TypeList

main.cpp

#include <iostream>
#include <limits>
#include <type_traits>

/**
 * @brief Keep list of types provided by variadic template arguments.
 * Provide constexpr function to get index of specific type in the list.
 */

template <typename... Ts> class TypeList {
private:
  static constexpr auto no_index = std::numeric_limits<size_t>::max();

  template <typename T> static constexpr size_t getIndexImpl() {
    auto seq = {same<T, Ts>()...};
    size_t i = 0u;
    for (auto s : seq) {
      if (s) {
        return i;
      }
      i++;
    }
    return no_index;
  }
  template <typename U, typename V> static constexpr bool same() {
    return std::is_same<U, V>::value;
  }

public:
  template <typename T> static constexpr size_t getIndex() {
    constexpr size_t index = getIndexImpl<T>();
    static_assert(index != no_index, "Type T not found in TypeList");
    return index;
  }
  template <typename U> static constexpr size_t getIndex(U object) {
    using type = std::remove_cv_t<std::remove_pointer_t<U>>;
    return getIndexImpl<type>();
  }
};

class C1;
class C2;
using ClassList = TypeList<C1, C2>;

// Sample classes
class Base {
public:
  virtual int id() const = 0;
  virtual ~Base() {}
};

class C1 : public Base {
public:
  int id() const override {
    return static_cast<int>(ClassList::getIndex(this));
  }
};

class C2 : public Base {
public:
  int id() const override {
    return static_cast<int>(ClassList::getIndex(this));
  }
};

static_assert(ClassList::getIndex<C1>() == 0, "Test failed");
static_assert(ClassList::getIndex<C2>() == 1, "Test failed");

size_t test() { return ClassList::getIndex<C2>(); }

int main() {
  std::cout << "Hello World!" << std::endl;

  // Must raise compile error when uncommented:
  // std::cout << ClassList::getIndex<Base>() << std::endl;

  std::cout << ClassList::getIndex<C1>() << std::endl;
  std::cout << ClassList::getIndex<C2>() << std::endl;

  Base *b;
  b = new C1();
  std::cout << b->id() << std::endl;
  b = new C2();
  std::cout << b->id() << std::endl;

  return 0;
}