Pointer to class member as template argument

Question

How to implement a template that gets a class and a pointer to a member of this class?

I want to template a sort of Accessor of the form

template<??? T, ??? P>
class Accessor
{
    // ...

    T& setMember(const Vector3d& point)
    {
        T& obj = getObject<T>();
        obj.*P = point;
        return obj;
    }
};

Here, there is a class T with a member P, both template variables. I know that P is always of type Vector3d. But I have to stay within C++17. Of course, if you have a C++20 technique... I want to know!

Also, Accessor does not have any reference to a T object. But the function getObject magically handles it here. Therefore do not need to care here.

I'd like to use it like this:

class Accessed
{
    Vector3d one_point;
    Vector3d another_point;

    class OneAccessor : public Accessor<&one_point> {};
    class AnotherAccessor : public Accessor<&another_point> {};
}

Notice the format : public Accessor<&one_point>.

Maybe public Accessor<&Accessed::one_point>.

I would prefer

public Accessor<one_point> // or 
public Accessor<Accessed::one_point>

without the &. I'd like Accessed:: to be inferred.

If you have an answer to my problem that you want to deliver, I would very much like to see it.

Answer 1

template <typename T>
struct class_type;

template<typename T>
struct class_type<Vector3d T::*> {
    using type = T;
};

template<auto P>
struct Accessor
{
    using T = typename class_type<decltype(P)>::type;

    T& setMember(const Vector3d& point)
    {
        T& obj = getObject<T>();
        obj.*P = point;
        return obj;
    }
};

class Accessed
{
    Vector3d one_point;
    Vector3d another_point;

    class OneAccessor : public Accessor<&Accessed::one_point> {};
    class AnotherAccessor : public Accessor<&Accessed::another_point> {};
};

Another version using variables:

template<typename T>
struct Accessor
{
    Vector3d T::* p;

    constexpr Accessor(Vector3d T::* p) :p{p} {}

    T& setMember(const Vector3d& point)
    {
        T& obj = getObject<T>();
        obj.*p = point;
        return obj;
    }
};

struct Accessed
{
    Vector3d one_point;
    Vector3d another_point;

    static constexpr Accessor OneAccessor{&Accessed::one_point};
    static constexpr Accessor AnotherAccessor{&Accessed::another_point};
};