ceph/src/boost/libs/mpl/doc/src/refmanual/Metafunctions.rst

   1
   2 The MPL includes a number of predefined metafunctions that can be roughly
   3 classified in two categories: `general purpose metafunctions`, dealing with
   4 conditional |type selection| and higher-order metafunction |invocation|,
   5 |composition|, and |argument binding|, and `numeric metafunctions`,
   6 incapsulating built-in and user-defined |arithmetic|, |comparison|,
   7 |logical|, and |bitwise| operations.
   8
   9 Given that it is possible to perform integer numeric computations at
  10 compile time using the conventional operators notation, the need for the
  11 second category might be not obvious, but it in fact plays a cental role in
  12 making programming with MPL seemingly effortless. In
  13 particular, there are at least two contexts where built-in language
  14 facilities fall short [#portability]_\ :
  15
  16 1) Passing a computation to an algorithm.
  17 2) Performing a computation on non-integer data.
  18
  19 The second use case deserves special attention. In contrast to the built-in,
  20 strictly integer compile-time arithmetics, the MPL numeric metafunctions are
  21 *polymorphic*, with support for *mixed-type arithmetics*. This means that they
  22 can operate on a variety of numeric types |--| for instance, rational,
  23 fixed-point or complex numbers, |--| and that, in general, you are allowed to
  24 freely intermix these types within a single expression. See |Numeric
  25 Metafunction| concept for more details on the MPL numeric infrastructure.
  26
  27 .. The provided `infrastructure`__ allows easy plugging of user-defined numeric
  28    types
  29    Naturally, they also , meaning that you can perform a computation on the
  30    arguments of different types, and the result will yeild the largest/most general
  31    of them. For user-defined numeric types, they provide an `infrastructure`__ that
  32    allows easy plugging and seemless integration with predefined library
  33    types. details.
  34
  35    __ `Numeric Metafunction`_
  36
  37
  38 To reduce a negative syntactical impact of the metafunctions notation
  39 over the infix operator notation, all numeric metafunctions
  40 allow to pass up to N arguments, where N is defined by the value of
  41 |BOOST_MPL_LIMIT_METAFUNCTION_ARITY| configuration macro.
  42
  43
  44 .. [#portability] All other considerations aside, as of the time of this writing
  45    (early 2004), using built-in operators on integral constants still often
  46    present a portability problem |--| many compilers cannot handle particular
  47    forms of expressions, forcing us to use conditional compilation. Because MPL
  48    numeric metafunctions work on types and encapsulate these kind of workarounds
  49    internally, they elude these problems, so if you aim for portability, it is
  50    generally adviced to use them in the place of the conventional operators, even
  51    at the price of slightly decreased readability.
  52
  53
  54 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
  55    Distributed under the Boost Software License, Version 1.0. (See accompanying
  56    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)