Re: [Boost-users] Bug in enable_if?

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2008-05-06 08:26:50

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.
>
>
>