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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="math_toolkit.tutorial.templ"></a><a class="link" href="templ.html" title="Use in template code">Use in template code</a>
  28 </h3></div></div></div>
  29 <p>
  30         When using the constants inside a function template, we need to ensure that
  31         we use a constant of the correct precision for our template parameters. We
  32         can do this by calling the function-template versions, <code class="computeroutput"><span class="identifier">pi</span><span class="special">&lt;</span><span class="identifier">FPType</span><span class="special">&gt;()</span></code>, of the constants like this:
  33       </p>
  34 <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">constants</span><span class="special">/</span><span class="identifier">constants</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  35
  36 <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Real</span><span class="special">&gt;</span>
  37 <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>
  38 <span class="special">{</span>
  39    <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>
  40    <span class="keyword">return</span> <span class="identifier">pi</span><span class="special">&lt;</span><span class="identifier">Real</span><span class="special">&gt;()</span> <span class="special">*</span> <span class="identifier">r</span> <span class="special">*</span> <span class="identifier">r</span><span class="special">;</span>
  41 <span class="special">}</span>
  42 </pre>
  43 <p>
  44         Although this syntax is a little less "cute" than the non-template
  45         version, the code is no less efficient (at least for the built-in types
  46         <code class="computeroutput"><span class="keyword">float</span></code>, <code class="computeroutput"><span class="keyword">double</span></code>
  47         and <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>)
  48         : the function template versions of the constants are simple inline functions
  49         that return a constant of the correct precision for the type used. In addition,
  50         these functions are declared <code class="computeroutput"><span class="identifier">constexp</span></code>
  51         for those compilers that support this, allowing the result to be used in
  52         constant-expressions provided the template argument is a literal type.
  53       </p>
  54 <div class="tip"><table border="0" summary="Tip">
  55 <tr>
  56 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
  57 <th align="left">Tip</th>
  58 </tr>
  59 <tr><td align="left" valign="top"><p>
  60           Keep in mind the difference between the variable version, just <code class="computeroutput"><span class="identifier">pi</span></code>, and the template-function version:
  61           the template-function requires both a &lt;<em class="replaceable"><code>floating-point-type</code></em>&gt;
  62           and function call <code class="computeroutput"><span class="special">()</span></code> brackets,
  63           for example: <code class="computeroutput"><span class="identifier">pi</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;()</span></code>.
  64           You cannot write <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">p</span>
  65           <span class="special">=</span> <span class="identifier">pi</span><span class="special">&lt;&gt;()</span></code>, nor <code class="computeroutput"><span class="keyword">double</span>
  66           <span class="identifier">p</span> <span class="special">=</span>
  67           <span class="identifier">pi</span><span class="special">()</span></code>.
  68         </p></td></tr>
  69 </table></div>
  70 <div class="note"><table border="0" summary="Note">
  71 <tr>
  72 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
  73 <th align="left">Note</th>
  74 </tr>
  75 <tr><td align="left" valign="top">
  76 <p>
  77           You can always use <span class="bold"><strong>both</strong></span> variable and template-function
  78           versions <span class="bold"><strong>provided calls are fully qualified</strong></span>,
  79           for example:
  80         </p>
  81 <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">&lt;</span><span class="keyword">double</span><span class="special">&gt;();</span>
  82 <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>
  83 </pre>
  84 </td></tr>
  85 </table></div>
  86 <div class="warning"><table border="0" summary="Warning">
  87 <tr>
  88 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td>
  89 <th align="left">Warning</th>
  90 </tr>
  91 <tr><td align="left" valign="top">
  92 <p>
  93           It may be tempting to simply define
  94         </p>
  95 <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>
  96 <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>
  97 </pre>
  98 <p>
  99           but if you do define two namespaces, this will, of course, create ambiguity!
 100         </p>
 101 <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>
 102 <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>
 103 </pre>
 104 <p>
 105           Although the mistake above is fairly obvious, it is also not too difficult
 106           to do this accidentally, or worse, create it in someone elses code.
 107         </p>
 108 <p>
 109           Therefore is it prudent to avoid this risk by <span class="bold"><strong>localising
 110           the scope of such definitions</strong></span>, as shown above.
 111         </p>
 112 </td></tr>
 113 </table></div>
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 115 <tr>
 116 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
 117 <th align="left">Tip</th>
 118 </tr>
 119 <tr><td align="left" valign="top">
 120 <p>
 121           Be very careful with the type provided as parameter. For example, providing
 122           an <span class="bold"><strong>integer</strong></span> instead of a floating-point
 123           type can be disastrous (a C++ feature).
 124         </p>
 125 <pre class="programlisting"><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Area = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">area</span><span class="special">(</span><span class="number">2</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span> <span class="comment">// Area = 12!!!</span></pre>
 126 <p>
 127           You should get a compiler warning
 128         </p>
 129 <pre class="programlisting">warning : 'return' : conversion from 'double' to 'int', possible loss of data
 130 </pre>
 131 <p>
 132           Failure to heed this warning can lead to very wrong answers!
 133         </p>
 134 <p>
 135           You can also avoid this by being explicit about the type of <code class="computeroutput"><span class="identifier">Area</span></code>.
 136         </p>
 137 <pre class="programlisting"><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Area = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">area</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="number">2</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span> <span class="comment">// Area = 12.566371</span></pre>
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