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<div class="titlepage"><div><div><h4 class="title">
<a name="math_toolkit.policy.pol_tutorial.ad_hoc_sf_policies"></a><a class="link" href="ad_hoc_sf_policies.html" title="Changing the Policy on an Ad Hoc Basis for the Special Functions">
Changing the Policy on an Ad Hoc Basis for the Special Functions</a>
</h4></div></div></div>
<p>
All of the special functions in this library come in two overloaded forms,
one with a final "policy" parameter, and one without. For example:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">math</span><span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">RealType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Policy</span><span class="special">></span>
<span class="identifier">RealType</span> <span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">RealType</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">Policy</span><span class="special">&);</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">RealType</span><span class="special">></span>
<span class="identifier">RealType</span> <span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">RealType</span><span class="special">);</span>
<span class="special">}}</span> <span class="comment">// namespaces
</span></pre>
<p>
Normally, the second version is just a forwarding wrapper to the first
like this:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">RealType</span><span class="special">></span>
<span class="keyword">inline</span> <span class="identifier">RealType</span> <span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">RealType</span> <span class="identifier">x</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">x</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="special">}</span>
</pre>
<p>
So calling a special function with a specific policy is just a matter of
defining the policy type to use and passing it as the final parameter.
For example, suppose we want <code class="computeroutput"><span class="identifier">tgamma</span></code>
to behave in a C-compatible fashion and set <code class="computeroutput"><span class="special">::</span><span class="identifier">errno</span></code> when an error occurs, and never
throw an exception:
</p>
<p>
</p>
<p>
</p>
<pre class="programlisting"><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">special_functions</span><span class="special">/</span><span class="identifier">gamma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="comment">//
</span><span class="comment">// Define the policy to use:
</span><span class="comment">//
</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="keyword">typedef</span> <span class="identifier">policy</span><span class="special"><</span>
<span class="identifier">domain_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>,</span>
<span class="identifier">pole_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>,</span>
<span class="identifier">overflow_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>,</span>
<span class="identifier">evaluation_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">></span>
<span class="special">></span> <span class="identifier">c_policy</span><span class="special">;</span>
<span class="comment">//
</span><span class="comment">// Now use the policy when calling tgamma:
</span><span class="comment">//
</span><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</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">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Result of tgamma(30000) is: "</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="number">30000</span><span class="special">,</span> <span class="identifier">c_policy</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="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"errno = "</span> <span class="special"><<</span> <span class="identifier">errno</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="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Result of tgamma(-10) is: "</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="number">10</span><span class="special">,</span> <span class="identifier">c_policy</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="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"errno = "</span> <span class="special"><<</span> <span class="identifier">errno</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>
</pre>
<p>
</p>
<p>
</p>
<p>
which outputs:
</p>
<pre class="programlisting">Result of tgamma(30000) is: 1.#INF
errno = 34
Result of tgamma(-10) is: 1.#QNAN
errno = 33
</pre>
<p>
Alternatively, for ad hoc use, we can use the <code class="computeroutput"><span class="identifier">make_policy</span></code>
helper function to create a policy for us: this usage is more verbose,
so is probably only preferred when a policy is going to be used once only:
</p>
<p>
</p>
<p>
</p>
<pre class="programlisting"><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">special_functions</span><span class="special">/</span><span class="identifier">gamma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</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="identifier">errno</span> <span class="special">=</span> <span class="number">0</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">"Result of tgamma(30000) is: "</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="number">30000</span><span class="special">,</span>
<span class="identifier">make_policy</span><span class="special">(</span>
<span class="identifier">domain_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>(),</span>
<span class="identifier">pole_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>(),</span>
<span class="identifier">overflow_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>(),</span>
<span class="identifier">evaluation_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>()</span>
<span class="special">)</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="comment">// Check errno was set:
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"errno = "</span> <span class="special"><<</span> <span class="identifier">errno</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="comment">// and again with evaluation at a pole:
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Result of tgamma(-10) is: "</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="special">-</span><span class="number">10</span><span class="special">,</span>
<span class="identifier">make_policy</span><span class="special">(</span>
<span class="identifier">domain_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>(),</span>
<span class="identifier">pole_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>(),</span>
<span class="identifier">overflow_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>(),</span>
<span class="identifier">evaluation_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>()</span>
<span class="special">)</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="comment">// Check errno was set:
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"errno = "</span> <span class="special"><<</span> <span class="identifier">errno</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>
</pre>
<p>
</p>
<p>
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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>)
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