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| <div class="titlepage"><div><div><h3 class="title"> |
| <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> |
| </h3></div></div></div> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| The features described in this section make heavy use of C++11 language |
| features, currently (as of May 2013) only GCC-4.7 and later, and Clang |
| 3.3 and later have the support required to make these features work. |
| </p></td></tr> |
| </table></div> |
| <p> |
| There is limited support for <code class="computeroutput"><span class="keyword">constexpr</span></code> |
| and user-defined literals in the library, currently the <code class="computeroutput"><span class="identifier">number</span></code> |
| front end supports <code class="computeroutput"><span class="keyword">constexpr</span></code> |
| on default construction and all forwarding constructors, but not on any of |
| the non-member operators. So if some type <code class="computeroutput"><span class="identifier">B</span></code> |
| 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> |
| is also a literal type, and you will be able to compile-time-construct such |
| a type from any literal that <code class="computeroutput"><span class="identifier">B</span></code> |
| is compile-time-constructible from. However, you will not be able to perform |
| compile-time arithmetic on such types. |
| </p> |
| <p> |
| Currently the only backend type provided by the library that is also a literal |
| type are instantiations of <code class="computeroutput"><span class="identifier">cpp_int_backend</span></code> |
| where the Allocator parameter is type <code class="computeroutput"><span class="keyword">void</span></code>, |
| 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>. |
| </p> |
| <p> |
| For example: |
| </p> |
| <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> |
| |
| <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> |
| <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> |
| |
| <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> |
| <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> |
| </pre> |
| <p> |
| There is also limited support for user defined-literals - these are limited |
| to unchecked, fixed precision <code class="computeroutput"><span class="identifier">cpp_int</span></code>'s |
| which are specified in hexadecimal notation. The suffixes supported are: |
| </p> |
| <div class="informaltable"><table class="table"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Suffix |
| </p> |
| </th> |
| <th> |
| <p> |
| Meaning |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| _cppi |
| </p> |
| </td> |
| <td> |
| <p> |
| Specifies a value of type: <code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">cpp_int_backend</span><span class="special"><</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">></span> <span class="special">></span></code>, |
| where N is chosen to contain just enough digits to hold the number |
| specified. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| _cppui |
| </p> |
| </td> |
| <td> |
| <p> |
| Specifies a value of type: <code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">cpp_int_backend</span><span class="special"><</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">></span> <span class="special">></span></code>, |
| where N is chosen to contain just enough digits to hold the number |
| specified. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| _cppi<span class="emphasis"><em>N</em></span> |
| </p> |
| </td> |
| <td> |
| <p> |
| Specifies a value of type <code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">cpp_int_backend</span><span class="special"><</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">></span> <span class="special">></span></code>. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| _cppui<span class="emphasis"><em>N</em></span> |
| </p> |
| </td> |
| <td> |
| <p> |
| Specifies a value of type <code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">cpp_int_backend</span><span class="special"><</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">></span> <span class="special">></span></code>. |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| <p> |
| In each case, use of these suffixes with hexadecimal values produces a <code class="computeroutput"><span class="keyword">constexpr</span></code> result. |
| </p> |
| <p> |
| Examples: |
| </p> |
| <pre class="programlisting"><span class="comment">//</span> |
| <span class="comment">// Any use of user defined literals requires that we import the literal-operators</span> |
| <span class="comment">// into current scope first:</span> |
| <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> |
| <span class="comment">//</span> |
| <span class="comment">// To keep things simple in the example, we'll make our types used visible to this scope as well:</span> |
| <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="comment">//</span> |
| <span class="comment">// The value zero as a number<cpp_int_backend<4,4,signed_magnitude,unchecked,void> >:</span> |
| <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> |
| <span class="comment">// The type of each constant has 4 bits per hexadecimal digit,</span> |
| <span class="comment">// so this is of type uint256_t (ie number<cpp_int_backend<256,256,unsigned_magnitude,unchecked,void> >):</span> |
| <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> |
| <span class="comment">//</span> |
| <span class="comment">// Smaller values can be assigned to larger values:</span> |
| <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> |
| <span class="comment">//</span> |
| <span class="comment">// However, this does not currently work in constexpr contexts:</span> |
| <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> |
| <span class="comment">//</span> |
| <span class="comment">// Constants can be padded out with leading zeros to generate wider types:</span> |
| <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> |
| <span class="comment">//</span> |
| <span class="comment">// However, specific width types are best produced with specific-width suffixes,</span> |
| <span class="comment">// ones supported by default are `_cpp[u]i128`, `_cpp[u]i256`, `_cpp[u]i512`, `_cpp[u]i1024`.</span> |
| <span class="comment">//</span> |
| <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> |
| <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> |
| <span class="comment">//</span> |
| <span class="comment">// If other specific width types are required, then there is a macro for generating the operators</span> |
| <span class="comment">// for these. The macro can be used at namespace scope only:</span> |
| <span class="comment">//</span> |
| <span class="identifier">BOOST_MP_DEFINE_SIZED_CPP_INT_LITERAL</span><span class="special">(</span><span class="number">2048</span><span class="special">);</span> |
| <span class="comment">//</span> |
| <span class="comment">// Now we can create 2048-bit literals as well:</span> |
| <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<cpp_int_backend<2048,2048,signed_magnitude,unchecked,void> ></span> |
| <span class="comment">//</span> |
| <span class="comment">// Finally negative values are handled via the unary minus operator:</span> |
| <span class="comment">//</span> |
| <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> |
| <span class="comment">//</span> |
| <span class="comment">// Which means this also works:</span> |
| <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> |
| </pre> |
| </div> |
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