Writing a remove_all_pointers type trait, part 2

Last time, we wrote a remove_all_pointers type trait, but I noted that even though we found a solution, we weren’t finished yet.

We can bring back the one-liner by using a different trick to delay the recursion: Don’t ask for the type until we know we really are recursing.

The sketch is

template<typename T>
struct dummy { using type = T; };

template<typename T>
struct remove_all_pointers
{
    if (std::is_pointer_v<T>) {
        using type_holder =
            remove_all_pointers<
                std::remove_pointer_t<T>
            >;
    } else {
        using type_holder = dummy<T>;
    }
    using type =
        typename type_holder::type;
};

We first define a type_holder to be a type which has a type member type that holds our answer. If T is a pointer, then the type holder is the recursive call. Otherwise, the type holder is a dummy type whose sole purpose is to have a type member type that produces T again.

We can now pack up that if into a std::conditional.

template<typename T>
struct remove_all_pointers
{
    using type_holder =
        std::conditional_t<
            std::is_pointer_v<T>,
            remove_all_pointers<
                std::remove_pointer_t<T>
            >,
            dummy<T>>;
    using type =
        typename type_holder::type;
};

It turns out that we don’t need to define a dummy: The C++ standard library comes with one built in! It’s called std::type_identity<T>, available starting in C++20. (We looked at std::type_identity<T> a little while ago.)

template<typename T>
struct remove_all_pointers
{
    using type_holder =
        std::conditional_t<
            std::is_pointer_v<T>,
            remove_all_pointers<
                std::remove_pointer_t<T>
            >,
            std::type_identity<T>>;
    using type =
        typename type_holder::type;
};

Now we can inline the type_holder.

template<typename T>
struct remove_all_pointers
{
    using type =
        typename std::conditional_t<
            std::is_pointer_v<T>,
            remove_all_pointers<
                std::remove_pointer_t<T>
            >,
            std::type_identity<T>>::type;
};

Or even better, just derive from the type_holder!

template<typename T>
struct remove_all_pointers :
        std::conditional_t<
            std::is_pointer_v<T>,
            remove_all_pointers<
                std::remove_pointer_t<T>
            >,
            std::type_identity<T>>
{
};