Different variadic template expansion

Question

I want to prepare list of pairs of values from structure using variadic templates.

#include <vector>

struct foo
{
    foo(int a, int b)
        : a(a), b(b) {}
    int a;
    int b;
};

struct Msg
{
    std::vector<int> valueArray;    
};

template<typename... Args>
Msg func(Args... args)
{
    Msg msg;
    msg.valueArray = { sizeof...(args), (args.a)..., (args.b)... };
    return msg;
}

int main() {
    Msg msg = func(foo{1,2}, foo{3,4}, foo{5,6});
}

Msg which func will return will have valueArray = [3, 1, 3, 5, 2, 4, 6].

Is there any simple way to expand variadic paremeters in a way, where valueArray will look like valueArray = [3, 1, 2, 3, 4, 5, 6]?

Answer 1

The following is not as generic as I'd like to, but maybe it is sufficient for you:

template<typename Arr, std::size_t... Is>
Msg func2( const Arr& arr, std::index_sequence<Is...> )
{
    Msg msg;
    msg.valueArray = {
        sizeof...(Is) / 2,
        ( ( Is % 2 == 0 ) ? std::get< Is / 2 >( arr ).a
                          : std::get< Is / 2 >( arr ).b )... };
    return msg;
}

template<typename... Args>
Msg func(Args... args)
{
    return func2( std::forward_as_tuple( args... ),
                  std::make_index_sequence< 2*sizeof...(Args) >() );
}

Live example