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

   1 .. Macros/Introspection//BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF
   2
   3 BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF
   4 =================================
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     #define BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(trait, name, default\_) \\
  12         |unspecified-token-seq| \\
  13     /\*\*/
  14
  15
  16 Description
  17 -----------
  18
  19 Expands into a definition of a boolean unary |Metafunction| ``trait``
  20 such that for any type ``x`` ``trait<x>::value == true`` if and only
  21 if ``x`` is a class type and has a nested type memeber ``x::name``.
  22
  23 On the deficient compilers not capabale of performing the detection,
  24 ``trait<x>::value`` always returns a fallback value ``default_``.
  25 A boolean configuraion macro, |BOOST_MPL_CFG_NO_HAS_XXX|, is provided
  26 to signal or override the "deficient" status of a particular compiler.
  27 |Note:| The fallback value can also be provided at the point of the
  28 metafunction invocation; see the `Expression semantics` section for
  29 details |-- end note|
  30
  31
  32 Header
  33 ------
  34
  35 .. parsed-literal::
  36
  37     #include <boost/mpl/has_xxx.hpp>
  38
  39
  40 Parameters
  41 ----------
  42
  43 +---------------+-------------------------------+---------------------------------------------------+
  44 | Parameter     | Requirement                   | Description                                       |
  45 +===============+===============================+===================================================+
  46 | ``trait``     | A legal identifier token      | A name of the metafunction to be generated.       |
  47 +---------------+-------------------------------+---------------------------------------------------+
  48 | ``name``      | A legal identifier token      | A name of the member being detected.              |
  49 +---------------+-------------------------------+---------------------------------------------------+
  50 | ``default_``  | An boolean constant           | A fallback value for the deficient compilers.     |
  51 +---------------+-------------------------------+---------------------------------------------------+
  52
  53
  54 Expression semantics
  55 --------------------
  56
  57 For any legal C++ identifiers ``trait`` and ``name``, boolean constant expression ``c1``,
  58 boolean |Integral Constant| ``c2``, and arbitrary type ``x``:
  59
  60 .. parsed-literal::
  61
  62     BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(trait, name, c1)
  63
  64 :Precondition:
  65     Appears at namespace scope.
  66
  67 :Return type:
  68     None.
  69
  70 :Semantics:
  71     Expands into an equivalent of the following class template definition
  72
  73     .. parsed-literal::
  74
  75         template< typename X, typename fallback = boost::mpl::bool_<c1> >
  76         struct trait
  77         {
  78             // |unspecified|
  79             // ...
  80         };
  81
  82     where ``trait`` is a boolean |Metafunction| with the following semantics:
  83
  84     .. parsed-literal::
  85
  86         typedef trait<x>::type r;
  87
  88     :Return type:
  89         |Integral Constant|.
  90
  91     :Semantics:
  92         If |BOOST_MPL_CFG_NO_HAS_XXX| is defined, ``r::value == c1``;
  93         otherwise, ``r::value == true`` if and only if ``x`` is a class type
  94         that has a nested type memeber ``x::name``.
  95
  96
  97     .. parsed-literal::
  98
  99         typedef trait< x,c2 >::type r;
 100
 101     :Return type:
 102         |Integral Constant|.
 103
 104     :Semantics:
 105         If |BOOST_MPL_CFG_NO_HAS_XXX| is defined, ``r::value == c2::value``;
 106         otherwise, equivalent to
 107
 108         .. parsed-literal::
 109
 110             typedef trait<x>::type r;
 111
 112
 113 Example
 114 -------
 115
 116 .. parsed-literal::
 117
 118     BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(has_xxx, xxx, false)
 119
 120     struct test1 {};
 121     struct test2 { void xxx(); };
 122     struct test3 { int xxx; };
 123     struct test4 { static int xxx(); };
 124     struct test5 { template< typename T > struct xxx {}; };
 125     struct test6 { typedef int xxx; };
 126     struct test7 { struct xxx; };
 127     struct test8 { typedef void (\*xxx)(); };
 128     struct test9 { typedef void (xxx)(); };
 129
 130     BOOST_MPL_ASSERT_NOT(( has_xxx<test1> ));
 131     BOOST_MPL_ASSERT_NOT(( has_xxx<test2> ));
 132     BOOST_MPL_ASSERT_NOT(( has_xxx<test3> ));
 133     BOOST_MPL_ASSERT_NOT(( has_xxx<test4> ));
 134     BOOST_MPL_ASSERT_NOT(( has_xxx<test5> ));
 135
 136     #if !defined(BOOST_MPL_CFG_NO_HAS_XXX)
 137     BOOST_MPL_ASSERT(( has_xxx<test6> ));
 138     BOOST_MPL_ASSERT(( has_xxx<test7> ));
 139     BOOST_MPL_ASSERT(( has_xxx<test8> ));
 140     BOOST_MPL_ASSERT(( has_xxx<test9> ));
 141     #endif
 142
 143     BOOST_MPL_ASSERT(( has_xxx<test6,true\_> ));
 144     BOOST_MPL_ASSERT(( has_xxx<test7,true\_> ));
 145     BOOST_MPL_ASSERT(( has_xxx<test8,true\_> ));
 146     BOOST_MPL_ASSERT(( has_xxx<test9,true\_> ));
 147
 148
 149 See also
 150 --------
 151
 152 |Macros|, |BOOST_MPL_HAS_XXX_TRAIT_DEF|, |BOOST_MPL_CFG_NO_HAS_XXX|
 153
 154
 155 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
 156    Distributed under the Boost Software License, Version 1.0. (See accompanying
 157    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)