ceph/src/boost/libs/tti/doc/tti_nested_type_and_signatures.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_func_sig Nested Types and Function Signatures]
   9
  10 The strength of `BOOST_TTI_MEMBER_TYPE` to represent a type which may or may not exist, and which
  11 then can be subsequently used in other macro metafunctions whenever a type is needed as a template
  12 parameter without producing a compiler error, should not be underestimated. It is one of the
  13 reasons why we have two different ways of using our generated metafunction when introspecting
  14 for member data, a member function, or a static member function of an enclosing type.
  15
  16 In the cases where we specify a composite syntax when using `BOOST_TTI_HAS_MEMBER_DATA`,
  17 `BOOST_TTI_HAS_MEMBER_FUNCTION`, or `BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION`, the signature
  18 for the member data, member function, or static member function is a single type. For
  19 `BOOST_TTI_HAS_MEMBER_DATA` the signature is a pointer to member data, for
  20 `BOOST_TTI_HAS_MEMBER_FUNCTION` the signature is a pointer to a member function, and for
  21 `BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION` the signature  is divided between an enclosing type
  22 and a function in composite format. This makes for a syntactical notation which is natural
  23 to specify, but because of the notation we can not use the nested type functionality in
  24 `BOOST_TTI_MEMBER_TYPE` for potential parts of these composite types. If any part of this
  25 signature, which specifies a composite of various types, is invalid, a compiler time error
  26 will occur.
  27
  28 But in the more specific cases, when we use `BOOST_TTI_HAS_MEMBER_DATA`,
  29 `BOOST_TTI_HAS_MEMBER_FUNCTION`, and `BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION`, our composite
  30 type in our signatures is broken down into their individual types so that using
  31 `BOOST_TTI_MEMBER_TYPE` for any one of the individual types will not lead to a compile time
  32 error if the type specified does not actually exist.
  33
  34 A few examples will suffice.
  35
  36 Given known types T and U, and the supposed type Ntype as a
  37 nested type of U, we want to find out if type T has a member function whose signature is
  38 `void aMemberFunction(U::Ntype)`.
  39
  40 First using `BOOST_TTI_HAS_MEMBER_FUNCTION` using our composite form we would code:
  41
  42  #include <boost/tti/has_member_function.hpp>
  43
  44  BOOST_TTI_HAS_MEMBER_FUNCTION(aMemberFunction)
  45
  46  has_member_function_aMemberFunction<void (T::*)(U::Ntype)>::value;
  47
  48 If the nested type U::Ntype does not exist, this leads to a compiler error.
  49 We really want to avoid this situation, so let's try our alternative.
  50
  51 Second using `BOOST_TTI_HAS_MEMBER_FUNCTION` using our specific form we would code:
  52
  53  #include <boost/tti/member_type.hpp>
  54  #include <boost/tti/has_member_function.hpp>
  55
  56  BOOST_TTI_HAS_MEMBER_TYPE(Ntype)
  57  BOOST_TTI_HAS_MEMBER_FUNCTION(aMemberFunction)
  58
  59  typedef typename has_member_type_Ntype<U>::type OurType;
  60  has_member_function_aMemberFunction<T,void,boost::mpl::vector<OurType> >::value;
  61
  62 If the nested type U::Ntype does exist and T does have a member function
  63 whose signature is `void aMemberFunction(U::Ntype)` our 'value' is true,
  64 otherwise it is false. We will never get a compiler error in this case.
  65
  66 As a second example we will once again use the suppositions of our first
  67 example; given known types T and U, and the supposed type Ntype as a
  68 nested type of U. But this time let us look for a static member function
  69 whose signature is `void aStaticMemberFunction(U::Ntype)`.
  70
  71 First using `BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION` using our composite form we would code:
  72
  73  #include <boost/tti/has_static_member_function.hpp>
  74
  75  BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION(aStaticMemberFunction)
  76
  77  has_static_member_function_aStaticMemberFunction<T,void (U::Ntype)>::value;
  78
  79 Once again if the nested type U::Ntype does not exist, this leads to a compiler error,
  80 so let's try our alternative.
  81
  82 Second using `BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION` using our specific form we would code:
  83
  84  #include <boost/tti/member_type.hpp>
  85  #include <boost/tti/has_static_member_function.hpp>
  86
  87  BOOST_TTI_HAS_MEMBER_TYPE(Ntype)
  88  BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION(aStaticMemberFunction)
  89
  90  typedef typename has_member_type_Ntype<U>::type OurType;
  91  has_static_member_function_aStaticMemberFunction<T,void,boost::mpl::vector<OurType> >::value;
  92
  93 If the nested type U::Ntype does exist and T does have a member function
  94 whose signature is `void aMemberFunction(U::Ntype)` our 'value' is true,
  95 otherwise it is false. We will never get a compiler error in this case.
  96
  97 [endsect]