Re: Type traits and accessibility

From:
=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Tue, 13 Apr 2010 15:57:35 CST
Message-ID:
<b1fc6c65-f1dc-4d5d-927f-c0d08a5be045@r1g2000yqb.googlegroups.com>
On 13 Apr., 18:41, Nikolay Ivchenkov <ts...@mail.ru> wrote:

According to N3092 - 11/5,
"The interpretation of a given construct is established without regard
to access control. If the interpretation established makes use of
inaccessible member names or base classes, the construct is ill-
formed".


This is the general core language rule and applies to
everything which is does not say otherwise.

Does it mean that the following program is ill-formed?

     #include <type_traits>

     class X
     {
     public:
         X() {}
     private:
         X(int) {}
     };

     int main()
     {
         int bool value =
             std::is_constructible<X, int>::value;
     }


This is well-formed, because std::is_constructible
needs compiler-support (same as is_convertible
does) to realize it's specification, see
[meta.unary.prop]/6:

"Given the following function prototype:

template <class T>
typename add_rvalue_reference<T>::type create();

the predicate condition for a template specialization
is_constructible<T, Args...> shall be satisfied if and
only if the following expression CE would be well-formed:

? if sizeof...(Args) == 1, the expression:
     static_cast<T>(create<Args>()...)
? otherwise, the expression:
     T(create<Args>()...)"

Note the usage of "would be well-formed". For your
example the effective test-expression is

static_cast<X>(create<int>())

This expression *would* be ill-formed, because
attempting to evaluate it inside main would
stumble across the lack of access here. The result
is, that the program is well-formed and that
"value" will evaluate to false.

It may well be that your current compiler declares
the program as ill-formed because it just simulates
the semantics via "normal" code. But this is not
a compliant implementation.

HTH & Greetings from Bremen,

Daniel Kr?gler

P.S.: Note that there is a open library issue in regard
to is_constructible, but it does not affect your example:

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#1260

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