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26 <div class="titlepage"><div><div><h5 class="title">
27 <a name="boost_multiprecision.tut.ints.egs.bitops"></a><a class="link" href="bitops.html" title="Bit Operations">Bit Operations</a>
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30 In this example we'll show how individual bits within an integer may
31 be manipulated, we'll start with an often needed calculation of <span class="emphasis"><em>2<sup>n</sup> -
32 1</em></span>, which we could obviously implement like this:
34 <pre class="programlisting"><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_int</span><span class="special">;</span>
36 <span class="identifier">cpp_int</span> <span class="identifier">b1</span><span class="special">(</span><span class="keyword">unsigned</span> <span class="identifier">n</span><span class="special">)</span>
37 <span class="special">{</span>
38 <span class="identifier">cpp_int</span> <span class="identifier">r</span><span class="special">(</span><span class="number">1</span><span class="special">);</span>
39 <span class="keyword">return</span> <span class="special">(</span><span class="identifier">r</span> <span class="special"><<</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">-</span> <span class="number">1</span><span class="special">;</span>
40 <span class="special">}</span>
45 <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">hex</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">showbase</span> <span class="special"><<</span> <span class="identifier">b1</span><span class="special">(</span><span class="number">200</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
50 <pre class="programlisting">0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF</pre>
52 However, we could equally just set the n'th bit in the result, like this:
54 <pre class="programlisting"><span class="identifier">cpp_int</span> <span class="identifier">b2</span><span class="special">(</span><span class="keyword">unsigned</span> <span class="identifier">n</span><span class="special">)</span>
55 <span class="special">{</span>
56 <span class="identifier">cpp_int</span> <span class="identifier">r</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
57 <span class="keyword">return</span> <span class="special">--</span><span class="identifier">bit_set</span><span class="special">(</span><span class="identifier">r</span><span class="special">,</span> <span class="identifier">n</span><span class="special">);</span>
58 <span class="special">}</span>
61 Note how the <code class="computeroutput"><span class="identifier">bit_set</span></code>
62 function sets the specified bit in its argument and then returns a reference
63 to the result - which we can then simply decrement. The result from a
64 call to <code class="computeroutput"><span class="identifier">b2</span></code> is the same
65 as that to <code class="computeroutput"><span class="identifier">b1</span></code>.
68 We can equally test bits, so for example the n'th bit of the result returned
69 from <code class="computeroutput"><span class="identifier">b2</span></code> shouldn't be
70 set unless we increment it first:
72 <pre class="programlisting"><span class="identifier">BOOST_ASSERT</span><span class="special">(!</span><span class="identifier">bit_test</span><span class="special">(</span><span class="identifier">b1</span><span class="special">(</span><span class="number">200</span><span class="special">),</span> <span class="number">200</span><span class="special">));</span> <span class="comment">// OK</span>
73 <span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">bit_test</span><span class="special">(++</span><span class="identifier">b1</span><span class="special">(</span><span class="number">200</span><span class="special">),</span> <span class="number">200</span><span class="special">));</span> <span class="comment">// OK</span>
76 And of course if we flip the n'th bit after increment, then we should
79 <pre class="programlisting"><span class="identifier">BOOST_ASSERT</span><span class="special">(!</span><span class="identifier">bit_flip</span><span class="special">(++</span><span class="identifier">b1</span><span class="special">(</span><span class="number">200</span><span class="special">),</span> <span class="number">200</span><span class="special">));</span> <span class="comment">// OK</span>
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