c++ - Determine if a type is an STL container at compile time

Question

I would like to write a template that will determine if a type is an stl container at compile time.  

I've got the following bit of code:

struct is_cont{};
struct not_cont{};

template <typename T>
struct is_cont { typedef not_cont result_t; };

but I'm not sure how to create the necessary specializations for std::vector<T,Alloc>, deque<T,Alloc>, set<T,Alloc,Comp> etc...

Answer 1

Note: the following code is taken from an excellent utility called pretty-print written by @Kerrek SB (a topic on it at stackoverflow).

Disclaimer : I don't know if I'm allowed to copy and paste this code here without taking permission from the original author. @Kerrek, let me know if you've any issue. :-)

---

You can use this classs template:

  template<typename T> 
  struct is_container : std::integral_constant<bool, has_const_iterator<T>::value && has_begin_end<T>::beg_value && has_begin_end<T>::end_value> 
  { };

Usage:

 std::cout << is_container<std::vector<int>>::value << std::endl; //true
 std::cout << is_container<std::list<int>>::value << std::endl;   //true 
 std::cout << is_container<std::map<int>>::value << std::endl;    //true
 std::cout << is_container<std::set<int>>::value << std::endl;    //true
 std::cout << is_container<int>::value << std::endl;              //false

Note that is_container needs following helper class templates:

template<typename T>
struct has_const_iterator
{
private:
    typedef char                      yes;
    typedef struct { char array[2]; } no;

    template<typename C> static yes test(typename C::const_iterator*);
    template<typename C> static no  test(...);
public:
    static const bool value = sizeof(test<T>(0)) == sizeof(yes);
    typedef T type;
};

template <typename T>
struct has_begin_end
{
    template<typename C> static char (&f(typename std::enable_if<
      std::is_same<decltype(static_cast<typename C::const_iterator (C::*)() const>(&C::begin)),
      typename C::const_iterator(C::*)() const>::value, void>::type*))[1];

    template<typename C> static char (&f(...))[2];

    template<typename C> static char (&g(typename std::enable_if<
      std::is_same<decltype(static_cast<typename C::const_iterator (C::*)() const>(&C::end)),
      typename C::const_iterator(C::*)() const>::value, void>::type*))[1];

    template<typename C> static char (&g(...))[2];

    static bool const beg_value = sizeof(f<T>(0)) == 1;
    static bool const end_value = sizeof(g<T>(0)) == 1;
};