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Full SFINAE type trait for containers
#include <vector>
#include <string>
#include <utility>
#include <iostream>
template<typename T>
struct has_const_iterator
{
private:
typedef char one;
typedef struct { char array[2]; } two;
template<typename C> static one test(typename C::const_iterator*);
template<typename C> static two test(...);
public:
static const bool value = sizeof(test<T>(0)) == sizeof(one);
typedef T type;
};
template <typename T>
struct has_begin_end
{
struct Dummy { typedef void const_iterator; };
typedef typename std::conditional<has_const_iterator<T>::value, T, Dummy>::type TType;
typedef typename TType::const_iterator iter;
struct Fallback { iter begin() const; iter end() const; };
struct Derived : TType, Fallback { };
template<typename C, C> struct ChT;
template<typename C> static char (&f(ChT<iter (Fallback::*)() const, &C::begin>*))[1];
template<typename C> static char (&f(...))[2];
template<typename C> static char (&g(ChT<iter (Fallback::*)() const, &C::end>*))[1];
template<typename C> static char (&g(...))[2];
static bool const beg_value = sizeof(f<Derived>(0)) == 2;
static bool const end_value = sizeof(g<Derived>(0)) == 2;
};
template <typename T>
struct is_container
{
static const bool value = has_const_iterator<T>::value &&
has_begin_end<T>::beg_value && has_begin_end<T>::end_value;
};
struct Foo { typedef int const_iterator; };
struct Goo { typedef int const_iterator; const_iterator begin() const; };
typedef std::pair<int, int> PT;
int main()
{
std::cout << is_container<int>::value << std::endl
<< is_container<Foo>::value << std::endl
<< is_container<Goo>::value << std::endl
<< is_container<PT>::value << std::endl
<< is_container<std::vector<int>>::value << std::endl
<< is_container<std::string>::value << std::endl
;
}
@louisdx
Owner
louisdx commented Jul 11, 2011

To be extremely pedantic on Line 24, I could have said:

typedef typename std::conditional<std::is_class<T>::value && has_const_iterator<T>::value, T, Dummy>::type TType;

However, I assume that anything that passes has_const_iterator must already be of class type, so I believe that's superfluous.

Also, why isn't static bool evaluation short-circuited?

@sytelus
sytelus commented Jun 5, 2016

The prettyprint library has much more extensive definitions. I've isolated its code so these traits are in just one include file. See my blog post: http://shitalshah.com/p/writing-generic-container-function-in-c11/

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