C++, pass two parameter packs to constructor - c++

Question

I have the following problem. I have a class (mixin), that has two template bases.
template <typename T>
class Id
{
using result = T;
};
template <typename Printer1, typename Printer2>
class SeveralPrinters : public Printer1, public Printer2
{
template <typename... Args1, typename... Args2>
SeveralPrinters(dummy, helper<Args1...>, helper<Args2...>,
typename Id<Args1>::result... args1,
typename Id<Args2>::result... args2)
: Printer1(std::forward<Args1>(args1)..., std::forward<Args2>(args2)...)
{}
public:
template <typename... Args, typename =
std::enable_if_t<!contains<dummy, Args...>::result>>
SeveralPrinters(Args&&... args)
: SeveralPrinters(dummy(), typename Printer1::ArgsCtor(),
typename Printer2::ArgsCtor(), std::forward<Args>(args)...)
{
}
};
All names of classes are fictitious. So, imagine its first base accepts int as constructor argument and second base accepts double. What I want to do is to be able to call constructor of SeveralPrinters like SeveralPrinters(1, 2.). The problem here, is that Args1 and Args2 are deduced not from helper structure, but from args, passed after helper structure. As you can see, I tried to wrap template arguments into Id structure, and that didn't help. I know, it is called smth like Non-deduced contexts, but I couldn't manage to make it work. Can anyone help with it (if it possible), and maybe explain a little bit more on this topic (why it doesn't work now).
Example of base classes:
class BasicPrinter1
{
public:
BasicPrinter1(int)
{}
void f()
{
}
using ArgsCtor = helper<int>;
};
class BasicPrinter2
{
public:
BasicPrinter2(int*)
{}
void g()
{
}
using ArgsCtor = helper<int*>;
};

Answer 1

It doesn't work mainly because the alias result in Id is private (default accessibility for classes), and so not accessible from the private constructor of SeveralPrinters, leading to a substitution failure (typename Id<Args1>::result) with no other viable candidate constructor to call. There were also a couple of typos in your code.
template <typename T>
struct Id
{
using result = T;
};
template <typename Printer1, typename Printer2>
class SeveralPrinters : public Printer1, public Printer2
{
template <typename... Args1, typename... Args2>
SeveralPrinters(dummy, helper<Args1...>, helper<Args2...>
, typename Id<Args1>::result... args1
, typename Id<Args2>::result... args2)
: Printer1(std::forward<Args1>(args1)...)
, Printer2(std::forward<Args2>(args2)...)
{}
public:
template <typename... Args>
SeveralPrinters(Args&&... args)
: SeveralPrinters(dummy{}
, typename Printer1::ArgsCtor{}
, typename Printer2::ArgsCtor{}
, std::forward<Args>(args)...)
{}
};
DEMO
In order to perfectly-forward the arguments to base classes, you should instead declare the number of parameters (ArgsCount) and use the below implementation:
template <typename Printer1, typename Printer2>
class SeveralPrinters : public Printer1, public Printer2
{
template <std::size_t... Is
, std::size_t... Js
, typename... Args>
SeveralPrinters(std::index_sequence<Is...>
, std::index_sequence<Js...>
, std::tuple<Args...>&& t)
: Printer1(std::get<Is>(std::move(t))...)
, Printer2(std::get<sizeof...(Is) + Js>(std::move(t))...)
{}
public:
SeveralPrinters() = default;
SeveralPrinters(const SeveralPrinters&) = default;
SeveralPrinters(SeveralPrinters& rhs)
: SeveralPrinters(static_cast<const SeveralPrinters&>(rhs))
{}
template <typename... Args>
SeveralPrinters(Args&&... args)
: SeveralPrinters(std::make_index_sequence<Printer1::ArgsCount>{}
, std::make_index_sequence<Printer2::ArgsCount>{}
, std::forward_as_tuple(std::forward<Args>(args)...))
{}
};
struct BasicPrinter1
{
BasicPrinter1(int) {}
static constexpr ArgsCount = 1;
};
struct BasicPrinter2
{
BasicPrinter2(int*, char&) {}
static constexpr ArgsCount = 2;
};
DEMO 2
Also notice how I'm protecting the copy-constructor from being overshadowed by the forwarding-references constructor.