How to find maximum dereferenceable-level of the parameter T using template

Question

I am designing a "dereferencer" class, for fun.

I wrote some structs and aliass :

template <class _T>
using deref_type = decltype(*std::declval<_T>());

template <class _T, class _SFINAE>
struct is_derefable : std::false_type {};

template <class _T>
struct is_derefable< _T, deref_type<_T> > : std::true_type
{
    using return_type = deref_type<_T>;
};

template<class _T>
using check_derefable = is_derefable<T, deref_type<T>>;

and let's say that there is a variable with type T = std::vector<int**>::iterator, which is the iterator dereferenced into a level-2 pointer, thus has a 3-level dereferenceability.

Here, I want to know the maximum level of "dereferenceability" of an arbitrary type T, at the compile-time.

std::cout << deref_level<std::vector<int**>::iterator>::max << std::endl; // this should prints 3

I thought that it would be way similar to generating a sequence at the compile-time: Template tuple - calling a function on each element , but I can't draw a concrete picture of it.

Here are what I've tried:

template<class _TF, class _T>
struct derefability {};

template<int _N, class _derefability>
struct deref_level;

template<int _N, class _T>
struct deref_level<_N, derefability<std::false_type, _T>>
{
    static const int max = _N;
};

template<int _N, class _T>
struct deref_level<_N, derefability<std::true_type, _T>> : 
    deref_level<_N + 1, derefability<typename check_derefable<deref_type<_T>>::type, deref_type<_T>>>{};

deref_level<0, derefability<check_derefable<T>::type, T>::max;

but it does not work...(compiler says that max is not a member of tje class) What went wrong?

Answer 1

After a few days of work, I was able to write code that works without causing an error in MSVC13.

First of all, I needed a robust module to check the dereferenceability of the type.

Since the struct-level SFINAE check fails, I took another method that deduces the return type from overloaded functions with auto->decltype expression, based on the answer: link

template<class T>
struct is_dereferenceable
{
private:
    template<class _type>
    struct dereferenceable : std::true_type
    {
        using return_type = _type;
    };

    struct illegal_indirection : std::false_type
    {
        using return_type = void*;
    };

    template<class _type>
    static auto dereference(int)->dereferenceable<
        decltype(*std::declval<_type>())>;

    template<class>
    static auto dereference(bool)->illegal_indirection;

    using dereferenced_result = decltype(dereference<T>(0));

public:
    using return_type = typename dereferenced_result::return_type;
    static const bool value = dereferenced_result::value;
};

Now I have a robust dereferenceability-checker, the remaining part becomes far easy.

template< class T,
    class D = typename is_dereferenceable<T>::return_type >
struct dereferenceability;

template< class T >
struct dereferenceability<T, void*>
{
    using level = std::integral_constant<int, 0>;
};

template< class T, class D >
struct dereferenceability<T, D&>
{
    using level = std::integral_constant<int, dereferenceability<D>::level::value + 1>;
};

int main()
{
    static_assert(dereferenceability<int>::level::value == 0, "something went wrong");
    static_assert(dereferenceability<int****>::iterator>::level::value == 4, "something went wrong");
    return 0;
}

I've tested codes above in Visual Studio 2013, and no error occured.

Answer 2

Here is a recursive implementation using SFINAE directly:

template <class T, class = void>
struct deref_level {
    enum : std::size_t { value = 0 };
};

template <class T>
struct deref_level<T, decltype(void(*std::declval<T const &>()))> {
    enum : std::size_t { value = deref_level<decltype(*std::declval<T const &>())>::value + 1 };
};

See it live on Wandbox

Answer 3

I don't know what went wrong with your template example, but here's an implementation using a recursive consteval function:

#include <type_traits>

template<typename T, int N = 0>
consteval int deref_level()
{
    if constexpr (std::is_pointer<T>::value) {
        typedef typename std::remove_pointer<T>::type U;
        return deref_level<U, N + 1>();
    } else {
        return N;
    }
}

int main() {
    return deref_level<int****>(); // Returns 4
}