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

   1 .. Metafunctions/Concepts//Metafunction |10
   2
   3 Metafunction
   4 ============
   5
   6 Description
   7 -----------
   8
   9 .. _`nullary-metafunction`:
  10
  11 A *metafunction* is a class or a class template that represents a
  12 function invocable at compile-time. An non-nullary metafunction is
  13 invoked by instantiating the class template with particular
  14 template parameters (metafunction arguments); the result of the
  15 metafunction application is accessible through the instantiation's
  16 nested ``type`` typedef. All metafunction's arguments must be types
  17 (i.e. only *type template parameters* are allowed). A metafunction
  18 can have a variable number of parameters. A *nullary metafunction* is
  19 represented as a (template) class with a nested ``type`` typename
  20 member.
  21
  22 .. |nullary metafunction| replace:: `nullary-metafunction`_
  23
  24
  25 Expression requirements
  26 -----------------------
  27
  28 |In the following table...| ``f`` is a |Metafunction|.
  29
  30 +-------------------------------+-----------------------+---------------------------+
  31 | Expression                    | Type                  | Complexity                |
  32 +===============================+=======================+===========================+
  33 | ``f::type``                   | Any type              | Unspecified.              |
  34 +-------------------------------+-----------------------+---------------------------+
  35 | ``f<>::type``                 | Any type              | Unspecified.              |
  36 +-------------------------------+-----------------------+---------------------------+
  37 | ``f<a1,..,an>::type``         | Any type              | Unspecified.              |
  38 +-------------------------------+-----------------------+---------------------------+
  39
  40
  41 Expression semantics
  42 --------------------
  43
  44 .. parsed-literal::
  45
  46     typedef f::type x;
  47
  48 :Precondition:
  49     ``f`` is a nullary |Metafunction|; ``f::type`` is a *type-name*.
  50
  51 :Semantics:
  52     ``x`` is the result of the metafunction invocation.
  53
  54
  55 .. ...................................................................................
  56
  57 .. parsed-literal::
  58
  59     typedef f<>::type x;
  60
  61 :Precondition:
  62     ``f`` is a nullary |Metafunction|; ``f<>::type`` is a *type-name*.
  63
  64 :Semantics:
  65     ``x`` is the result of the metafunction invocation.
  66
  67
  68 .. ...................................................................................
  69
  70 .. parsed-literal::
  71
  72     typedef f<a1,\ |...| \a\ *n*\>::type x;
  73
  74 :Precondition:
  75     ``f`` is an *n*-ary |Metafunction|; |a1...an| are types;
  76     ``f<a1,...an>::type`` is a *type-name*.
  77
  78 :Semantics:
  79     ``x`` is the result of the metafunction invocation
  80     with the actual arguments |a1...an|.
  81
  82
  83 Models
  84 ------
  85
  86 * |identity|
  87 * |plus|
  88 * |begin|
  89 * |insert|
  90 * |fold|
  91
  92
  93 See also
  94 --------
  95
  96 |Metafunctions|, |Metafunction Class|, |Lambda Expression|, |Invocation|, |apply|, |lambda|, |bind|
  97
  98
  99 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
 100    Distributed under the Boost Software License, Version 1.0. (See accompanying
 101    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)