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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <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="keyword">constexpr</span></code> Support</a>
  28 </h3></div></div></div>
  29 <div class="note"><table border="0" summary="Note">
  30 <tr>
  31 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
  32 <th align="left">Note</th>
  33 </tr>
  34 <tr><td align="left" valign="top"><p>
  35           The features described in this section make heavy use of C++11 language
  36           features, currently (as of May 2013) only GCC-4.7 and later, and Clang
  37           3.3 and later have the support required to make these features work.
  38         </p></td></tr>
  39 </table></div>
  40 <p>
  41         There is limited support for <code class="computeroutput"><span class="keyword">constexpr</span></code>
  42         and user-defined literals in the library, currently the <code class="computeroutput"><span class="identifier">number</span></code>
  43         front end supports <code class="computeroutput"><span class="keyword">constexpr</span></code>
  44         on default construction and all forwarding constructors, but not on any of
  45         the non-member operators. So if some type <code class="computeroutput"><span class="identifier">B</span></code>
  46         is a literal type, then <code class="computeroutput"><span class="identifier">number</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">&gt;</span></code>
  47         is also a literal type, and you will be able to compile-time-construct such
  48         a type from any literal that <code class="computeroutput"><span class="identifier">B</span></code>
  49         is compile-time-constructible from. However, you will not be able to perform
  50         compile-time arithmetic on such types.
  51       </p>
  52 <p>
  53         Currently the only backend type provided by the library that is also a literal
  54         type are instantiations of <code class="computeroutput"><span class="identifier">cpp_int_backend</span></code>
  55         where the Allocator parameter is type <code class="computeroutput"><span class="keyword">void</span></code>,
  56         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>.
  57       </p>
  58 <p>
  59         For example:
  60       </p>
  61 <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>
  62
  63 <span class="keyword">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>
  64 <span class="keyword">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>
  65
  66 <span class="keyword">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>
  67 <span class="keyword">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>
  68 </pre>
  69 <p>
  70         There is also limited support for user defined-literals - these are limited
  71         to unchecked, fixed precision <code class="computeroutput"><span class="identifier">cpp_int</span></code>'s
  72         which are specified in hexadecimal notation. The suffixes supported are:
  73       </p>
  74 <div class="informaltable"><table class="table">
  75 <colgroup>
  76 <col>
  77 <col>
  78 </colgroup>
  79 <thead><tr>
  80 <th>
  81                 <p>
  82                   Suffix
  83                 </p>
  84               </th>
  85 <th>
  86                 <p>
  87                   Meaning
  88                 </p>
  89               </th>
  90 </tr></thead>
  91 <tbody>
  92 <tr>
  93 <td>
  94                 <p>
  95                   _cppi
  96                 </p>
  97               </td>
  98 <td>
  99                 <p>
 100                   Specifies a value of type: <code class="computeroutput"><span class="identifier">number</span><span class="special">&lt;</span><span class="identifier">cpp_int_backend</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">,</span><span class="identifier">N</span><span class="special">,</span><span class="identifier">signed_magnitude</span><span class="special">,</span><span class="identifier">unchecked</span><span class="special">,</span><span class="keyword">void</span><span class="special">&gt;</span> <span class="special">&gt;</span></code>,
 101                   where N is chosen to contain just enough digits to hold the number
 102                   specified.
 103                 </p>
 104               </td>
 105 </tr>
 106 <tr>
 107 <td>
 108                 <p>
 109                   _cppui
 110                 </p>
 111               </td>
 112 <td>
 113                 <p>
 114                   Specifies a value of type: <code class="computeroutput"><span class="identifier">number</span><span class="special">&lt;</span><span class="identifier">cpp_int_backend</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">,</span><span class="identifier">N</span><span class="special">,</span><span class="identifier">unsigned_magnitude</span><span class="special">,</span><span class="identifier">unchecked</span><span class="special">,</span><span class="keyword">void</span><span class="special">&gt;</span> <span class="special">&gt;</span></code>,
 115                   where N is chosen to contain just enough digits to hold the number
 116                   specified.
 117                 </p>
 118               </td>
 119 </tr>
 120 <tr>
 121 <td>
 122                 <p>
 123                   _cppi<span class="emphasis"><em>N</em></span>
 124                 </p>
 125               </td>
 126 <td>
 127                 <p>
 128                   Specifies a value of type <code class="computeroutput"><span class="identifier">number</span><span class="special">&lt;</span><span class="identifier">cpp_int_backend</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">,</span><span class="identifier">N</span><span class="special">,</span><span class="identifier">signed_magnitude</span><span class="special">,</span><span class="identifier">unchecked</span><span class="special">,</span><span class="keyword">void</span><span class="special">&gt;</span> <span class="special">&gt;</span></code>.
 129                 </p>
 130               </td>
 131 </tr>
 132 <tr>
 133 <td>
 134                 <p>
 135                   _cppui<span class="emphasis"><em>N</em></span>
 136                 </p>
 137               </td>
 138 <td>
 139                 <p>
 140                   Specifies a value of type <code class="computeroutput"><span class="identifier">number</span><span class="special">&lt;</span><span class="identifier">cpp_int_backend</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">,</span><span class="identifier">N</span><span class="special">,</span><span class="identifier">signed_magnitude</span><span class="special">,</span><span class="identifier">unchecked</span><span class="special">,</span><span class="keyword">void</span><span class="special">&gt;</span> <span class="special">&gt;</span></code>.
