[boost] disable_if conundrum

Subject: [boost] disable_if conundrum
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2011-12-05 14:27:48

I have a function that returns a pair of iterators.
There's also a version that takes a comparison predicate.

        template<class ForwardIterator, class Compare>
        std::pair<ForwardIterator, ForwardIterator>
        foo ( ForwardIterator first, ForwardIterator last );

        template<class ForwardIterator, class Compare>
        std::pair<ForwardIterator, ForwardIterator>
        foo ( ForwardIterator first, ForwardIterator last, Compare comp );

I want to provide a range-based version of it.

        template<class Range>
        std::pair<typename boost::range_iterator<const Range>::type, typename boost::range_iterator<const Range>::type>
        foo ( const Range &r );

        template<class Range, class Compare>
        std::pair<typename boost::range_iterator<const Range>::type, typename boost::range_iterator<const Range>::type>
        foo ( const Range &r, Compare comp );

Ok. There's a problem.
If I call:
        foo ( first, last )
I get an error, because there are two perfectly good two argument candidates.

Fine. Been there, seen that. I can use disable_if to make sure that the second range based version is only "active" when the arguments are different types. It's verbose, but it (usually) works

        template<class Range, class Compare>
        typename boost::disable_if_c<boost::is_same<Range, Compare>::value,
                std::pair<typename boost::range_iterator<const Range>::type, typename boost::range_iterator<const Range>::type>
		>::type
        foo ( const Range &r, Compare comp );

But this time it does not! I get a compile error telling me that the compiler can't deal with boost::range_iterator<XXX>::type, when XXX = some random iterator. It seems that the compiler wants to evaluate all the parameters of disable_if_c before deciding whether or not to SFINAE it (I guess that's not unreasonable, but it's not what I want)

Apparently, all the other times that I did this, the return type of the function was not a dependent type of the template arguments.

Any suggestions for a workaround here?

-- Marshall

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists_at_[hidden]>

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