Finding size of dynamically allocated array

Question

Why is it not possible to get the length of a buffer allocated in this fashion.

AType * pArr = new AType[nVariable];

When the same array is

Answer 1

why not a bit of extra info like this:

template  class AType
{
public:

    AType(size_t s) : data(0)
    {
        a_size = s;
        data = new T[s];
    }
    ~AType() {
        if (data != nullptr)
            delete [] data;
    }

    size_t getSize() const
    {
        return a_size * sizeof(T);
    }

private:
    size_t a_size;
    T* data;
};