confusion between std::[tr1::]ref and boost::ref

Question

Beware: This is GCC 4.1.2. We're on a proprietary embedded platform. We cannot update to a new compiler. So C++03 + TR1 it is.

We somewhere have a function like this:

template<typename T>
void foo(const boost::any& x)
{
  bar(boost::any_cast<T>(x));
}

which is then later used in a bind expression:

std::tr1::bind( &foo<T>, _1);

This generates the following error:

error: call of overloaded 'ref(const boost::any&)' is ambiguous
note: candidates are: std::tr1::reference_wrapper<_Tp> std::tr1::ref(_Tp&) [with _Tp = const boost::any]
note: const boost::reference_wrapper boost::ref(T&) [with T = const boost::any]

I do understand that this is Koenig lookup hitting us. However, I lack ideas about how to circumvent this problem.

Anyone out there?

Archibald

Is the lack of ref in your code intentional? – Grosgrain

@Simple: Where do you see a lacking ref? – Archibald

Your two code samples don't have a call to ref, whereas the error does. I was asking whether that is intentional or not. By your mention of ADL I wasn't sure. – Grosgrain

@Simple: It's deep down in the implementation of std::tr1::bind that an unqualified call to ref() is made with a boost::any as a parameter. Koenig lookup seems to make boost::ref() just as good a match as std::tr1::ref() for this call. Hence the error. Anyway, Dietmar's nice idea employing a special overload solved this. – Archibald

@sbi: it seems odd that the implementation seems to use an unqualified ref(). If it uses ref() it should really qualify it. – Bourke

@Dietmar: Yes, it does call it unqualifiedly. Note that we're stuck with GCC4.1, though, which is quite old. I suppose this is fixed in newer versions. – Archibald

Answer 1

Define a version of boost::ref() specifically taking a boost::any and have it return the proper type. Probably

namespace boost {
    std::tr1::reference_wrapper<boost::any const>
    ref(boost::any const& o) {
        return std::tr1::ref(o);
    }
}

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