[boost] ccref

From: alnsn_at_[hidden]
Date: 2004-06-29 16:13:12

This is not a typo, I'm trying to emphasize that ref, cref, ... sequence
lacks one element which imposes more strict requirements on its argument.
According to IQ tests logic that sequence should be continued with ccref :)
If seriously, cref allows silent binding of a temporary to a reference.
Apparently, this is not always fine.
Inspired by N1610 (thanks to Dave and Gary) I've tried to detect both
const and non-const rvalues. I didn't yet opened my copy of the standard
to check it but the code seems to work on g++ 3.2.2, g++ 3.4.0, Intel 8.0
(all under linux) and Comeau online.

#include <boost/ref.hpp>

template<class T1, class T2>
struct enable_if_same
{
};

template<class T>
struct enable_if_same<T,T>
{
    typedef int type;
};

template<class T>
boost::reference_wrapper<T const> const ccref(T volatile& r,
                typename enable_if_same<T,T const>::type = 0)
{
    return boost::reference_wrapper<T const>(const_cast<T const&>(r));
}

struct convertible_to_int
{
    operator int() const { return 0; }
};

template<class T>
boost::reference_wrapper<T const> const ccref(T const volatile& r)
{
    return boost::reference_wrapper<T const>(const_cast<T const&>(r));
}

int rvalue() { return 0; }
int const const_rvalue() { return 0; }

int main()
{
    int i;
    int const j = 0;

    ccref(i);
    ccref(j);

// Try uncommenting any of these lines:
//    ccref(rvalue());
//    ccref(const_rvalue());
}

-- 
Alexander Nasonov