Are objects inside rvalue referenced object, also rvalue referenced?

问题

Are objects inside rvalue referenced object, also rvalue referenced?

struct A{
};

struct B{
   A a2;
};

//template<class B>
void test(B &&b){

    // 1. Is this the correct way?
    auto &&in3 = std::forward<B>(b).a2;
    std::cout << std::is_rvalue_reference<decltype(in3)>::value;
    // return true

    // 2. or this?
    auto &&in4 = b.a2;
    std::cout << std::is_rvalue_reference<decltype(in4)>::value;    
    // return false        
}

test(B());

http://coliru.stacked-crooked.com/a/bcf0f7dc4cc0440e

回答1:

Yes, members of rvalues are themselves rvalues. This was clarified by DR 421

But that is irrelevant here:

auto &&in4 = b.a2;

b is not an rvalue, it's an lvalue (simple rule of thumb: it has a name).

To restore the value category it had when passed to the function you need to forward it

来源：https://stackoverflow.com/questions/24380677/are-objects-inside-rvalue-referenced-object-also-rvalue-referenced