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<div class="titlepage"><div><div><h3 class="title">
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<a name="math_toolkit.tutorial.templ"></a><a class="link" href="templ.html" title="Use in template code">Use in template code</a>
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</h3></div></div></div>
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<p>
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When using the constants inside a function template, we need to ensure that
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we use a constant of the correct precision for our template parameters. We
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can do this by calling the function-template versions, <code class="computeroutput"><span class="identifier">pi</span><span class="special"><</span><span class="identifier">FPType</span><span class="special">>()</span></code>, of the constants like this:
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</p>
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<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">constants</span><span class="special">/</span><span class="identifier">constants</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Real</span><span class="special">></span>
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<span class="identifier">Real</span> <span class="identifier">area</span><span class="special">(</span><span class="identifier">Real</span> <span class="identifier">r</span><span class="special">)</span>
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<span class="special">{</span>
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<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">constants</span><span class="special">;</span>
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<span class="keyword">return</span> <span class="identifier">pi</span><span class="special"><</span><span class="identifier">Real</span><span class="special">>()</span> <span class="special">*</span> <span class="identifier">r</span> <span class="special">*</span> <span class="identifier">r</span><span class="special">;</span>
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<span class="special">}</span>
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</pre>
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<p>
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Although this syntax is a little less "cute" than the non-template
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version, the code is no less efficient (at least for the built-in types
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<code class="computeroutput"><span class="keyword">float</span></code>, <code class="computeroutput"><span class="keyword">double</span></code>
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and <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>)
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: the function template versions of the constants are simple inline functions
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that return a constant of the correct precision for the type used. In addition,
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these functions are declared <code class="computeroutput"><span class="identifier">constexp</span></code>
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for those compilers that support this, allowing the result to be used in
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constant-expressions provided the template argument is a literal type.
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</p>
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<div class="tip"><table border="0" summary="Tip">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
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<th align="left">Tip</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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Keep in mind the difference between the variable version, just <code class="computeroutput"><span class="identifier">pi</span></code>, and the template-function version:
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the template-function requires both a <<em class="replaceable"><code>floating-point-type</code></em>>
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and function call <code class="computeroutput"><span class="special">()</span></code> brackets,
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for example: <code class="computeroutput"><span class="identifier">pi</span><span class="special"><</span><span class="keyword">double</span><span class="special">>()</span></code>.
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You cannot write <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">p</span>
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<span class="special">=</span> <span class="identifier">pi</span><span class="special"><>()</span></code>, nor <code class="computeroutput"><span class="keyword">double</span>
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<span class="identifier">p</span> <span class="special">=</span>
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<span class="identifier">pi</span><span class="special">()</span></code>.
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</p></td></tr>
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</table></div>
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<div class="note"><table border="0" summary="Note">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
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<th align="left">Note</th>
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</tr>
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<tr><td align="left" valign="top">
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<p>
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You can always use <span class="bold"><strong>both</strong></span> variable and template-function
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versions <span class="bold"><strong>provided calls are fully qualified</strong></span>,
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for example:
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</p>
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<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">my_pi1</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">constants</span><span class="special">::</span><span class="identifier">pi</span><span class="special"><</span><span class="keyword">double</span><span class="special">>();</span>
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<span class="keyword">double</span> <span class="identifier">my_pi2</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">double_constants</span><span class="special">::</span><span class="identifier">pi</span><span class="special">;</span>
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</pre>
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</td></tr>
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</table></div>
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<div class="warning"><table border="0" summary="Warning">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td>
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<th align="left">Warning</th>
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</tr>
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<tr><td align="left" valign="top">
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<p>
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It may be tempting to simply define
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</p>
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<pre class="programlisting"><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">double_constants</span><span class="special">;</span>
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<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">constants</span><span class="special">;</span>
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</pre>
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<p>
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but if you do define two namespaces, this will, of course, create ambiguity!
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</p>
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<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">my_pi</span> <span class="special">=</span> <span class="identifier">pi</span><span class="special">();</span> <span class="comment">// error C2872: 'pi' : ambiguous symbol</span>
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<span class="keyword">double</span> <span class="identifier">my_pi2</span> <span class="special">=</span> <span class="identifier">pi</span><span class="special">;</span> <span class="comment">// Context does not allow for disambiguation of overloaded function</span>
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</pre>
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<p>
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Although the mistake above is fairly obvious, it is also not too difficult
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to do this accidentally, or worse, create it in someone elses code.
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</p>
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<p>
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Therefore is it prudent to avoid this risk by <span class="bold"><strong>localising
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the scope of such definitions</strong></span>, as shown above.
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</p>
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</td></tr>
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</table></div>
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<div class="tip"><table border="0" summary="Tip">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
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<th align="left">Tip</th>
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</tr>
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<tr><td align="left" valign="top">
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<p>
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Be very careful with the type provided as parameter. For example, providing
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an <span class="bold"><strong>integer</strong></span> instead of a floating-point
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type can be disastrous (a C++ feature).
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</p>
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<pre class="programlisting"><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Area = "</span> <span class="special"><<</span> <span class="identifier">area</span><span class="special">(</span><span class="number">2</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> <span class="comment">// Area = 12!!!</span></pre>
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<p>
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You should get a compiler warning
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</p>
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<pre class="programlisting">warning : 'return' : conversion from 'double' to 'int', possible loss of data
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</pre>
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<p>
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Failure to heed this warning can lead to very wrong answers!
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</p>
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<p>
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You can also avoid this by being explicit about the type of <code class="computeroutput"><span class="identifier">Area</span></code>.
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</p>
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<pre class="programlisting"><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Area = "</span> <span class="special"><<</span> <span class="identifier">area</span><span class="special"><</span><span class="keyword">double</span><span class="special">>(</span><span class="number">2</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> <span class="comment">// Area = 12.566371</span></pre>
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</td></tr>
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</table></div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2006-2019 Nikhar
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Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos,
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Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan
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Råde, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg,
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Daryle Walker and Xiaogang Zhang<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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