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@gmail.com> 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.