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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>
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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>)
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