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25 | <div class="section"> | |
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 = π * 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 π as shown below: | |
41 | </p> | |
42 | <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> | |
43 | ||
44 | <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</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> | |
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"><</span><span class="identifier">iostream</span><span class="special">></span> | |
64 | <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iomanip</span><span class="special">></span> | |
65 | <span class="preprocessor">#include</span> <span class="special"><</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">></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"><<</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"><</span><span class="keyword">float</span><span class="special">>::</span><span class="identifier">digits10</span><span class="special">)</span> | |
83 | <span class="special"><<</span> <span class="identifier">a_f</span> | |
84 | <span class="special"><<</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"><<</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"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">digits10</span><span class="special">)</span> | |
89 | <span class="special"><<</span> <span class="identifier">a_d</span> | |
90 | <span class="special"><<</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"><<</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"><</span><span class="identifier">cpp_dec_float_50</span><span class="special">>::</span><span class="identifier">digits10</span><span class="special">)</span> | |
95 | <span class="special"><<</span> <span class="identifier">a_mp</span> | |
96 | <span class="special"><<</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 © 2002-2013 John Maddock and Christopher Kormanyos<p> | |
109 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
110 | 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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