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

   1 .. Metafunctions/Concepts//Placeholder Expression |40
   2
   3 Placeholder Expression
   4 ======================
   5
   6 Description
   7 -----------
   8
   9 A |Placeholder Expression| is a type that is either a |placeholder| or a class
  10 template specialization with at least one argument that itself is a
  11 |Placeholder Expression|.
  12
  13
  14 Expression requirements
  15 -----------------------
  16
  17 If ``X`` is a class template, and ``a1``,... ``an`` are arbitrary types, then
  18 ``X<a1,...,an>`` is a |Placeholder Expression| if and only if all of the following
  19 conditions hold:
  20
  21 * At least one of the template arguments ``a1``,... ``an`` is a |placeholder|
  22   or a |Placeholder Expression|.
  23
  24 * All of ``X``\ 's template parameters, including the default ones, are types.
  25
  26 * The number of ``X``\ 's template parameters, including the default ones, is
  27   less or equal to the value of ``BOOST_MPL_LIMIT_METAFUNCTION_ARITY``
  28   `configuration macro`__.
  29
  30 __ `Configuration`_
  31
  32
  33 Models
  34 ------
  35
  36 * |_1|
  37 * ``plus<_, int_<2> >``
  38 * ``if_< less<_1, int_<7> >, plus<_1,_2>, _1 >``
  39
  40
  41 See also
  42 --------
  43
  44 |Lambda Expression|, |Placeholders|, |Metafunctions|, |apply|, |lambda|
  45
  46
  47 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
  48    Distributed under the Boost Software License, Version 1.0. (See accompanying
  49    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)