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

   1 [/
   2   (C) Copyright Edward Diener 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_function Introspecting an inner function]
   9
  10 The TTI macro [macroref BOOST_TTI_HAS_FUNCTION] introspects
  11 an inner function of a class. The function can be either a member
  12 function or a static member function.
  13
  14 BOOST_TTI_HAS_FUNCTION takes a single
  15 parameter which is the name of an inner function whose existence
  16 the programmer wants to check. The macro generates a metafunction
  17 called 'has_function_'name_of_inner_function'.
  18
  19 The metafunction can be invoked by passing it the enclosing type to introspect and a
  20 signature for the function as separate template arguments. The signature for the
  21 function consists of a return type, optional parameter types in the form of a boost::mpl
  22 forward sequence of types, and an optional Boost FunctionTypes tag type. A
  23 typical boost::mpl forward sequence of types is a boost::mpl::vector<>.
  24
  25 The metafunction returns a single type called 'type', which is a
  26 boost::mpl::bool_. As a convenience the metafunction
  27 returns the value of this type directly as a compile time bool constant
  28 called 'value'. This is true or false depending on whether the inner
  29 function, of the specified signature, exists or not.
  30
  31 [heading Generating the metafunction]
  32
  33 You generate the metafunction by invoking the macro with the name
  34 of an inner function:
  35
  36   BOOST_TTI_HAS_FUNCTION(AnInnerFunction)
  37
  38 generates a metafunction called 'has_function_AnInnerFunction' in the current scope.
  39
  40 [heading Invoking the metafunction]
  41
  42 You invoke the metafunction by instantiating the template with an enclosing
  43 type to introspect and the signature of the function as a series of template
  44 parameters.
  45
  46 A return value called 'value' is a compile time bool constant.
  47
  48   has_function_AnInnerFunction
  49     <
  50     Enclosing_Type,
  51     Function_ReturnType,
  52     boost::mpl::vector<Function_ParameterTypes>, // optional, can be any mpl forward sequence
  53     boost::function_types::SomeTagType           // optional, can be any FunctionTypes tag type
  54     >::value
  55
  56 [heading Examples]
  57
  58 First we generate metafunctions for various inner function names:
  59
  60  #include <boost/tti/has_function.hpp>
  61
  62  BOOST_TTI_HAS_FUNCTION(function1)
  63  BOOST_TTI_HAS_FUNCTION(function2)
  64  BOOST_TTI_HAS_FUNCTION(function3)
  65
  66 Next let us create some user-defined types we want to introspect.
  67
  68  struct AClass { };
  69  struct Top
  70    {
  71    static int function1();
  72    AClass function2(double,short *);
  73    };
  74  struct Top2
  75    {
  76    long function2(Top &,int,bool,short,float);
  77    static Top * function3(long,int,AClass &);
  78    };
  79
  80 Finally we invoke our metafunction and return our value.
  81 This all happens at compile time, and can be used by
  82 programmers doing compile time template metaprogramming.
  83
  84  has_function_function1<Top,int>::value; // true
  85  has_function_function1<Top2,int>::value; // false
  86
  87  has_function_function2<Top,AClass,boost::mpl::vector<double,short *> >::value; // true
  88  has_function_function2<Top2,AClass,boost::mpl::vector<double,short *> >::value; // false
  89
  90  has_function_function3<Top2,int>::value; // false
  91  has_function_function3<Top2,Top *,boost::mpl::vector<long,int,AClass &> >::value; // true;
  92
  93 [heading Metafunction re-use]
  94
  95 The macro encodes only the name of the function
  96 for which we are searching and the fact that we are
  97 introspecting for a function within an enclosing type.
  98
  99 Because of this, once we create our metafunction for
 100 introspecting a function by name, we can reuse the
 101 metafunction for introspecting any enclosing type, having any
 102 function, for that name.
 103
 104 [endsect]