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| <div class="titlepage"><div><div><h3 class="title"> |
| <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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| <p> |
| When using the constants inside a function template, we need to ensure that |
| we use a constant of the correct precision for our template parameters. We |
| 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: |
| </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">constants</span><span class="special">/</span><span class="identifier">constants</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Real</span><span class="special">></span> |
| <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> |
| <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">constants</span><span class="special">;</span> |
| <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| Although this syntax is a little less "cute" than the non-template |
| version, the code is no less efficient (at least for the built-in types |
| <code class="computeroutput"><span class="keyword">float</span></code>, <code class="computeroutput"><span class="keyword">double</span></code> |
| and <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>) |
| : the function template versions of the constants are simple inline functions |
| that return a constant of the correct precision for the type used. In addition, |
| these functions are declared <code class="computeroutput"><span class="identifier">constexp</span></code> |
| for those compilers that support this, allowing the result to be used in |
| constant-expressions provided the template argument is a literal type. |
| </p> |
| <div class="tip"><table border="0" summary="Tip"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td> |
| <th align="left">Tip</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| Keep in mind the difference between the variable version, just <code class="computeroutput"><span class="identifier">pi</span></code>, and the template-function version: |
| the template-function requires both a <<em class="replaceable"><code>floating-point-type</code></em>> |
| and function call <code class="computeroutput"><span class="special">()</span></code> brackets, |
| for example: <code class="computeroutput"><span class="identifier">pi</span><span class="special"><</span><span class="keyword">double</span><span class="special">>()</span></code>. |
| You cannot write <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">p</span> |
| <span class="special">=</span> <span class="identifier">pi</span><span class="special"><>()</span></code>, nor <code class="computeroutput"><span class="keyword">double</span> |
| <span class="identifier">p</span> <span class="special">=</span> |
| <span class="identifier">pi</span><span class="special">()</span></code>. |
| </p></td></tr> |
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| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
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| <p> |
| You can always use <span class="bold"><strong>both</strong></span> variable and template-function |
| versions <span class="bold"><strong>provided calls are fully qualified</strong></span>, |
| for example: |
| </p> |
| <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> |
| <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> |
| </pre> |
| </td></tr> |
| </table></div> |
| <div class="warning"><table border="0" summary="Warning"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td> |
| <th align="left">Warning</th> |
| </tr> |
| <tr><td align="left" valign="top"> |
| <p> |
| It may be tempting to simply define |
| </p> |
| <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> |
| <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> |
| </pre> |
| <p> |
| but if you do define two namespaces, this will, of course, create ambiguity! |
| </p> |
| <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> |
| <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> |
| </pre> |
| <p> |
| Although the mistake above is fairly obvious, it is also not too difficult |
| to do this accidentally, or worse, create it in someone elses code. |
| </p> |
| <p> |
| Therefore is it prudent to avoid this risk by <span class="bold"><strong>localising |
| the scope of such definitions</strong></span>, as shown above. |
| </p> |
| </td></tr> |
| </table></div> |
| <div class="tip"><table border="0" summary="Tip"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td> |
| <th align="left">Tip</th> |
| </tr> |
| <tr><td align="left" valign="top"> |
| <p> |
| Be very careful with the type provided as parameter. For example, providing |
| an <span class="bold"><strong>integer</strong></span> instead of a floating-point |
| type can be disastrous (a C++ feature). |
| </p> |
| <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> |
| <p> |
| You should get a compiler warning |
| </p> |
| <pre class="programlisting">warning : 'return' : conversion from 'double' to 'int', possible loss of data |
| </pre> |
| <p> |
| Failure to heed this warning can lead to very wrong answers! |
| </p> |
| <p> |
| You can also avoid this by being explicit about the type of <code class="computeroutput"><span class="identifier">Area</span></code>. |
| </p> |
| <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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| Holin, Bruno Lalande, John Maddock, Johan Råde, Gautam Sewani, Benjamin Sobotta, |
| Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
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