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| <div class="section" id="metafunctions"> |
| <h1><a class="toc-backref" href="../refmanual.html#id1510">Metafunctions</a></h1> |
| <p id="label-metafunctions">The MPL includes a number of predefined metafunctions that can be roughly |
| classified in two categories: <cite>general purpose metafunctions</cite>, dealing with |
| 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>, |
| <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>, |
| 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>, |
| <a class="reference internal" href="./logical-operations.html">logical</a>, and <a class="reference internal" href="./bitwise-operations.html">bitwise</a> operations.</p> |
| <p>Given that it is possible to perform integer numeric computations at |
| compile time using the conventional operators notation, the need for the |
| second category might be not obvious, but it in fact plays a cental role in |
| making programming with MPL seemingly effortless. In |
| particular, there are at least two contexts where built-in language |
| facilities fall short <a class="footnote-reference" href="#portability" id="id820">[3]</a>:</p> |
| <ol class="arabic simple"> |
| <li>Passing a computation to an algorithm.</li> |
| <li>Performing a computation on non-integer data.</li> |
| </ol> |
| <p>The second use case deserves special attention. In contrast to the built-in, |
| strictly integer compile-time arithmetics, the MPL numeric metafunctions are |
| <em>polymorphic</em>, with support for <em>mixed-type arithmetics</em>. This means that they |
| can operate on a variety of numeric types — for instance, rational, |
| fixed-point or complex numbers, — and that, in general, you are allowed to |
| 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> |
| <!-- The provided `infrastructure`__ allows easy plugging of user-defined numeric |
| types |
| Naturally, they also , meaning that you can perform a computation on the |
| arguments of different types, and the result will yeild the largest/most general |
| of them. For user-defined numeric types, they provide an `infrastructure`__ that |
| allows easy plugging and seemless integration with predefined library |
| types. details. |
| |
| __ `Numeric Metafunction`_ --> |
| <p>To reduce a negative syntactical impact of the metafunctions notation |
| over the infix operator notation, all numeric metafunctions |
| allow to pass up to N arguments, where N is defined by the value of |
| <a class="reference internal" href="./limit-metafunction-arity.html">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</a> configuration macro.</p> |
| <table class="docutils footnote" frame="void" id="portability" rules="none"> |
| <colgroup><col class="label" /><col /></colgroup> |
| <tbody valign="top"> |
| <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 |
| (early 2004), using built-in operators on integral constants still often |
| present a portability problem — many compilers cannot handle particular |
| forms of expressions, forcing us to use conditional compilation. Because MPL |
| numeric metafunctions work on types and encapsulate these kind of workarounds |
| internally, they elude these problems, so if you aim for portability, it is |
| generally adviced to use them in the place of the conventional operators, even |
| at the price of slightly decreased readability.</td></tr> |
| </tbody> |
| </table> |
| <ul class="toc simple" id="outline"> |
| <li><a class="reference internal" href="./metafunctions-concepts.html" id="id1511">Concepts</a></li> |
| <li><a class="reference internal" href="./type-selection.html" id="id1519">Type Selection</a></li> |
| <li><a class="reference internal" href="./invocation.html" id="id1524">Invocation</a></li> |
| <li><a class="reference internal" href="./composition-and-argument-binding.html" id="id1528">Composition and Argument Binding</a></li> |
| <li><a class="reference internal" href="./arithmetic-operations.html" id="id1535">Arithmetic Operations</a></li> |
| <li><a class="reference internal" href="./comparisons.html" id="id1542">Comparisons</a></li> |
| <li><a class="reference internal" href="./logical-operations.html" id="id1549">Logical Operations</a></li> |
| <li><a class="reference internal" href="./bitwise-operations.html" id="id1553">Bitwise Operations</a></li> |
| <li><a class="reference internal" href="./trivial.html" id="id1559">Trivial</a></li> |
| <li><a class="reference internal" href="./string-operations.html" id="id1561">String Operations</a></li> |
| <li><a class="reference internal" href="./miscellaneous.html" id="id1563">Miscellaneous</a></li> |
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