ceph/src/boost/libs/tti/doc/tti_detail_has_member_data.qbk

   1 [/
   2   (C) Copyright Edward Diener 2011,2012
   3   Distributed under the Boost Software License, Version 1.0.
   4   (See accompanying file LICENSE_1_0.txt or copy at
   5   http://www.boost.org/LICENSE_1_0.txt).
   6 ]
   7
   8 [section:tti_detail_has_member_data Introspecting member data]
   9
  10 The TTI macro [macroref BOOST_TTI_HAS_MEMBER_DATA] introspects
  11 member data of a class.
  12
  13 BOOST_TTI_HAS_MEMBER_DATA macro takes a single
  14 parameter which is the name of an inner member data whose existence
  15 the programmer wants to check. The macro generates a metafunction
  16 called 'has_member_data_'name_of_inner_member_data'.
  17
  18 The metafunction can be invoked in two different ways.
  19
  20 The first way is by passing it two parameters. The first parameter
  21 is the enclosing type to introspect and the second parameter is the
  22 type of the member data.
  23
  24 The second way is by passing it a single parameter, which is a pointer
  25 to member type. This type has the form of:
  26
  27  MemberData_Type Enclosing_Type::*
  28
  29 The metafunction returns a single type called 'type', which is a
  30 boost::mpl::bool_. As a convenience the metafunction
  31 returns the value of this type directly as a compile time bool constant
  32 called 'value'. This value is true or false depending on whether the
  33 inner member data, of the specified type, exists or not.
  34
  35 [heading Generating the metafunction]
  36
  37 You generate the metafunction by invoking the macro with the name
  38 of an inner member data:
  39
  40   BOOST_TTI_HAS_MEMBER_DATA(AMemberData)
  41
  42 generates a metafunction called 'has_member_data_AMemberData' in the current scope.
  43
  44 [heading Invoking the metafunction]
  45
  46 You invoke the metafunction by instantiating the template with an enclosing
  47 type to introspect and the type of the member data, or by instantiating the
  48 template with a pointer to member data type. The return value called 'value'
  49 is a compile time bool constant telling you whether or not the member data .
  50 exists.
  51
  52   has_member_data_AMemberData<Enclosing_Type,MemberData_Type>::value
  53
  54   OR
  55
  56   has_member_data_AMemberData<MemberData_Type Enclosing_Type::*>::value
  57
  58 [heading Examples]
  59
  60 First we generate metafunctions for various inner member data names:
  61
  62  #include <boost/tti/has_member_data.hpp>
  63
  64  BOOST_TTI_HAS_MEMBER_DATA(data1)
  65  BOOST_TTI_HAS_MEMBER_DATA(data2)
  66  BOOST_TTI_HAS_MEMBER_DATA(data3)
  67
  68 Next let us create some user-defined types we want to introspect.
  69
  70  struct AClass
  71    {
  72    };
  73  struct Top
  74    {
  75    int data1;
  76    AClass * data2;
  77    };
  78  struct Top2
  79    {
  80    long data1;
  81    Top data3;
  82    };
  83
  84 Finally we invoke our metafunction and return our value.
  85 This all happens at compile time, and can be used by
  86 programmers doing compile time template metaprogramming.
  87
  88 We will show both forms in the following examples.
  89 Both forms are completely interchangeable as to the result
  90 desired.
  91
  92  has_member_data_data1<Top,int>::value; // true
  93  has_member_data_data1<Top,long>::value; // false
  94
  95  has_member_data_data1<Top2,int>::value; // false
  96  has_member_data_data1<long Top2::*>::value; // true
  97
  98  has_member_data_data2<AClass * Top::*>::value; // true
  99  has_member_data_data2<Top,int *>::value; // false
 100
 101  has_member_data_data3<int Top2::*>::value; // false
 102  has_member_data_data3<Top Top2::*>::value; // true;
 103
 104 [heading Metafunction re-use]
 105
 106 The macro encodes only the name of the member data for which
 107 we are searching and the fact that we are introspecting for
 108 member data within an enclosing type.
 109
 110 Because of this, once we create our metafunction for
 111 introspecting an inner member data by name, we can reuse the
 112 metafunction for introspecting any enclosing type, having any
 113 inner member data type, for that name.
 114
 115 [endsect]