From: David Abrahams (dave_at_[hidden])
Date: 2002-10-14 09:53:53
"Kevin S. Van Horn" <Kevin.VanHorn_at_[hidden]> writes:
> I've been looking over some of the type traits stuff, and I had a question
> about the implementation of is_enum. Here is the code in question:
> template <typename T> struct is_enum
> typedef ::boost::add_reference<T> ar_t;
> typedef typename ar_t::type r_type;
> BOOST_STATIC_CONSTANT(bool, selector =
> , ::boost::is_reference<T>::value
> // We MUST do this on conforming compilers in order to
> // correctly deduce that noncopyable types are not enums (dwa
> // 2002/04/15)...
> , ::boost::is_class<T>::value
> typedef ::boost::detail::is_enum_helper<selector> se_t;
> typedef typename se_t::template type<r_type> helper;
> BOOST_STATIC_CONSTANT(bool, value = helper::value);
> I'm quite mystified by the comment about ORing is_class<T>::value into
> selector. How is the fact that a class C is noncopyable going to make
> is_enum_helper<false>::type<r_type>::value true?
It doesn't help make anything true. It makes sure that the return
value is taken from
In the case where T is a class type. Otherwise,
gets instantiated, which is only legal if T is copyable.
> I tried to look up the mentioned message
Message? Mentioned where?
> in the mailing list archives, and it didn't shed any
-- David Abrahams * Boost Consulting dave_at_[hidden] * http://www.boost-consulting.com
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