Re: [Boost-users] Bug in enable_if?

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2008-05-06 08:31:55

Sorry the location below deleted '<' '>' characters which are essential for
templates. Take a look at this location:

http://lists.boost.org/boost-users/2007/11/31849.php

On Tue, May 6, 2008 at 2:26 PM, Ovanes Markarian <om_boost_at_[hidden]>
wrote:

> Hi!
>
> I had a similar question already. Please take a look at my post and answer
> from Dave Abrahams
>
> http://archive.netbsd.se/?ml=boost-users&a=2007-10&t=5555956
>
>
>
> With Kind Regards,
> Ovanes
>
>
>
> On Tue, May 6, 2008 at 1:23 PM, Germán Diago Gómez <germandiago_at_[hidden]>
> wrote:
>
> > Hello. I'm trying to compile this code, but it doesn't do the trick and
> > I don't know why.
> >
> > struct print
> > {
> >        typedef void result_type;
> >
> >        template <class T>
> >        void operator()(const T & elem) const
> >        {
> >                std::cout << elem << std::endl;
> >        }
> > };
> >
> >
> > template <class Tuple, class UnaryF, int i = 0>
> > struct tuple_for_each_helper
> > {
> > private:
> >        Tuple & t_;
> >        UnaryF f_;
> > public:
> >        typedef typename UnaryF::result_type result_type;
> >
> >        tuple_for_each_helper(Tuple & t, UnaryF f) : t_(t), f_(f)
> >        {
> >        }
> >
> >
> >        result_type operator()(typename
> > boost::enable_if<is_same<result_of<UnaryF (const Tuple &)>, void>
> > >::type * = 0)
> >        {
> >                f_(get<i>(t_));
> >                typename boost::mpl::if_c<i == tuple_size<Tuple>::value -
> > 1,
> >                        tuple_for_each_helper<Tuple, UnaryF, -1>,
> >                        tuple_for_each_helper<Tuple, UnaryF, i + 1>
> > >::type(t_, f_)();
> >        }
> > };
> >
> >
> > template <class Tuple, class UnaryF>
> > struct tuple_for_each_helper<Tuple, UnaryF, -1>
> > {
> >        typedef typename UnaryF::result_type result_type;
> >
> >        tuple_for_each_helper(Tuple & t, UnaryF f) {}
> >        result_type operator()() {}
> > };
> >
> >
> > template <class Tuple, class UnaryF>
> > typename
> > result_of<UnaryF (const Tuple &)>::type tuple_for_each (Tuple & t,
> > UnaryF f,
> > typename boost::enable_if<is_same<result_of<UnaryF (const Tuple &)>,
> > void> >::type * = 0)
> > {
> >    tuple_for_each_helper<Tuple, UnaryF>(t, f)();
> > }
> >
> > int main(int argc, char * argv[])
> > {
> >        tuple<int, float, string, string> t(3, 5.3, "hello", "goodbye");
> >        tuple_for_each(t, print());
> > }
> >
> > And the error is:
> >
> > tuple_each.cpp: In function 'int main(int, char**)':
> > tuple_each.cpp:99: error: no matching function for call to
> > 'tuple_for_each(std::tr1::tuple<int, float, std::basic_string<char,
> > std::char_traits<char>, std::allocator<char> >, std::basic_string<char,
> > std::char_traits<char>, std::allocator<char> >, std::tr1::_NullClass,
> > std::tr1::_NullClass, std::tr1::_NullClass, std::tr1::_NullClass,
> > std::tr1::_NullClass, std::tr1::_NullClass>&, print)'
> >
> > If I change boost::enable_if by boost::disable_if the code compiles. But
> > if I check typeid(typename print::result_type).name() and
> > typeid(result_of<UnaryF (const Tuple &)>::type).name() they are the
> > same. I'm using g++ 4.2.3 and boost 1.34.1.
> >
> > Thanks in advance.
> >
> >
> >
>