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| 7 | <title>The MPL Reference Manual: Metafunctions</title> |
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| 12 | <td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a></td> |
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| 14 | <div class="section" id="metafunctions"> |
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| 15 | <h1><a class="toc-backref" href="../refmanual.html#id436" name="metafunctions">Metafunctions</a></h1> |
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| 16 | <a class="target" id="label-metafunctions" name="label-metafunctions"></a><p>The MPL includes a number of predefined metafunctions that can be roughly |
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| 17 | classified in two categories: <cite>general purpose metafunctions</cite>, dealing with |
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| 18 | conditional <a class="reference" href="./type-selection.html">type selection</a> and higher-order metafunction <a class="reference" href="./invocation.html">invocation</a>, |
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| 19 | <a class="reference" href="./composition-and-argument.html">composition</a>, and <a class="reference" href="./composition-and-argument.html">argument binding</a>, and <cite>numeric metafunctions</cite>, |
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| 20 | incapsulating built-in and user-defined <a class="reference" href="./arithmetic-operations.html">arithmetic</a>, <a class="reference" href="./comparisons.html">comparison</a>, |
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| 21 | <a class="reference" href="./logical-operations.html">logical</a>, and <a class="reference" href="./bitwise-operations.html">bitwise</a> operations.</p> |
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| 22 | <p>Given that it is possible to perform integer numeric computations at |
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| 23 | compile time using the conventional operators notation, the need for the |
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| 24 | second category might be not obvious, but it in fact plays a cental role in |
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| 25 | making programming with MPL seemingly effortless. In |
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| 26 | particular, there are at least two contexts where built-in language |
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| 27 | facilities fall short <a class="footnote-reference" href="#portability" id="id200" name="id200">[3]</a>:</p> |
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| 28 | <ol class="arabic simple"> |
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| 29 | <li>Passing a computation to an algorithm.</li> |
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| 30 | <li>Performing a computation on non-integer data.</li> |
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| 31 | </ol> |
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| 32 | <p>The second use case deserves special attention. In contrast to the built-in, |
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| 33 | strictly integer compile-time arithmetics, the MPL numeric metafunctions are |
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| 34 | <em>polymorphic</em>, with support for <em>mixed-type arithmetics</em>. This means that they |
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| 35 | can operate on a variety of numeric types — for instance, rational, |
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| 36 | fixed-point or complex numbers, — and that, in general, you are allowed to |
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| 37 | freely intermix these types within a single expression. See <a class="reference" href="./numeric-metafunction.html">Numeric Metafunction</a> concept for more details on the MPL numeric infrastructure.</p> |
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| 38 | <!-- The provided `infrastructure`__ allows easy plugging of user-defined numeric |
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| 39 | types |
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| 40 | Naturally, they also , meaning that you can perform a computation on the |
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| 41 | arguments of different types, and the result will yeild the largest/most general |
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| 42 | of them. For user-defined numeric types, they provide an `infrastructure`__ that |
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| 43 | allows easy plugging and seemless integration with predefined library |
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| 44 | types. details. |
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| 45 | |
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| 46 | __ `Numeric Metafunction`_ --> |
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| 47 | <p>To reduce a negative syntactical impact of the metafunctions notation |
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| 48 | over the infix operator notation, all numeric metafunctions |
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| 49 | allow to pass up to N arguments, where N is defined by the value of |
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| 50 | <a class="refentry reference" href="./limit-metafunction-arity.html"><tt class="refentry literal"><span class="pre">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</span></tt></a> configuration macro.</p> |
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| 51 | <table class="footnote" frame="void" id="portability" rules="none"> |
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| 52 | <colgroup><col class="label" /><col /></colgroup> |
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| 53 | <tbody valign="top"> |
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| 54 | <tr><td class="label"><a class="fn-backref" href="#id200" name="portability">[3]</a></td><td>All other considerations aside, as of the time of this writing |
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| 55 | (early 2004), using built-in operators on integral constants still often |
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| 56 | present a portability problem — many compilers cannot handle particular |
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| 57 | forms of expressions, forcing us to use conditional compilation. Because MPL |
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| 58 | numeric metafunctions work on types and encapsulate these kind of workarounds |
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| 59 | internally, they elude these problems, so if you aim for portability, it is |
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| 60 | generally adviced to use them in the place of the conventional operators, even |
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| 61 | at the price of slightly decreased readability.</td></tr> |
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| 62 | </tbody> |
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| 63 | </table> |
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| 64 | <!-- modtime: November 04, 2004 09:37:30 +0000 --> |
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| 65 | <ul class="toc simple" id="outline"> |
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| 66 | <li><a class="reference" href="./metafunctions-concepts.html" id="id437" name="id437">Concepts</a></li> |
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| 67 | <li><a class="reference" href="./type-selection.html" id="id445" name="id445">Type Selection</a></li> |
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| 68 | <li><a class="reference" href="./invocation.html" id="id450" name="id450">Invocation</a></li> |
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| 69 | <li><a class="reference" href="./composition-and-argument.html" id="id454" name="id454">Composition and Argument Binding</a></li> |
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| 70 | <li><a class="reference" href="./arithmetic-operations.html" id="id461" name="id461">Arithmetic Operations</a></li> |
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| 71 | <li><a class="reference" href="./comparisons.html" id="id468" name="id468">Comparisons</a></li> |
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| 72 | <li><a class="reference" href="./logical-operations.html" id="id475" name="id475">Logical Operations</a></li> |
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| 73 | <li><a class="reference" href="./bitwise-operations.html" id="id479" name="id479">Bitwise Operations</a></li> |
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| 74 | <li><a class="reference" href="./trivial.html" id="id485" name="id485">Trivial</a></li> |
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| 75 | <li><a class="reference" href="./miscellaneous.html" id="id487" name="id487">Miscellaneous</a></li> |
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| 76 | </ul> |
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