c++ - How to check if a template parameter is an iterator type or not? -

template<class t> struct is_iterator {     static const bool value = ??? // write ??? };  int main() {     assert(false == is_iterator<int>::value);     assert(true == is_iterator<vector<int>::iterator>::value);     assert(true == is_iterator<list<int>::iterator>::value);     assert(true == is_iterator<string::iterator>::value);     assert(true == is_iterator<char*>::value); // raw pointer iterator } 

the question is: how make 5 assert statements pass?

template<class t> struct is_iterator {        static t maket();     typedef void * twoptrs[2];  // sizeof(twoptrs) > sizeof(void *)     static twoptrs & test(...); // common case     template<class r> static typename r::iterator_category * test(r); // iterator     template<class r> static void * test(r *); // pointer      static const bool value = sizeof(test(maket())) == sizeof(void *);  };