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

   1 .. Metafunctions/Composition and Argument Binding//_1,_2,..._n |10
   2
   3 Placeholders
   4 ============
   5 .. _`placeholder`:
   6
   7
   8 Synopsis
   9 --------
  10
  11 .. parsed-literal::
  12
  13     namespace placeholders {
  14     typedef |unspecified| _;
  15     typedef arg<1>      _1;
  16     typedef arg<2>      _2;
  17     |...|
  18     typedef arg<\ *n*\ >      _\ *n*\ ;
  19     }
  20
  21     using placeholders::_;
  22     using placeholders::_1;
  23     using placeholders::_2;
  24     |...|
  25     using placeholders::_\ *n*\ ;
  26
  27
  28 Description
  29 -----------
  30
  31 A placeholder in a form ``_``\ *n* is simply a synonym for the corresponding
  32 ``arg<n>`` specialization. The unnamed placeholder ``_`` (underscore) carries
  33 `special meaning`__ in bind and lambda expressions, and does not have
  34 defined semantics outside of these contexts.
  35
  36 Placeholder names can be made available in the user namespace through
  37 ``using namespace mpl::placeholders;`` directive.
  38
  39 __ `bind semantics`_
  40
  41 Header
  42 ------
  43
  44 .. parsed-literal::
  45
  46     #include <boost/mpl/placeholders.hpp>
  47
  48 |Note:| The include might be omitted when using placeholders to construct a |Lambda
  49 Expression| for passing it to MPL's own algorithm or metafunction: any library
  50 component that is documented to accept a lambda expression makes the placeholders
  51 implicitly available for the user code |-- end note|
  52
  53
  54 Parameters
  55 ----------
  56
  57 None.
  58
  59
  60 Expression semantics
  61 --------------------
  62
  63 For any integral constant ``n`` in the range [1, |BOOST_MPL_LIMIT_METAFUNCTION_ARITY|\] and
  64 arbitrary types |a1...an|:
  65
  66
  67 .. parsed-literal::
  68
  69     typedef apply_wrap\ *n*\<_\ *n*\,a1,\ |...|\a\ *n*\ >::type x;
  70
  71 :Return type:
  72     A type.
  73
  74 :Semantics:
  75     Equivalent to
  76
  77     .. parsed-literal::
  78
  79         typedef apply_wrap\ *n*\< arg<\ *n*\ >,a1,\ |...|\a\ *n* >::type x;
  80
  81
  82 Example
  83 -------
  84
  85 .. parsed-literal::
  86
  87     typedef apply_wrap\ ``5``\< _1,bool,char,short,int,long >::type t1;
  88     typedef apply_wrap\ ``5``\< _3,bool,char,short,int,long >::type t3;
  89
  90     BOOST_MPL_ASSERT(( is_same< t1, bool > ));
  91     BOOST_MPL_ASSERT(( is_same< t3, short > ));
  92
  93
  94 See also
  95 --------
  96
  97 |Composition and Argument Binding|, |arg|, |lambda|, |bind|, |apply|, |apply_wrap|
  98
  99
 100 .. |placeholder| replace:: `placeholder`_
 101
 102 .. |_1| replace:: `_1`_
 103 .. |_2| replace:: `_2`_
 104 .. |_3| replace:: `_3`_
 105 .. |_4| replace:: `_4`_
 106 .. |_5| replace:: `_5`_
 107
 108 .. _`_1`: `Placeholders`_
 109 .. _`_2`: `Placeholders`_
 110 .. _`_3`: `Placeholders`_
 111 .. _`_4`: `Placeholders`_
 112 .. _`_5`: `Placeholders`_
 113
 114 .. |_1,_2,..._n| replace:: |_1|, |_2|, |_3|,\ |...|
 115
 116
 117 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
 118    Distributed under the Boost Software License, Version 1.0. (See accompanying
 119    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)