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27<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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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">
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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>
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73<th align="left">Note</th>
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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>
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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>
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117<th align="left">Tip</th>
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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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