template structure constructor with reference parameter

Question

I've a template structure like this

template<typename ... Args> struct A;
template<>
struct A<>{

};
template<typename First, typename ... Args>
struct A<First, Args...>:private A<Args...>{
    typedef A<Args...> child;
    typedef A<First, Args...> self;
    typedef First type;
    static const size_t size = 1 + sizeof...(Args);
    type value;
    template<typename ... _Args>
    A(First && first, _Args && ... args) :child(forward<_Args>(args)...),value(move(first)){}//move
    template<typename ... _Args>
    A(const First & first, _Args && ... args) :child(forward<_Args>(args)...), value(first){}//assign
};

If I call this structure like below;

int f= 6;  
 A<int, int, int> g(f, 5, f); //it is working   
 A<int&> i(f);//not working  compile error

Second one not working cause rvalue constructor can't assign first to value.
What should I do for second one work?

Answer 1

Your bigger problem is that when First is a lvalue reference (say, T &), both First && and const First & becomes T &, meaning that your two constructors have the same signature, which is obviously illegal. (const First & is "reference to const First", but since references themselves are immutable , when First is a reference type itself the const is meaningless.)

One possible fix would be making your constructor take universal references for the first parameter as well, i.e.:

template<typename F, typename ... Cargs>
A(F&& first, Cargs && ... args) :child(forward<Cargs>(args)...),value(forward<F>(first)){}

Demo.