static_assert for unique_ptr of any type

Question

How can I statically assert that an expression is a std::unique_ptr i.e. std::unique_ptr<T> for any T.

static_assert (std::is_pointer<decltype(exp)>()), "not a smart pointer")

Above does not work. If nothing straight forward, I am only interested if bool() operator is defined for the type.

Answer 1

Create your own trait, with the appropriate partial specialisation:

template <class T>
struct is_unique_ptr : std::false_type
{};

template <class T, class D>
struct is_unique_ptr<std::unique_ptr<T, D>> : std::true_type
{};

Answer 2

You can create a trait for that:

template <typename T, typename D>
std::true_type is_unique_ptr_impl(const std::unique_ptr<T, D>&, int);

template <typename T>
std::false_type is_unique_ptr_impl(const T&, ...);

template <typename T>
using is_unique_ptr = decltype(is_unique_ptr_impl(std::declval<T>(), 0));