how do i delete a dynamic array with my function class?

You cannot use delete with memory allocated with new[], and vice versa using delete[] with memory allocated with new. This is exactly why std::unique_ptr and std::shared_ptr have specializations to decide when to use delete[] vs delete for array and non-array memory, respectively (via the std::default_delete template).

You are just going to have to specialize your Deleter in a similar manner, too. Only the user of your Deleter, not the Deleter itself, can decide which specialization to use, since only the user has information about how the memory was allocated to begin with, and thus how it needs to be freed.

For example:

template <typename T>
class Deleter
{
public:
    void operator()(T* ptr) const {
        std::cout << "freeing memory using 'delete'...\n";
        delete ptr;
    }
};

template <typename T>
class Deleter<T[]>
{
public:
    template <typename U>
    void operator()(U* ptr) const {
        std::cout << "freeing memory using 'delete[]'...\n";
        delete[] ptr;
    }
};

int main()
{
    int* pi = new int(7);
    char* cp = new char[100];
    strcpy(cp, "hi there!");

    Deleter<int> del1;
    del1(pi); // uses 'delete'...

    Deleter<char[]> del2;
    del2(cp); // uses 'delete[]'...

    std::cout << "\ndone\n";
}

Live Demo