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

   1 .. Metafunctions/Composition and Argument Binding//lambda |20
   2
   3 lambda
   4 ======
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           typename X
  13         , typename Tag = |unspecified|
  14         >
  15     struct lambda
  16     {
  17         typedef |unspecified| type;
  18     };
  19
  20
  21
  22 Description
  23 -----------
  24
  25 If ``X`` is a |placeholder expression|, transforms ``X`` into a corresponding
  26 |Metafunction Class|, otherwise ``X`` is returned unchanged.
  27
  28
  29 Header
  30 ------
  31
  32 .. parsed-literal::
  33
  34     #include <boost/mpl/lambda.hpp>
  35
  36
  37 Parameters
  38 ----------
  39
  40 +---------------+-----------------------+-----------------------------------------------+
  41 | Parameter     | Requirement           | Description                                   |
  42 +===============+=======================+===============================================+
  43 | ``X``         | Any type              | An expression to transform.                   |
  44 +---------------+-----------------------+-----------------------------------------------+
  45 | ``Tag``       | Any type              | A tag determining transform semantics.        |
  46 +---------------+-----------------------+-----------------------------------------------+
  47
  48 Expression semantics
  49 --------------------
  50
  51 For arbitrary types ``x`` and ``tag``:
  52
  53
  54 .. parsed-literal::
  55
  56     typedef lambda<x>::type f;
  57
  58 :Return type:
  59     |Metafunction Class|.
  60
  61 :Semantics:
  62     If ``x`` is a |placeholder expression| in a general form ``X<a1,...an>``, where
  63     ``X`` is a class template and ``a1``,... ``an`` are arbitrary types, equivalent
  64     to
  65
  66     .. parsed-literal::
  67
  68         typedef protect< bind<
  69               quote\ *n*\ <X>
  70             , lambda<a1>::type,\ |...| lambda<a\ *n*\ >::type
  71             > > f;
  72
  73     otherwise, ``f`` is identical to ``x``.
  74
  75 .. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  76
  77 .. parsed-literal::
  78
  79     typedef lambda<x,tag>::type f;
  80
  81 :Return type:
  82     |Metafunction Class|.
  83
  84 :Semantics:
  85     If ``x`` is a |placeholder expression| in a general form ``X<a1,...an>``, where
  86     ``X`` is a class template and ``a1``,... ``an`` are arbitrary types, equivalent
  87     to
  88
  89     .. parsed-literal::
  90
  91         typedef protect< bind<
  92               quote\ *n*\ <X,tag>
  93             , lambda<a1,tag>::type,\ |...| lambda<a\ *n*\ ,tag>::type
  94             > > f;
  95
  96     otherwise, ``f`` is identical to ``x``.
  97
  98
  99 Example
 100 -------
 101
 102 .. parsed-literal::
 103
 104     template< typename N1, typename N2 > struct int_plus
 105         : int_<( N1::value + N2::value )>
 106     {
 107     };
 108
 109     typedef lambda< int_plus<_1, int_<42> > >::type f1;
 110     typedef bind< quote\ ``2``\ <int_plus>, _1, int_<42> > f2;
 111
 112     typedef f1::apply<42>::type r1;
 113     typedef f2::apply<42>::type r2;
 114
 115     BOOST_MPL_ASSERT_RELATION( r1::value, ==, 84 );
 116     BOOST_MPL_ASSERT_RELATION( r2::value, ==, 84 );
 117
 118
 119 See also
 120 --------
 121
 122 |Composition and Argument Binding|, |Invocation|, |Placeholders|, |bind|, |quote|, |protect|, |apply|
 123
 124
 125 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
 126    Distributed under the Boost Software License, Version 1.0. (See accompanying
 127    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)