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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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<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>
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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:
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>.
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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>
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<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>
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<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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