Determine if a type is an STL container at compile time
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...
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;
};
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) { ... }