value_of.cpp

#include <iostream>
#include <memory>
#include <vector>
#include <algorithm>
#include <type_traits>

template< class ... > using void_t = void;

template< class T , class = void  >
struct is_dereferenceable  : std::false_type
{ };

template< class T  >
struct is_dereferenceable<T, void_t<decltype( &std::remove_reference_t<T>::operator* )>> : std::true_type
{
  typedef std::result_of_t<decltype(&std::remove_reference_t<T>::operator*)(T)> result;
};

template< class T >
struct is_dereferenceable<T, 
     std::enable_if_t<
       !std::is_void<std::remove_pointer_t<T>>::value 
       && std::is_pointer<std::remove_reference_t<T>>::value
     >
   > : std::true_type
{
  typedef std::add_lvalue_reference_t<std::remove_pointer_t<std::remove_reference_t<T>>> result;
};




template< class T >
constexpr bool is_dereferenceable_v = is_dereferenceable<T>::value;

template< class T , class = void  >
struct has_arrow_operator  : std::false_type
{ };

template< class T  >
struct has_arrow_operator<T, void_t<decltype( &std::decay_t<T>::operator-> )>> : std::true_type
{
  typedef typename std::result_of_t<decltype(&std::decay_t<T>::operator->)(T)> result;
};

template< class T >
struct has_arrow_operator<T, std::enable_if_t<std::is_pointer<std::remove_reference_t<T>>::value>> : std::true_type
{
  typedef std::remove_reference_t<T> result;
};

template< class T >
constexpr bool has_arrow_operator_v = has_arrow_operator<T>::value;

template< class T , class = void >
struct is_model_of_pointer : std::false_type
{ };

template< class T >
struct is_model_of_pointer<T, 
  std::enable_if_t<
    is_dereferenceable<T>()
    && has_arrow_operator<T>()
    && std::is_same<
          std::remove_pointer_t<typename has_arrow_operator<T>::result>,
          std::remove_reference_t<typename is_dereferenceable<T>::result>
       >::value
    >
  > : std::true_type
{ };

template< class T >
constexpr bool is_model_of_pointer_v = is_model_of_pointer<T>::value;


template<typename V>
auto value_of(V &&v) -> std::enable_if_t<is_model_of_pointer_v<V>, decltype(*v)> {
  return *v;
}

template<typename V>
auto value_of(V &&v) -> std::enable_if_t<!is_model_of_pointer<V>::value, V> {
  return std::forward<V>(v);
}


int main() {
  static_assert(is_dereferenceable<std::vector<int>::iterator>(), "iter");
  static_assert(is_dereferenceable<std::vector<int>::const_iterator>(), "const_iter");
  static_assert(is_dereferenceable<int*>(), "ptr");
  static_assert(!is_dereferenceable<int>(), "value");
  static_assert(is_dereferenceable<std::shared_ptr<int>>(), "shared_ptr");
  static_assert(is_dereferenceable<std::unique_ptr<int>>(), "unique_ptr");
  static_assert(!is_dereferenceable<void *>(), "void *");
  static_assert(!is_dereferenceable<const void *>(), "const void *");

  const std::array<int, 3> a{1,2,3};
  const std::vector<int> v1{4,5,6};
  const std::vector<std::shared_ptr<int>> v2{std::make_shared<int>(7),std::make_shared<int>(8),std::make_shared<int>(9)};
  const std::vector<int *> v3{new int(10), new int(11), new int(12)};
  const std::vector<std::unique_ptr<int>> v4 =
    [](){
      std::vector<std::unique_ptr<int>> v;
      v.emplace_back(std::make_unique<int>(13));
      v.emplace_back(std::make_unique<int>(14));
      v.emplace_back(std::make_unique<int>(15));
      return v;
    }();

  const std::vector<std::vector<int>::const_iterator> v5{v1.begin()+2, v1.begin()+1, v1.begin()};

  const auto printer = [](const auto &v){ std::cout << value_of(v) << '\n'; };
  const auto print_all = [printer](const auto &c) { std::for_each(std::begin(c), std::end(c), printer); };

  print_all(a);
  print_all(v1);
  print_all(v2);
  print_all(v3);
  print_all(v4);
  print_all(v5);

}