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

   1 .. Macros/Introspection//BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF
   2
   3 .. Copyright Daniel Walker 2007.
   4 .. Distributed under the Boost
   5 .. Software License, Version 1.0. (See accompanying
   6 .. file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   7
   8 BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF
   9 ====================================
  10
  11 Synopsis
  12 --------
  13
  14 .. parsed-literal::
  15
  16     #define BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF(trait, name, default\_) \\
  17         |unspecified-token-seq| \\
  18     /\*\*/
  19
  20
  21 Description
  22 -----------
  23
  24 Expands into the definition of a boolean |Metafunction| ``trait`` such
  25 that for any type ``x`` ``trait<x>::value == true`` if and only if
  26 ``x`` is a class type and has a nested template member ``x::template
  27 name`` with no more than |BOOST_MPL_LIMIT_METAFUNCTION_ARITY|
  28 parameters.
  29
  30 On deficient compilers not capable of performing the detection,
  31 ``trait<x>::value`` always returns a fallback value ``default_``.  A
  32 boolean configuration macro, |BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE|, is
  33 provided to signal or override the "deficient" status of a particular
  34 compiler.  |Note:| The fallback value can also be provided at the
  35 point of the metafunction invocation; see the `Expression semantics`
  36 section for details |-- end note|
  37
  38
  39 Header
  40 ------
  41
  42 .. parsed-literal::
  43
  44     #include <boost/mpl/has_xxx.hpp>
  45
  46
  47 Parameters
  48 ----------
  49
  50 +---------------+-------------------------------+---------------------------------------------------+
  51 | Parameter     | Requirement                   | Description                                       |
  52 +===============+===============================+===================================================+
  53 | ``trait``     | A legal identifier token      | A name of the metafunction to be generated.       |
  54 +---------------+-------------------------------+---------------------------------------------------+
  55 | ``name``      | A legal identifier token      | A name of the member being detected.              |
  56 +---------------+-------------------------------+---------------------------------------------------+
  57 | ``default_``  | An boolean constant           | A fallback value for the deficient compilers.     |
  58 +---------------+-------------------------------+---------------------------------------------------+
  59
  60
  61 Expression semantics
  62 --------------------
  63
  64 For any legal C++ identifiers ``trait`` and ``name``, boolean constant
  65 expression ``c1``, boolean |Integral Constant| ``c2``, and arbitrary
  66 type ``x``:
  67
  68 .. parsed-literal::
  69
  70     BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF(trait, name, c1)
  71
  72 :Precondition:
  73     Appears at namespace scope.
  74
  75 :Return type:
  76     None.
  77
  78 :Semantics:
  79     Expands into an equivalent of the following class template
  80     definition
  81
  82     .. parsed-literal::
  83
  84         template<
  85             typename X
  86           , typename fallback = boost::mpl::bool\_<c1>
  87         >
  88         struct trait
  89         {
  90             // |unspecified|
  91             // ...
  92         };
  93
  94     where ``trait`` is a boolean |Metafunction| with the following
  95     semantics:
  96
  97     .. parsed-literal::
  98
  99         typedef trait<x>::type r;
 100
 101     :Return type:
 102         |Integral Constant|.
 103
 104     :Semantics:
 105         If |BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE| is defined, ``r::value
 106         == c1``; otherwise, ``r::value == true`` if and only if ``x``
 107         is a class type that has a nested template member ``x::template
 108         name`` with no more than |BOOST_MPL_LIMIT_METAFUNCTION_ARITY|.
 109
 110
 111     .. parsed-literal::
 112
 113         typedef trait< x, c2 >::type r;
 114
 115     :Return type:
 116         |Integral Constant|.
 117
 118     :Semantics:
 119         If |BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE| is defined, ``r::value
 120         == c2::value``; otherwise, equivalent to
 121
 122         .. parsed-literal::
 123
 124             typedef trait<x>::type r;
 125
 126
 127 Example
 128 -------
 129
 130 .. parsed-literal::
 131
 132     BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF(
 133         has_xxx, xxx, false
 134     )
 135
 136     struct test1  {};
 137     struct test2  { void xxx(); };
 138     struct test3  { int xxx; };
 139     struct test4  { static int xxx(); };
 140     struct test5  { typedef int xxx; };
 141     struct test6  { struct xxx; };
 142     struct test7  { typedef void (\*xxx)(); };
 143     struct test8  { typedef void (xxx)(); };
 144     struct test9  { template< class T > struct xxx {}; };
 145
 146     BOOST_MPL_ASSERT_NOT(( has_xxx<test1> ));
 147     BOOST_MPL_ASSERT_NOT(( has_xxx<test2> ));
 148     BOOST_MPL_ASSERT_NOT(( has_xxx<test3> ));
 149     BOOST_MPL_ASSERT_NOT(( has_xxx<test4> ));
 150     BOOST_MPL_ASSERT_NOT(( has_xxx<test5> ));
 151     BOOST_MPL_ASSERT_NOT(( has_xxx<test6> ));
 152     BOOST_MPL_ASSERT_NOT(( has_xxx<test7> ));
 153     BOOST_MPL_ASSERT_NOT(( has_xxx<test8> ));
 154
 155     #if !defined(BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE)
 156     BOOST_MPL_ASSERT(( has_xxx<test9> ));
 157     #endif
 158
 159     BOOST_MPL_ASSERT(( has_xxx<test9, true\_> ));
 160
 161
 162 See also
 163 --------
 164
 165 |Macros|, |BOOST_MPL_HAS_XXX_TEMPLATE_DEF|,
 166 |BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE|, |BOOST_MPL_LIMIT_METAFUNCTION_ARITY|
 167