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<title>Literal Types and constexpr Support</title>
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<div class="section boost_multiprecision_tut_lits">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_multiprecision.tut.lits"></a><a class="link" href="lits.html" title="Literal Types and constexpr Support">Literal Types and <code class="computeroutput"><span class="identifier">constexpr</span></code> Support</a>
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</h3></div></div></div>
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<p>
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There is limited support for <code class="computeroutput"><span class="identifier">constexpr</span></code>
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in the library, currently the <code class="computeroutput"><span class="identifier">number</span></code>
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front end supports <code class="computeroutput"><span class="identifier">constexpr</span></code>
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on default construction and all forwarding constructors, but not on any of
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the non-member operators. So if some type <code class="computeroutput"><span class="identifier">B</span></code>
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is a literal type, then <code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">B</span><span class="special">></span></code>
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is also a literal type, and you will be able to compile-time-construct such
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a type from any literal that <code class="computeroutput"><span class="identifier">B</span></code>
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is compile-time-constructible from. However, you will not be able to perform
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compile-time arithmetic on such types.
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</p>
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Currently the only backend type provided by the library that is also a literal
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type are instantiations of <code class="computeroutput"><span class="identifier">cpp_int_backend</span></code>
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where the Allocator parameter is type <code class="computeroutput"><span class="keyword">void</span></code>,
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and the Checked parameter is <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">::</span><span class="identifier">unchecked</span></code>.
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For example:
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</p>
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<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">multiprecision</span><span class="special">;</span>
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<span class="identifier">constexpr</span> <span class="identifier">int128_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="comment">// OK, fixed precision int128_t has no allocator.</span>
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<span class="identifier">constexpr</span> <span class="identifier">uint1024_t</span> <span class="identifier">j</span> <span class="special">=</span> <span class="number">0</span><span class="identifier">xFFFFFFFF00000000uLL</span><span class="special">;</span> <span class="comment">// OK, fixed precision uint1024_t has no allocator.</span>
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<span class="identifier">constexpr</span> <span class="identifier">checked_uint128_t</span> <span class="identifier">k</span> <span class="special">=</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span> <span class="comment">// Error, checked type is not a literal type as we need runtime error checking.</span>
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<span class="identifier">constexpr</span> <span class="identifier">cpp_int</span> <span class="identifier">l</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> <span class="comment">// Error, type is not a literal as it performs memory management.</span>
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</pre>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2012 John Maddock and Christopher Kormanyos<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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