问题
Lets say I have classes Date and classes Year, Month and Day.
struct Date {
Date(Year year, Month month, Day day) : d(day), m(month), y(year) {};
Date(Month month, Day day, Year year) : d(day), m(month), y(year) {};
Date(Day day, Month month, Year year) : d(day), m(month), y(year) {};
Date(Day day, Year year, Month month) : d(day), m(month), y(year) {};
...
...
private:
Day d;
Month m;
Year y;
}
This allows me not to have a specific layout of arguments for Date as I have a lot of overloadings.
Am I able to generate all the permutations/overloadings automatically?
Just to be clear:
- Permutations are only of argument layout, nothing about them should change as I know that would not be possible to automate.
- All the generated overloadings should have the same code as only the layout of arguments changes not the logic itself.
回答1:
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
回答2:
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.
来源：https://stackoverflow.com/questions/30561407/c-how-to-generate-all-the-permutations-of-function-overloads