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. :-)

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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; }; 

Answer 2

First, you define your primary template, which will have a member which is false in the default case:

template <typename T> struct is_cont {   static const bool value = false; }; 

Then you will define partial specializations for your container types which have a value of true instead:

template <typename T,typename Alloc> struct is_cont<std::vector<T,Alloc> > {   static const bool value = true; }; 

Then for a type X that you want to check, use it like

if (is_cont<X>::value) { ... }