Unable to bind lvalue to A <Cv2> &&

Question

I thought that universal link ( T&&) should accept any links. But the following does not work.

I encounter this problem when I try to be const-correct in the library that I am writing. I am new to C ++ and have never seen anything like it before.

test.cpp:

enum Cv_qualifier {
    constant,
    non_const
};
template <Cv_qualifier Cv> class A;
template<>
class A<Cv_qualifier::constant> {
public:
    template<Cv_qualifier Cv2> 
    void t(const A<Cv2>&& out) {}
};

template <>
class A<Cv_qualifier::non_const> {
public:
    template<Cv_qualifier Cv2> 
    void t(const A<Cv2>&& out) {}
};

int main()
{
    A<Cv_qualifier::non_const> a;
    A<Cv_qualifier::constant> b;
    a.t(b);
}

Error (compiled with g++ test.cpp -std=c++11):

test.cpp: In function ‘int main()’:
test.cpp:24:10: error: cannot bind ‘A<(Cv_qualifier)0u>’ lvalue to ‘const A<(Cv_qualifier)0u>&&’
     a.t(b);
          ^
test.cpp:17:10: note:   initializing argument 1 of ‘void A<(Cv_qualifier)1u>::t(const A<Cv2>&&) [with Cv_qualifier Cv2 = (Cv_qualifier)0u]’
     void t(const A<Cv2>&& out) {}
          ^

By the way, in a real program it class Adoes not contain any real data and contains links to another class that actually stores data. Hopefully this means that I do not constantly create indirect / copies of data when I allow the member function tof class Ato accept temporary objects.

Answer 1

- . :

T&& & -> T&
T& && -> T&
T&& && -> T&&

, T&& , rvalue T. , , SomeType&& SomeType&& rvalue.

, , , :

template <Cv_qualifier Cv> struct A;

template<>
struct A<Cv_qualifier::constant> {
    template<typename T> 
    void t(T&& out) {}
};

template <>
struct A<Cv_qualifier::non_const> {
    template<typename T> 
    void t(T&& out) {}
};

, . Cv_qualifier T, , :

template<typename>
struct CvValue;

template<Cv_qualifier cv>
struct CvValue<A<cv>> {
    constexpr static Cv_qualifier value = cv;
};

T :

//                   v----- This is a good practice to apply a constraint
template<typename T, std::void_t<decltype(CvValue<std::decay_t<T>>::value)>* = 0> 
auto t(T&& out) {
    constexpr auto cv = CvValue<std::decay_t<T>>::value;

    // do whatever you want with cv
}

++ 17 std::void_t, :

template<typename...>
using void_t = void;

---

, , T A<...>, :

template<typename>
struct is_A : std::false_type {};

template<Cv_qualifier cv>
struct is_A<A<cv>> : std::true_type {};

std::decay_t:

template<typename T, std::enable_if_t<std::is_A<std::decay_t<T>>::value>* = 0> 
void t(T&& out) {}