How to define a class that can save variadic template arguments?

Question

template<typename... Args>
struct A
{
    Args&... args_;
    //
    // error : only function and template parameters can be parameter packs
    //

    A(Args&... args) : args_(args)
    {}
};

int main()
{
    auto a = A(1, 2, 3);
}

My compiler is clang 5.0 with -std=c++1z.

How to define a class that can save variadic template arguments in such a case?

Answer 1

That's not possible as far as I know. You have to use a std::tuple to store the parameters:

template<typename... Args>
struct A
{
    std::tuple<std::decay_t<Args>...> args_;

    A(Args&&... args) : args_(std::make_tuple(std::forward<Args>(args)...))
    {}
};

As of C++17, you can use std::apply to cal functions with args_ as parameters, instead of unpacking them:

std::apply(func, args_);