Re: Detecting a static member variable

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Sun, 7 Mar 2010 15:51:30 CST

Message-ID:

<a6e52de3-bbc2-476f-b37c-7a7b421a6d46@q23g2000yqd.googlegroups.com>

On 7 Mrz., 15:50, usenet only_tech_talk <usenet.tech.t...@gmail.com>
wrote:

I tested the code below on a boost::array<> which defines
   static const size_type static_size = N;
and it fails. Seehttp://www.boost.org/doc/libs/1_42_0/doc/html/boost/array.html

By comparison, the commented out code for size(), not static_size
works. I use OSX 10.6, gcc 4.2.

Any suggestion to solve this would be appreciated. Thanks.

template<typename T>
struct has_static_size{
        typedef typename T::size_type size_type;
        typedef char yes;
        typedef char (&no)[2];
        typedef const size_type* sig;
// typedef size_type (const T::*sig)();

        template<typename U,sig>
        struct sfinae { };

        template<typename U> static yes test(sfinae<U,
&U::static_size>*);
        //template<typename U> static yes test(sfinae<U,&U::size> *);
        template<typename U> static no test(...);

        BOOST_STATIC_CONSTANT(
                 bool,
                value = sizeof( test<T>(0) ) == sizeof(yes)
         );

         typedef boost::mpl::bool_<value> type;
    };

This is a particular problem in the concrete
definition of boost::array's static_size as

enum { static_size = N };

and you take the address of an enumerator or any
other literal (Your test code only works for
lvalues like static const data member static_size).

Your problem is easy to fix: Just replace your
typedef "sig" by

typedef const size_type sig;

and your positive test overload by

template<typename U> static yes
test(sfinae<U, U::static_size>*);

HTH & Greetings from Bremen,

Daniel Kr?gler

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