How to check if a type is a smart pointer or a reference to a smart pointer

Question

I am trying to implement a structure that checks at compile time if a given type is a smart pointer or a reference to a smart pointer.

I rewrote this solution (that does not work for references):

template<typename T, typename Enable = void>
struct IsSmartPointer : std::false_type {
};

template<typename T>
struct IsSmartPointer<T,
        typename std::enable_if<
                std::is_same<
                        typename std::decay_t<T>, std::unique_ptr<typename std::decay_t<T>::element_type>
                >::value
        >
> : std::true_type {
};

template<typename T>
struct IsSmartPointer<T,
        typename std::enable_if<
                std::is_same<
                        typename std::decay_t<T>, std::shared_ptr<typename std::decay_t<T>::element_type>
                >::value
        >
> : std::true_type {
};

template<typename T>
struct IsSmartPointer<T,
        typename std::enable_if<
                std::is_same<
                        typename std::decay_t<T>, std::weak_ptr<typename std::decay_t<T>::element_type>
                >::value
        >
> : std::true_type {
};

I feel this implementation is very close to the right solution. However, the code below prints zeros:

std::cout << IsSmartPointer<int>::value                          << '\n'
          << IsSmartPointer<const std::shared_ptr<int> &>::value << '\n'
          << IsSmartPointer<std::shared_ptr<int> &>::value       << '\n'
          << IsSmartPointer<const std::shared_ptr<int>>::value   << '\n'
          << IsSmartPointer<std::shared_ptr<int>>::value
          << std::endl;

Could you try to find a mistake because I have run out of ideas?

Answer 1

You wrote std::enable_if<...>, when it should be std::enable_if_t<...> or std::enable_if<...>::type.

std::enable_if<...> is a distinct type that is never going to be void. So your partial specializations will never be used if the second template argument is defaulted to void.

---

Also note that you are forgetting custom deleters. std::unique_ptr has a second template argument for a custom deleter type that you will not catch if it isn't specified to the default.