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

   1 [/
   2   (C) Copyright Edward Diener 2011,2012,2014
   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_function Introspecting member function]
   9
  10 The TTI macro [macroref BOOST_TTI_HAS_MEMBER_FUNCTION] introspects
  11 a member function of a class.
  12
  13 BOOST_TTI_HAS_MEMBER_FUNCTION takes a single
  14 parameter which is the name of an inner member function whose existence
  15 the programmer wants to check. The macro generates a metafunction
  16 called 'has_member_function_'name_of_inner_member_function'.
  17
  18 The metafunction can be invoked in two different ways.
  19
  20 The first way of invoking the metafunction is by passing it the enclosing
  21 type to introspect and a signature for the member function as a series of
  22 separate template arguments. The signature for the member function consists
  23 of the template arguments of a return type, of optional parameter types in
  24 the form of a boost::mpl forward sequence of types, and of an optional Boost
  25 FunctionTypes tag type. A typical boost::mpl forward sequence of types is
  26 a boost::mpl::vector<>.
  27
  28 The optional Boost FunctionTypes tag type may be used to specify
  29 cv-qualification. This means you can add 'const', 'volatile', or both by
  30 specifying an appropriate tag type. An alternate to using the tag type
  31 is to specify the enclosing type as 'const', 'volatile', or both.
  32 As an example if you specify the tag type as
  33 'boost::function_types::const_qualified' or if you specify the enclosing
  34 type as 'const T', the member function which you are introspecting
  35 must be a const function.
  36
  37 The second way of invoking the metafunction is by passing it a single
  38 parameter, which is a pointer to member function. This type has the form of:
  39
  40  Return_Type ( Enclosing_Type::* ) ( Parameter_Types ) cv_qualifier(s)
  41
  42 where the Parameter_Types may be empty, or a comma-separated
  43 list of parameter types if there are more than one parameter type.
  44 The cv-qualifier may be 'const', 'volatile', or 'const volatile'.
  45
  46 The metafunction returns a single type called 'type', which is a
  47 boost::mpl::bool_. As a convenience the metafunction
  48 returns the value of this type directly as a compile time bool constant
  49 called 'value'. This 'value' is true or false depending on whether the inner
  50 member function, of the specified signature, exists or not.
  51
  52 [heading Generating the metafunction]
  53
  54 You generate the metafunction by invoking the macro with the name
  55 of an inner member function:
  56
  57   BOOST_TTI_HAS_MEMBER_FUNCTION(AMemberFunction)
  58
  59 generates a metafunction called 'has_member_function_AMemberFunction' in the current scope.
  60
  61 [heading Invoking the metafunction]
  62
  63 You invoke the metafunction by instantiating the template with an enclosing
  64 type to introspect and the signature of the member function as a series of template
  65 parameters. Alternatively you can invoke the metafunction by passing it a single
  66 type which is a pointer to member function.
  67
  68 A return value called 'value' is a compile time bool constant.
  69
  70   has_member_function_AMemberFunction
  71     <
  72     Enclosing_Type,
  73     MemberFunction_ReturnType,
  74     boost::mpl::vector<MemberFunction_ParameterTypes>, // optional, can be any mpl forward sequence
  75     boost::function_types::SomeTagType                 // optional, can be any FunctionTypes tag type
  76     >::value
  77
  78   OR
  79
  80   has_member_function_AMemberFunction
  81     <
  82     MemberFunction_ReturnType (Enclosing_Type::*) (MemberFunction_ParameterTypes) optional_cv_qualification
  83     >::value
  84
  85 [heading Examples]
  86
  87 First we generate metafunctions for various inner member function names:
  88
  89  #include <boost/tti/has_member_function.hpp>
  90
  91  BOOST_TTI_HAS_MEMBER_FUNCTION(function1)
  92  BOOST_TTI_HAS_MEMBER_FUNCTION(function2)
  93  BOOST_TTI_HAS_MEMBER_FUNCTION(function3)
  94
  95 Next let us create some user-defined types we want to introspect.
  96
  97  struct AClass
  98    {
  99    };
 100  struct Top
 101    {
 102    int function1();
 103    AClass function2(double,short *);
 104    };
 105  struct Top2
 106    {
 107    long function2(Top &,int,bool,short,float);
 108    Top * function3(long,int,AClass &);
 109    };
 110
 111 Finally we invoke our metafunction and return our value.
 112 This all happens at compile time, and can be used by
 113 programmers doing compile time template metaprogramming.
 114
 115 We will show both forms in the following examples.
 116 Both forms are completely interchangeable as to the result
 117 desired.
 118
 119  has_member_function_function1<Top,int>::value; // true
 120  has_member_function_function1<Top,int,boost::mpl::vector<> >::value; // true
 121  has_member_function_function1<Top2,int>::value; // false
 122
 123  has_member_function_function2<AClass (Top::*) (double,short *)>::value; // true
 124  has_member_function_function2<AClass (Top2::*) (double,short *)>::value; // false
 125  has_member_function_function2<long (Top2::*) (Top &,int,bool,short,float)>::value; // true
 126
 127  has_member_function_function3<int (Top2::*) ()>::value; // false
 128  has_member_function_function3<Top2,Top *,boost::mpl::vector<long,int,AClass &> >::value; // true;
 129
 130 [heading Metafunction re-use]
 131
 132 The macro encodes only the name of the member function for which
 133 we are searching and the fact that we are introspecting for a
 134 member function within an enclosing type.
 135
 136 Because of this, once we create our metafunction for
 137 introspecting a member function by name, we can reuse the
 138 metafunction for introspecting any enclosing type, having any
 139 member function, for that name.
 140
 141 [endsect]