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62 lines
3.8 KiB
C++
62 lines
3.8 KiB
C++
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// Copyright (c) 2009-2016 Vladimir Batov.
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// Use, modification and distribution are subject to the Boost Software License,
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// Version 1.0. See http://www.boost.org/LICENSE_1_0.txt.
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#ifndef BOOST_CONVERT_HAS_MEMBER_HPP
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#define BOOST_CONVERT_HAS_MEMBER_HPP
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#include <boost/type_traits/remove_const.hpp>
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#include <boost/type_traits/detail/yes_no_type.hpp>
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// This macro allows to check if a type has a member named "__member_name__"...
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// ... regardless of the signature. If takes advantage of the following behavior related to
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// function resolution. Say, both, foo and base, declare a method with the same name "func":
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//
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// struct foo { int func (int, int) { return 0; } };
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// struct base { void func () {} };
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// struct mixin : public foo, public base {};
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//
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// Now, if we inherit from both -- foo and base -- classes, then the following calls will fail
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// mixin_ptr(0)->func();
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// mixin_ptr(0)->func(5, 5);
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// with the error message (gcc): request for member func is ambiguous
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// regardless if we provide any arguments or not even though one might expect that
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// arg-based signature resolution might kick in. The only way to deploy those methods is:
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//
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// mixin_ptr(0)->foo::func();
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// mixin_ptr(0)->base::func(5, 5);
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//
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// C2. The actual signature of __member_name__ is not taken into account. If
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// __T__::__member_name__(any-signature) exists, then the introduced base::__member_name__
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// will cause mixin->__member_name__() call to fail to compile (due to ambiguity).
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// C3. &U::__member_name__ (a.k.a. &mixin::__member_name__)
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// has the type of func_type only if __T__::__member_name__ does not exist.
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// If __T__::member_name does exist, then mixin::__member_name__ is ambiguous
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// and "yes_type test (...)" kicks in instead.
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// C4. Need to find some unique/ugly name so that it does not clash if this macro is
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// used inside some other template class;
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#define BOOST_DECLARE_HAS_MEMBER(__trait_name__, __member_name__) \
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\
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template <typename __boost_has_member_T__> /*C4*/ \
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class __trait_name__ \
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{ \
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typedef typename ::boost::remove_const<__boost_has_member_T__>::type check_type; \
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typedef ::boost::type_traits::yes_type yes_type; \
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typedef ::boost::type_traits:: no_type no_type; \
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\
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struct base { void __member_name__(/*C2*/) {}}; \
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struct mixin : public base, public check_type {}; \
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\
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template <void (base::*)()> struct aux {}; \
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\
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template <typename U> static no_type test(aux<&U::__member_name__>*); /*C3*/ \
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template <typename U> static yes_type test(...); \
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\
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public: \
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\
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BOOST_STATIC_CONSTANT(bool, value = (sizeof(yes_type) == sizeof(test<mixin>(0)))); \
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}
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#endif // BOOST_CONVERT_HAS_MEMBER_HPP
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