ceph/src/boost/libs/mpl/doc/refmanual/metafunctions.html

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  14 <div class="section" id="metafunctions">
  15 <h1><a class="toc-backref" href="../refmanual.html#id1510">Metafunctions</a></h1>
  16 <p id="label-metafunctions">The MPL includes a number of predefined metafunctions that can be roughly
  17 classified in two categories: <cite>general purpose metafunctions</cite>, dealing with
  18 conditional <a class="reference internal" href="./type-selection.html">type selection</a> and higher-order metafunction <a class="reference internal" href="./invocation.html">invocation</a>,
  19 <a class="reference internal" href="./composition-and-argument-binding.html">composition</a>, and <a class="reference internal" href="./composition-and-argument-binding.html">argument binding</a>, and <cite>numeric metafunctions</cite>,
  20 incapsulating built-in and user-defined <a class="reference internal" href="./arithmetic-operations.html">arithmetic</a>, <a class="reference internal" href="./comparisons.html">comparison</a>,
  21 <a class="reference internal" href="./logical-operations.html">logical</a>, and <a class="reference internal" href="./bitwise-operations.html">bitwise</a> operations.</p>
  22 <p>Given that it is possible to perform integer numeric computations at
  23 compile time using the conventional operators notation, the need for the
  24 second category might be not obvious, but it in fact plays a cental role in
  25 making programming with MPL seemingly effortless. In
  26 particular, there are at least two contexts where built-in language
  27 facilities fall short <a class="footnote-reference" href="#portability" id="id820">[3]</a>:</p>
  28 <ol class="arabic simple">
  29 <li>Passing a computation to an algorithm.</li>
  30 <li>Performing a computation on non-integer data.</li>
  31 </ol>
  32 <p>The second use case deserves special attention. In contrast to the built-in,
  33 strictly integer compile-time arithmetics, the MPL numeric metafunctions are
  34 <em>polymorphic</em>, with support for <em>mixed-type arithmetics</em>. This means that they
  35 can operate on a variety of numeric types — for instance, rational,
  36 fixed-point or complex numbers, — and that, in general, you are allowed to
  37 freely intermix these types within a single expression. See <a class="reference internal" href="./numeric-metafunction.html">Numeric Metafunction</a> concept for more details on the MPL numeric infrastructure.</p>
  38 <!-- The provided `infrastructure`__ allows easy plugging of user-defined numeric
  39 types
  40 Naturally, they also , meaning that you can perform a computation on the
  41 arguments of different types, and the result will yeild the largest/most general
  42 of them. For user-defined numeric types, they provide an `infrastructure`__ that
  43 allows easy plugging and seemless integration with predefined library
  44 types. details.
  45
  46 __ `Numeric Metafunction`_ -->
  47 <p>To reduce a negative syntactical impact of the metafunctions notation
  48 over the infix operator notation, all numeric metafunctions
  49 allow to pass up to N arguments, where N is defined by the value of
  50 <a class="reference internal" href="./limit-metafunction-arity.html">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</a> configuration macro.</p>
  51 <table class="docutils footnote" frame="void" id="portability" rules="none">
  52 <colgroup><col class="label" /><col /></colgroup>
  53 <tbody valign="top">
  54 <tr><td class="label"><a class="fn-backref" href="#id820">[3]</a></td><td>All other considerations aside, as of the time of this writing
  55 (early 2004), using built-in operators on integral constants still often
  56 present a portability problem — many compilers cannot handle particular
  57 forms of expressions, forcing us to use conditional compilation. Because MPL
  58 numeric metafunctions work on types and encapsulate these kind of workarounds
  59 internally, they elude these problems, so if you aim for portability, it is
  60 generally adviced to use them in the place of the conventional operators, even
  61 at the price of slightly decreased readability.</td></tr>
  62 </tbody>
  63 </table>
  64 <ul class="toc simple" id="outline">
  65 <li><a class="reference internal" href="./metafunctions-concepts.html" id="id1511">Concepts</a></li>
  66 <li><a class="reference internal" href="./type-selection.html" id="id1519">Type Selection</a></li>
  67 <li><a class="reference internal" href="./invocation.html" id="id1524">Invocation</a></li>
  68 <li><a class="reference internal" href="./composition-and-argument-binding.html" id="id1528">Composition and Argument Binding</a></li>
  69 <li><a class="reference internal" href="./arithmetic-operations.html" id="id1535">Arithmetic Operations</a></li>
  70 <li><a class="reference internal" href="./comparisons.html" id="id1542">Comparisons</a></li>
  71 <li><a class="reference internal" href="./logical-operations.html" id="id1549">Logical Operations</a></li>
  72 <li><a class="reference internal" href="./bitwise-operations.html" id="id1553">Bitwise Operations</a></li>
  73 <li><a class="reference internal" href="./trivial.html" id="id1559">Trivial</a></li>
  74 <li><a class="reference internal" href="./string-operations.html" id="id1561">String Operations</a></li>
  75 <li><a class="reference internal" href="./miscellaneous.html" id="id1563">Miscellaneous</a></li>
  76 </ul>
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  81 <td><div class="copyright-footer"><div class="copyright">Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams</div>
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