c++ - 为什么add_const<T>中的T若为引用则add_const<T>::type仍为T？

Question

为什么add_const对引用无效，比如我认为，add_const<int &>::type应该是const int &才对，
而实际上，add_const<int &>::type仍旧是int &

Answer 1

References only have underlying const, not top-level const.
If add_const<int &> changes int &, its type will be int &const, which is obviously wrong

Answer 2

template <typename T, 
    bool = is_reference<T>::value ||
    is_function<T>::value ||
    is_const<T>::value
>
struct add_const;

Then I wrote specializations of true and false

To add a reference, there are one add_lvalue_reference and add_rvalue_reference