ceph/src/boost/boost/convert/detail/has_member.hpp

   1 // Copyright (c) 2009-2016 Vladimir Batov.
   2 // Use, modification and distribution are subject to the Boost Software License,
   3 // Version 1.0. See http://www.boost.org/LICENSE_1_0.txt.
   4
   5 #ifndef BOOST_CONVERT_HAS_MEMBER_HPP
   6 #define BOOST_CONVERT_HAS_MEMBER_HPP
   7
   8 #include <boost/type_traits/detail/yes_no_type.hpp>
   9
  10 // This macro allows to check if a type has a member named "__member_name__"...
  11 // ... regardless of the signature. If takes advantage of the following behavior related to
  12 // function resolution. Say, both, foo and base, declare a method with the same name "func":
  13 //
  14 //      struct   foo { int  func (int, int) { return 0; } };
  15 //      struct  base { void func () {} };
  16 //      struct mixin : public foo, public base {};
  17 //
  18 // Now, if we inherit from both -- foo and base -- classes, then the following calls will fail
  19 //      mixin_ptr(0)->func();
  20 //      mixin_ptr(0)->func(5, 5);
  21 // with the error message (gcc): request for member func is ambiguous
  22 // regardless if we provide any arguments or not even though one might expect that
  23 // arg-based signature resolution might kick in. The only way to deploy those methods is:
  24 //
  25 //      mixin_ptr(0)->foo::func();
  26 //      mixin_ptr(0)->base::func(5, 5);
  27 //
  28 // C2. The actual signature of __member_name__ is not taken into account. If
  29 //     __T__::__member_name__(any-signature) exists, then the introduced base::__member_name__
  30 //     will cause mixin->__member_name__() call to fail to compile (due to ambiguity).
  31 // C3. &U::__member_name__ (a.k.a. &mixin::__member_name__)
  32 //     has the type of func_type only if __T__::__member_name__ does not exist.
  33 //     If __T__::member_name does exist, then mixin::__member_name__ is ambiguous
  34 //     and "yes_type test (...)" kicks in instead.
  35 // C4. Need to find some unique/ugly name so that it does not clash if this macro is
  36 //     used inside some other template class;
  37
  38 #define BOOST_DECLARE_HAS_MEMBER(__trait_name__, __member_name__)                           \
  39                                                                                             \
  40     template <typename __boost_has_member_T__> /*C4*/                                       \
  41     class __trait_name__                                                                    \
  42     {                                                                                       \
  43         typedef typename boost::remove_const<__boost_has_member_T__>::type check_type;      \
  44         typedef ::boost::type_traits::yes_type                yes_type;                     \
  45         typedef ::boost::type_traits:: no_type                 no_type;                     \
  46                                                                                             \
  47         struct  base { void __member_name__(/*C2*/) {}};                                    \
  48         struct mixin : public base, public check_type {};                                   \
  49                                                                                             \
  50         template <void (base::*)()> struct aux {};                                          \
  51                                                                                             \
  52         template <typename U> static no_type  test(aux<&U::__member_name__>*); /*C3*/       \
  53         template <typename U> static yes_type test(...);                                    \
  54                                                                                             \
  55         public:                                                                             \
  56                                                                                             \
  57         BOOST_STATIC_CONSTANT(bool, value = (sizeof(yes_type) == sizeof(test<mixin>(0))));  \
  58     }
  59
  60 #endif // BOOST_CONVERT_HAS_MEMBER_HPP