the special functions in this scope to use the policy: </span>
<span class="identifier">BOOST_MATH_DECLARE_SPECIAL_FUNCTIONS</span><span class="special">(</span><span class="identifier">my_policy</span><span class="special">)</span>
<span class="special">}</span>
<span class="comment">// Now we can use myspace::tgamma etc.</span>
<span class="comment">// They will automatically use "my_policy":</span>
<span class="comment">//</span>
<span class="keyword">double</span> <span class="identifier">t</span> <span class="special">=</span> <span class="identifier">myspace</span><span class="special">::</span><span class="identifier">tgamma</span><span class="special">(</span><span class="number">30.0</span><span class="special">);</span> <span class="comment">// Will *not* throw on overflow,</span>
<span class="comment">// despite the large value of factorial 30 = 265252859812191058636308480000000</span>
<span class="comment">// unlike default policy boost::math::tgamma;</span>
</pre>
<p>
In this example, using BOOST_MATH_DECLARE_SPECIAL_FUNCTIONS results in a
set of thin inline forwarding functions being defined:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">a</span><span class="special">){</span> <span class="keyword">return</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">tgamma</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">mypolicy</span><span class="special">());</span> <span class="special">}</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">lgamma</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">a</span><span class="special">)</span> <span class="special">(</span> <span class="keyword">return</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">lgamma</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">mypolicy</span><span class="special">());</span> <span class="special">}</span>
</pre>
<p>
and so on. Note that while a forwarding function is defined for all the special
functions, however, unless you include the specific header for the special
function you use (or boost/math/special_functions.hpp to include everything),
you will get linker errors from functions that are forward declared, but
not defined.
</p>
<p>
We can do the same thing with the distributions, but this time we need to
specify the floating-point type to use:
</p>
<pre class="programlisting"><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">cauchy</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">myspace</span>
<span class="special">{</span> <span class="comment">// using namespace boost::math::policies; // May be convenient in myspace.</span>
<span class="comment">// Define a policy called my_policy to use.</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">policies</span><span class="special">::</span><span class="identifier">policy</span><span class="special">;</span>
<span class="comment">// In this case we want all the distribution accessor functions to compile,</span>
<span class="comment">// even if they are mathematically undefined, so</span>
<span class="comment">// make the policy assert_undefined.</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">policies</span><span class="special">::</span><span class="identifier">assert_undefined</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special">&lt;</span><span class="identifier">assert_undefined</span><span class="special">&lt;</span><span class="keyword">false</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">my_policy</span><span class="special">;</span>
<span class="comment">// Finally apply this policy to type double.</span>
<span class="identifier">BOOST_MATH_DECLARE_DISTRIBUTIONS</span><span class="special">(</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">my_policy</span><span class="special">)</span>
<span class="special">}</span> <span class="comment">// namespace myspace</span>
<span class="comment">// Now we can use myspace::cauchy etc, which will use policy</span>
<span class="comment">// myspace::mypolicy:</span>
<span class="comment">//</span>
<span class="comment">// This compiles but throws a domain error exception at runtime.</span>
<span class="comment">// Caution! If you omit the try'n'catch blocks, </span>
<span class="comment">// it will just silently terminate, giving no clues as to why! </span>
<span class="comment">// So try'n'catch blocks are very strongly recommended.</span>
<span class="keyword">void</span> <span class="identifier">test_cauchy</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">try</span>
<span class="special">{</span>
<span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">mean</span><span class="special">(</span><span class="identifier">myspace</span><span class="special">::</span><span class="identifier">cauchy</span><span class="special">());</span> <span class="comment">// Cauchy does not have a mean!</span>
<span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="identifier">d</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">domain_error</span><span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">cout</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">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
In this example the result of BOOST_MATH_DECLARE_DISTRIBUTIONS is to declare
a typedef for each distribution like this:
</p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">cauchy_distribution</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">my_policy</span><span class="special">&gt;</span> <span class="identifier">cauchy</span><span class="special">;</span>
<span class="identifier">tyepdef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">gamma_distribution</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">my_policy</span><span class="special">&gt;</span> <span class="identifier">gamma</span><span class="special">;</span>
</pre>
<p>
and so on. The name given to each typedef is the name of the distribution
with the "_distribution" suffix removed.
</p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2006-2019 Nikhar
Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos,
Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan
R&#229;de, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg,
Daryle Walker and Xiaogang Zhang<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>)
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