C++ how to generate all the permutations of function overloads?

Question

Lets say I have classes Date and classes Year, Month and Day.

struct Date {
  Date(Year year, Month month         


        

Answer 1

In C++14, take 3 generic arguments, forward them to a tuple, forward that tuple to a new constructor (possibly with a tag type to aid dispatch), and use the type-based std::get to exctract each type. Forward that to yet another constructor, with a tag to aid in dispatchimg.

SFINAE checks to provide for early failure optional.

struct Date {
private:
  struct as_tuple{};
  struct in_order{}; 
public:
  template<class A,class B,class C,
    // SFINAE test based on type_index below:
    class=decltype(
      type_index<Year,A,B,C>{}+type_index<Month,A,B,C>{}+type_index<Day,A,B,C>{}
    )
  >
  Date(A a,B b,C c):
    Date(as_tuple{},
      std::make_tuple(std::move(a),std::move(b),std::move(c))
    )
  {}
private:
  template<class...Ts>
  Date(as_tuple, std::tuple<Ts...> t):
    Date(in_order{},
      std::get<Year>(t),std::get<Month>(t),std::get<Day>(t)
    )
  {}
  Date(in_order,Year y_,Month m_,Day d_):
    y(y_),m(m_),d(d_)
  {}
};

In C++11, you can implement your own equivalent of std::get<T>.

SFINAE checks that y/m/d are all present are harder, but maybe not needed.

Optimization (adding move/perfect forwarding) is another improvement that may not be needed if your y/m/d types are simple enough.

The technique of forwarding constructors and tags is based on the idea of doing one thing at a time, instead of all at once. The code is going to be already strange enough.

Implementing your own std::get<T> is easy. Making it SFINAE friendly a bit harder:

 // helpers to keep code clean:
 template<std::size_t n>
 using size=std::integral_constant<std::size_t, n>;
 template<class T>struct tag{using type=T;};

 template<class T, class...Ts>
 struct type_index_t{}; // SFINAE failure

 // client code uses this.  Everything else can go in namespace details:
 template<class T, class...Ts>
 using type_index = typename type_index_t<T,Ts...>::type;

 // found a match!
 template<class T, class...Ts>
 struct type_index_t<T, T, Ts...>:
   tag<size<0>>
 {};
 template<class T, class T0, class...Ts>
 struct type_index_t<T, T0, Ts...>:
   tag<size<type_index<T,Ts...>::value+1>>
 {};

// SFINAE (hopefully) std::get<T>:
template<class T, class...Ts>
auto my_get( std::tuple<Ts...>& tup )
-> decltype( std::get< type_index<T,Ts...>::value >(tup) ) {
  return std::get< type_index<T,Ts...>::value >(tup);
}
template<class T, class...Ts>
auto my_get( std::tuple<Ts...> const& tup )
-> decltype( std::get< type_index<T,Ts...>::value >(tup) ) {
  return std::get< type_index<T,Ts...>::value >(tup);
}
template<class T, class...Ts>
auto my_get( std::tuple<Ts...>&& tup )
-> decltype( std::get< type_index<T,Ts...>::value >(std::move(tup)) ) {
  return std::get< type_index<T,Ts...>::value >(std::move(tup));
}

but that is just an untested sketch. Looking at the proposals for C++14 std::get<Type> is probably a better idea.

Answer 2

With C++14, you may do:

struct Date {
public:
    Date(const Year& year, const Month& month, const Day& day) :
        d(day), m(month), y(year)
    {}

    template <typename T1, typename T2, typename T3>
    Date(const T1& t1, const T2& t2, const T3& t3) : 
        Date(std::get<Year>(std::tie(t1, t2, t3)),
             std::get<Month>(std::tie(t1, t2, t3)),
             std::get<Day>(std::tie(t1, t2, t3)))
    {}

private:
    Day d;
    Month m;
    Year y;
};

Edit: if you would also allow default argument, you may do something like:

namespace detail
{
    template <typename T, typename... Ts> struct has_T;

    template <typename T> struct has_T<T> : std::false_type {};

    template <typename T, typename... Ts> struct has_T<T, T, Ts...>
    : std::true_type {};

    template <typename T, typename Tail, typename... Ts>
    struct has_T<T, Tail, Ts...> : has_T<T, Ts...> {};

    template <typename T, typename... Ts>
    const T& get_or_default_impl(std::true_type,
                                 const std::tuple<Ts...>& t,
                                 const T&)
    {
        return std::get<T>(t);
    }

    template <typename T, typename... Ts>
    const T& get_or_default_impl(std::false_type,
                                 const std::tuple<Ts...>&,
                                 const T& default_value)
    {
        return default_value;
    }

    template <typename T1, typename T2> struct is_included;

    template <typename... Ts>
    struct is_included<std::tuple<>, std::tuple<Ts...>> : std::true_type {};

    template <typename T, typename... Ts, typename ... Ts2>
    struct is_included<std::tuple<T, Ts...>, std::tuple<Ts2...>> :
        std::conditional_t<has_T<T, Ts2...>::value,
                          is_included<std::tuple<Ts...>, std::tuple<Ts2...>>,
                          std::false_type> {};

}

template <typename T, typename... Ts>
const T& get_or_default(const std::tuple<Ts...>& t,
                        const T& default_value = T{})
{
    return detail::get_or_default_impl<T>(detail::has_T<T, Ts...>{}, t, default_value);
}

And then

struct Date {
public:
    Date(const Year& year, const Month& month, const Day& day) :
        d(day), m(month), y(year)
    {}

    template <typename ... Ts,
              typename std::enable_if_t<
                  detail::is_included<std::tuple<Ts...>,
                  std::tuple<Year, Month, Day>>::value>* = nullptr>
    Date(const Ts&... ts) :
        Date(get_or_default<const Year&>(std::tie(ts...)),
             get_or_default<const Month&>(std::tie(ts...)),
             get_or_default<const Day&>(std::tie(ts...)))
    {}

private:
    Day d;
    Month m;
    Year y;
};

Live Demo
Live Demo with invalid constructor call