an experiment on how to use structured bindings with prvalues (it took me a bit to realize the constness doesn't affect the double& embedded within)

unpacking_to_structured_bindings.cpp

#include <iostream>
#include <tuple>


//sample vector class
struct Vec3 {
    public:
        Vec3(const std::initializer_list<double>& a) { std::copy(a.begin(),a.end(),data); }
        double& operator()(size_t i) { return data[i]; }
    private:
        double data[3];
};



//unpacks things into tuples for me as references
//Mimics http://stackoverflow.com/a/40958644
template <int... Is>
auto unpack_impl(Vec3& a, std::integer_sequence<int,Is...>) {
    return std::tie( a(Is)... );
}

auto unpack(Vec3& a) {
    return unpack_impl(a,std::make_integer_sequence<int,3>{});
}


int main(int argc, char * argv[]) {
    Vec3 a{{1,2,3}};

    //constness is required for the prvalue std::tuple<double&,double&,double&> from unpack to by assignable
    //however, this just passes references down, which are still mutable
    const auto& [u,v,w] = unpack(a);

    //change u,v,w
    std::tie(u,v,w) = std::make_tuple<double,double,double>(5,12,13);

    //check that modifying u,v,w affected a
    auto [x,y,z] = unpack(a);
    std::cout << x << " " << y << " " << z << std::endl;

}