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  26 <div class="titlepage"><div><div><h5 class="title">
  27 <a name="boost_multiprecision.tut.floats.fp_eg.aos"></a><a class="link" href="aos.html" title="Area of Circle">Area of
  28           Circle</a>
  29 </h5></div></div></div>
  30 <p>
  31             Generic numeric programming employs templates to use the same code for
  32             different floating-point types and functions. Consider the area of a
  33             circle a of radius r, given by
  34           </p>
  35 <div class="blockquote"><blockquote class="blockquote"><p>
  36               <span class="emphasis"><em>a = &#960; * r<sup>2</sup></em></span>
  37             </p></blockquote></div>
  38 <p>
  39             The area of a circle can be computed in generic programming using Boost.Math
  40             for the constant &#960; as shown below:
  41           </p>
  42 <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>
  43
  44 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
  45 <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">area_of_a_circle</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">r</span><span class="special">)</span>
  46 <span class="special">{</span>
  47    <span class="keyword">using</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>
  48    <span class="keyword">return</span> <span class="identifier">pi</span><span class="special">&lt;</span><span class="identifier">T</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>
  49 <span class="special">}</span>
  50 </pre>
  51 <p>
  52             It is possible to use <code class="computeroutput"><span class="identifier">area_of_a_circle</span><span class="special">()</span></code> with built-in floating-point types
  53             as well as floating-point types from Boost.Multiprecision. In particular,
  54             consider a system with 4-byte single-precision float, 8-byte double-precision
  55             double and also the <code class="computeroutput"><span class="identifier">cpp_dec_float_50</span></code>
  56             data type from Boost.Multiprecision with 50 decimal digits of precision.
  57           </p>
  58 <p>
  59             We can compute and print the approximate area of a circle with radius
  60             123/100 for <code class="computeroutput"><span class="keyword">float</span></code>, <code class="computeroutput"><span class="keyword">double</span></code> and <code class="computeroutput"><span class="identifier">cpp_dec_float_50</span></code>
  61             with the program below.
  62           </p>
  63 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
  64 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iomanip</span><span class="special">&gt;</span>
  65 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">multiprecision</span><span class="special">/</span><span class="identifier">cpp_dec_float</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  66
  67 <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">::</span><span class="identifier">cpp_dec_float_50</span><span class="special">;</span>
  68
  69 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">char</span><span class="special">**)</span>
  70 <span class="special">{</span>
  71    <span class="keyword">const</span> <span class="keyword">float</span> <span class="identifier">r_f</span><span class="special">(</span><span class="keyword">float</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">/</span> <span class="number">100</span><span class="special">);</span>
  72    <span class="keyword">const</span> <span class="keyword">float</span> <span class="identifier">a_f</span> <span class="special">=</span> <span class="identifier">area_of_a_circle</span><span class="special">(</span><span class="identifier">r_f</span><span class="special">);</span>
  73
  74    <span class="keyword">const</span> <span class="keyword">double</span> <span class="identifier">r_d</span><span class="special">(</span><span class="keyword">double</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">/</span> <span class="number">100</span><span class="special">);</span>
  75    <span class="keyword">const</span> <span class="keyword">double</span> <span class="identifier">a_d</span> <span class="special">=</span> <span class="identifier">area_of_a_circle</span><span class="special">(</span><span class="identifier">r_d</span><span class="special">);</span>
  76
  77    <span class="keyword">const</span> <span class="identifier">cpp_dec_float_50</span> <span class="identifier">r_mp</span><span class="special">(</span><span class="identifier">cpp_dec_float_50</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">/</span> <span class="number">100</span><span class="special">);</span>
  78    <span class="keyword">const</span> <span class="identifier">cpp_dec_float_50</span> <span class="identifier">a_mp</span> <span class="special">=</span> <span class="identifier">area_of_a_circle</span><span class="special">(</span><span class="identifier">r_mp</span><span class="special">);</span>
  79
  80    <span class="comment">// 4.75292</span>
  81    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span>
  82       <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">setprecision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">float</span><span class="special">&gt;::</span><span class="identifier">digits10</span><span class="special">)</span>
  83       <span class="special">&lt;&lt;</span> <span class="identifier">a_f</span>
  84       <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
  85
  86    <span class="comment">// 4.752915525616</span>
  87    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span>
  88       <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">setprecision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">digits10</span><span class="special">)</span>
  89       <span class="special">&lt;&lt;</span> <span class="identifier">a_d</span>
  90       <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
  91
  92    <span class="comment">// 4.7529155256159981904701331745635599135018975843146</span>
  93    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span>
  94       <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">setprecision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">cpp_dec_float_50</span><span class="special">&gt;::</span><span class="identifier">digits10</span><span class="special">)</span>
  95       <span class="special">&lt;&lt;</span> <span class="identifier">a_mp</span>
  96       <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
  97 <span class="special">}</span>
  98 </pre>
  99 <p>
 100             In the next example we'll look at calling both standard library and Boost.Math
 101             functions from within generic code. We'll also show how to cope with
 102             template arguments which are expression-templates rather than number
 103             types.
 104           </p>
 105 </div>
 106 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 107 <td align="left"></td>
 108 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2019 John Maddock
 109       and Christopher Kormanyos<p>
 110         Distributed under the Boost Software License, Version 1.0. (See accompanying
 111         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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