sfinae - Check for existence of global operator<< in C++ -

hello want write 2 implementations of to_string member function follows:

template <typename t0> class foo { public: std::string     to_string(); public: t0              m_value; };  template <typename t0> std::string foo<t0>::to_string() { std::stringstream ss; ss << m_value; return ss.str(); }  template <typename t0> std::string foo<t0>::to_string() { return typeid(t0).name(); } 

i have seen this, don't know how use code, i'm not used enable_if , boost mpl @ all. how should define 2 to_string functions use second fallback?

thanks.

my take on this: can take metafunction found is, works nicely. let's still discuss briefly why working:

sizeof not evaluate expression; deduces type , returns size of type. type sizes implementation defined , cannot assume them, know sizeof(char) != sizeof(char[2]), use these types test for.

we define stream operator @ namespace level using any_t type, accept - guessed - type , let return (it's not important type, long it's not ostream &). fall if type not have stream operator defined. in class define 2 functions, 1 taking ostream &, result if stream operator defined, , 1 taking return type defined our fallback function.

we can test sizeof(test(s << c)) which, again, not evaluate expression, determine return type , return size.

now understand how works, there's left embed our application. there several approaches this; 1 way, works prior c++11 use functor:

template <bool, typename t> struct to_string_functor {     std::string operator()(t const & t) const     {         std::stringstream ss;         ss << t;         return ss.str();     } };  template <typename t> struct to_string_functor<false, t> {     std::string operator()(t const &) const     {         return typeid(t).name();     } };  template <typename t> struct foo {     std::string to_string() const     {         return to_string_functor<             has_insertion_operator<t>::value, t         >()(m_value);     }     /* ... */ }; 

there more ways this, 1 being enable_if, if c++11 available (you want partially specialized functions this); may want read this excellent blog post on matter.

in simple case however, functor should in opinion.