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Diffstat (limited to 'boost/convert/detail/has_member.hpp')
| -rw-r--r-- | boost/convert/detail/has_member.hpp | 61 |
1 files changed, 61 insertions, 0 deletions
diff --git a/boost/convert/detail/has_member.hpp b/boost/convert/detail/has_member.hpp new file mode 100644 index 0000000000..fafc39f5c4 --- /dev/null +++ b/boost/convert/detail/has_member.hpp @@ -0,0 +1,61 @@ +// Copyright (c) 2009-2014 Vladimir Batov. +// Use, modification and distribution are subject to the Boost Software License, +// Version 1.0. See http://www.boost.org/LICENSE_1_0.txt. + +#ifndef BOOST_CONVERT_HAS_MEMBER_HPP +#define BOOST_CONVERT_HAS_MEMBER_HPP + +#include <boost/type_traits/remove_const.hpp> +#include <boost/type_traits/detail/yes_no_type.hpp> + +// This macro allows to check if a type has a member named "__member_name__"... +// ... regardless of the signature. If takes advantage of the following behavior related to +// function resolution. Say, both, foo and base, declare a method with the same name "func": +// +// struct foo { int func (int, int) { return 0; } }; +// struct base { void func () {} }; +// struct mixin : public foo, public base {}; +// +// Now, if we inherit from both -- foo and base -- classes, then the following calls will fail +// mixin_ptr(0)->func(); +// mixin_ptr(0)->func(5, 5); +// with the error message (gcc): request for member func is ambiguous +// regardless if we provide any arguments or not even though one might expect that +// arg-based signature resolution might kick in. The only way to deploy those methods is: +// +// mixin_ptr(0)->foo::func(); +// mixin_ptr(0)->base::func(5, 5); +// +// C2. The actual signature of __member_name__ is not taken into account. If +// __T__::__member_name__(any-signature) exists, then the introduced base::__member_name__ +// will cause mixin->__member_name__() call to fail to compile (due to ambiguity). +// C3. &U::__member_name__ (a.k.a. &mixin::__member_name__) +// has the type of func_type only if __T__::__member_name__ does not exist. +// If __T__::member_name does exist, then mixin::__member_name__ is ambiguous +// and "yes_type test (...)" kicks in instead. +// C4. Need to find some unique/ugly name so that it does not clash if this macro is +// used inside some other template class; + +#define BOOST_DECLARE_HAS_MEMBER(__trait_name__, __member_name__) \ + \ + template <typename __boost_has_member_T__> /*C4*/ \ + class __trait_name__ \ + { \ + typedef typename ::boost::remove_const<__boost_has_member_T__>::type check_type; \ + typedef ::boost::type_traits::yes_type yes_type; \ + typedef ::boost::type_traits:: no_type no_type; \ + \ + struct base { void __member_name__(/*C2*/) {}}; \ + struct mixin : public base, public check_type {}; \ + \ + template <void (base::*)()> struct aux {}; \ + \ + template <typename U> static no_type test(aux<&U::__member_name__>*); /*C3*/ \ + template <typename U> static yes_type test(...); \ + \ + public: \ + \ + BOOST_STATIC_CONSTANT(bool, value = (sizeof(yes_type) == sizeof(test<mixin>(0)))); \ + } + +#endif // BOOST_CONVERT_HAS_MEMBER_HPP |