 141                 </p>
 142               </td>
 143 </tr>
 144 </tbody>
 145 </table></div>
 146 <p>
 147         In each case, use of these suffixes with hexadecimal values produces a <code class="computeroutput"><span class="keyword">constexpr</span></code> result.
 148       </p>
 149 <p>
 150         Examples:
 151       </p>
 152 <pre class="programlisting"><span class="comment">//</span>
 153 <span class="comment">// Any use of user defined literals requires that we import the literal-operators</span>
 154 <span class="comment">// into current scope first:</span>
 155 <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><span class="identifier">literals</span><span class="special">;</span>
 156 <span class="comment">//</span>
 157 <span class="comment">// To keep things simple in the example, we'll make our types used visible to this scope as well:</span>
 158 <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>
 159 <span class="comment">//</span>
 160 <span class="comment">// The value zero as a number&lt;cpp_int_backend&lt;4,4,signed_magnitude,unchecked,void&gt; &gt;:</span>
 161 <span class="keyword">constexpr</span> <span class="keyword">auto</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">0x0</span><span class="identifier">_cppi</span><span class="special">;</span>
 162 <span class="comment">// The type of each constant has 4 bits per hexadecimal digit,</span>
 163 <span class="comment">// so this is of type uint256_t (ie number&lt;cpp_int_backend&lt;256,256,unsigned_magnitude,unchecked,void&gt; &gt;):</span>
 164 <span class="keyword">constexpr</span> <span class="keyword">auto</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">0</span><span class="identifier">xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF_cppui</span><span class="special">;</span>
 165 <span class="comment">//</span>
 166 <span class="comment">// Smaller values can be assigned to larger values:</span>
 167 <span class="identifier">int256_t</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">0x1234</span><span class="identifier">_cppi</span><span class="special">;</span> <span class="comment">// OK</span>
 168 <span class="comment">//</span>
 169 <span class="comment">// However, this does not currently work in constexpr contexts:</span>
 170 <span class="keyword">constexpr</span> <span class="identifier">int256_t</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">0x1</span><span class="identifier">_cppi</span><span class="special">;</span> <span class="comment">// Compiler error</span>
 171 <span class="comment">//</span>
 172 <span class="comment">// Constants can be padded out with leading zeros to generate wider types:</span>
 173 <span class="keyword">constexpr</span> <span class="identifier">uint256_t</span> <span class="identifier">e</span> <span class="special">=</span> <span class="number">0</span><span class="identifier">x0000000000000000000000000000000000000000000FFFFFFFFFFFFFFFFFFFFF_cppui</span><span class="special">;</span> <span class="comment">// OK</span>
 174 <span class="comment">//</span>
 175 <span class="comment">// However, specific width types are best produced with specific-width suffixes,</span>
 176 <span class="comment">// ones supported by default are `_cpp[u]i128`, `_cpp[u]i256`, `_cpp[u]i512`, `_cpp[u]i1024`.</span>
 177 <span class="comment">//</span>
 178 <span class="keyword">constexpr</span> <span class="identifier">int128_t</span> <span class="identifier">f</span> <span class="special">=</span> <span class="number">0x1234</span><span class="identifier">_cppi128</span><span class="special">;</span> <span class="comment">// OK, always produces an int128_t as the result.</span>
 179 <span class="keyword">constexpr</span> <span class="identifier">uint1024_t</span> <span class="identifier">g</span> <span class="special">=</span> <span class="number">0</span><span class="identifier">xaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbccccccccccccccccccccc_cppui1024</span><span class="special">;</span>
 180 <span class="comment">//</span>
 181 <span class="comment">// If other specific width types are required, then there is a macro for generating the operators</span>
 182 <span class="comment">// for these.  The macro can be used at namespace scope only:</span>
 183 <span class="comment">//</span>
 184 <span class="identifier">BOOST_MP_DEFINE_SIZED_CPP_INT_LITERAL</span><span class="special">(</span><span class="number">2048</span><span class="special">);</span>
 185 <span class="comment">//</span>
 186 <span class="comment">// Now we can create 2048-bit literals as well:</span>
 187 <span class="keyword">constexpr</span> <span class="keyword">auto</span> <span class="identifier">h</span> <span class="special">=</span> <span class="number">0xff</span><span class="identifier">_cppi2048</span><span class="special">;</span> <span class="comment">// h is of type number&lt;cpp_int_backend&lt;2048,2048,signed_magnitude,unchecked,void&gt; &gt;</span>
 188 <span class="comment">//</span>
 189 <span class="comment">// Finally negative values are handled via the unary minus operator:</span>
 190 <span class="comment">//</span>
 191 <span class="keyword">constexpr</span> <span class="identifier">int1024_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">-</span><span class="number">0</span><span class="identifier">xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF_cppui1024</span><span class="special">;</span>
 192 <span class="comment">//</span>
 193 <span class="comment">// Which means this also works:</span>
 194 <span class="keyword">constexpr</span> <span class="identifier">int1024_t</span> <span class="identifier">j</span> <span class="special">=</span> <span class="special">-</span><span class="identifier">g</span><span class="special">;</span>   <span class="comment">// OK: unary minus operator is constexpr.</span>
 195 </pre>
 196 </div>
 197 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 198 <td align="left"></td>
 199 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2013 John Maddock and Christopher Kormanyos<p>
 200         Distributed under the Boost Software License, Version 1.0. (See accompanying
 201         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>)
 202       </p>
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