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<title>Rounding Rules for Conversions</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_multiprecision.tut.rounding"></a><a class="link" href="rounding.html" title="Rounding Rules for Conversions">Rounding Rules for
      Conversions</a>
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<p>
        As a general rule, all conversions between unrelated types are performed
        using basic arithmetic operations, therefore conversions are either exact,
        or follow the same rounding rules as arithmetic for the type in question.
      </p>
<p>
        The following table summarises the situation for conversions from native
        types:
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
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</colgroup>
<thead><tr>
<th>
                <p>
                  Backend
                </p>
              </th>
<th>
                <p>
                  Rounding Rules
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <a class="link" href="ints/cpp_int.html" title="cpp_int">cpp_int</a>
                </p>
              </td>
<td>
                <p>
                  Conversions from integer types are exact if the target has sufficient
                  precision, otherwise they truncate to the first 2^MaxBits bits
                  (modulo arithmetic). Conversions from floating-point types are
                  truncating to the nearest integer.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="ints/gmp_int.html" title="gmp_int">gmp_int</a>
                </p>
              </td>
<td>
                <p>
                  Conversions are performed by the GMP library except for conversion
                  from <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>
                  which is truncating.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="ints/tom_int.html" title="tom_int">tom_int</a>
                </p>
              </td>
<td>
                <p>
                  Conversions from floating-point types are truncating, all others
                  are performed by libtommath and are exact.
                </p>
              </td>
</tr>
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<td>
                <p>
                  <a class="link" href="floats/gmp_float.html" title="gmp_float">gmp_float</a>
                </p>
              </td>
<td>
                <p>
                  Conversions are performed by the GMP library except for conversion
                  from <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>
                  which should be exact provided the target type has as much precision
                  as a <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>.
                </p>
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                <p>
                  <a class="link" href="floats/mpfr_float.html" title="mpfr_float">mpfr_float</a>
                </p>
              </td>
<td>
                <p>
                  All conversions are performed by the underlying MPFR library.
                </p>
              </td>
</tr>
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<td>
                <p>
                  <a class="link" href="floats/cpp_dec_float.html" title="cpp_dec_float">cpp_dec_float</a>
                </p>
              </td>
<td>
                <p>
                  All conversions are performed using basic arithmetic operations
                  and are truncating.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="rational/gmp_rational.html" title="gmp_rational">gmp_rational</a>
                </p>
              </td>
<td>
                <p>
                  See <a class="link" href="ints/gmp_int.html" title="gmp_int">gmp_int</a>
                </p>
              </td>
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                <p>
                  <a class="link" href="rational/cpp_rational.html" title="cpp_rational">cpp_rational</a>
                </p>
              </td>
<td>
                <p>
                  See <a class="link" href="ints/cpp_int.html" title="cpp_int">cpp_int</a>
                </p>
              </td>
</tr>
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<td>
                <p>
                  <a class="link" href="rational/tommath_rational.html" title="tommath_rational">tommath_rational</a>
                </p>
              </td>
<td>
                <p>
                  See <a class="link" href="ints/tom_int.html" title="tom_int">tom_int</a>
                </p>
              </td>
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      and Christopher Kormanyos<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
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