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

   1 .. Metafunctions/Composition and Argument Binding//protect |60
   2
   3 protect
   4 =======
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           typename F
  13         >
  14     struct protect
  15     {
  16         // |unspecified|
  17         // |...|
  18     };
  19
  20
  21
  22 Description
  23 -----------
  24
  25 ``protect`` is an identity wrapper for a |Metafunction Class| that prevents
  26 its argument from being recognized as a |bind expression|.
  27
  28
  29 Header
  30 ------
  31
  32 .. parsed-literal::
  33
  34     #include <boost/mpl/protect.hpp>
  35
  36
  37 Parameters
  38 ----------
  39
  40 +---------------+---------------------------+---------------------------------------+
  41 | Parameter     | Requirement               | Description                           |
  42 +===============+===========================+=======================================+
  43 | ``F``         | |Metafunction Class|      | A metafunction class to wrap.         |
  44 +---------------+---------------------------+---------------------------------------+
  45
  46
  47 Expression semantics
  48 --------------------
  49
  50 For any |Metafunction Class| ``f``:
  51
  52
  53 .. parsed-literal::
  54
  55     typedef protect<f> g;
  56
  57 :Return type:
  58     |Metafunction Class|.
  59
  60 :Semantics:
  61     If ``f`` is a |bind expression|, equivalent to
  62
  63     .. parsed-literal::
  64
  65         struct g
  66         {
  67             template<
  68                   typename U1 = |unspecified|\,\ |...| typename U\ *n* = |unspecified|
  69                 >
  70             struct apply
  71                 : apply_wrap\ *n*\<f,U1,\ |...|\ U\ *n*\ >
  72             {
  73             };
  74         };
  75
  76     otherwise equivalent to ``typedef f g;``.
  77
  78
  79 Example
  80 -------
  81
  82 .. parsed-literal::
  83
  84     struct f
  85     {
  86         template< typename T1, typename T2 > struct apply
  87         {
  88             typedef T2 type;
  89         };
  90     };
  91
  92     typedef bind< quote\ ``3``\<if\_>,_1,_2,bind<f,_1,_2> > b1;
  93     typedef bind< quote\ ``3``\<if\_>,_1,_2,protect< bind<f,_1,_2> > > b2;
  94
  95     typedef apply_wrap\ ``2``\< b1,false\_,char >::type r1;
  96     typedef apply_wrap\ ``2``\< b2,false\_,char >::type r2;
  97
  98     BOOST_MPL_ASSERT(( is_same<r1,char> ));
  99     BOOST_MPL_ASSERT(( is_same<r2,protect< bind<f,_1,_2> > > ));
 100
 101
 102 See also
 103 --------
 104
 105 |Composition and Argument Binding|, |Invocation|, |bind|, |quote|, |apply_wrap|
 106
 107
 108 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
 109    Distributed under the Boost Software License, Version 1.0. (See accompanying
 110    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)