How can I call the specialized template overloaded function from the main template one?

Question

I have a method inside a templated class that creates a hash of a variable. I have specialized its template to int, double and std::string like this

template<>
class Hash<int>
{
public:
    unsigned long operator()(const int& d) const noexcept
    {
        return d * d;
    }
};

template<>
class Hash<double>
{
public:
    unsigned long operator()(const double& d) const noexcept
    {
        long intera = floor(d);
        long frazionaria = pow(2, 24) * (d - intera);
        return intera * frazionaria;
    }
};

template<>
class Hash<std::string>
{
public:
    unsigned long operator()(const std::string& d) const noexcept
    {
        unsigned long hash = 5381;
        for (unsigned long i = 0; i << d.length(); i++)
        {
            hash = (hash << 5) + d[i];
        }
        return hash;
    }
};

I then created a generic version of the same method which takes a template parameter and tries to convert it to std::string and then tries to call operator() on it (which I'd expect would call the specialized template one)

unsigned long operator()(const Data& d) const noexcept
    {
        std::cout<<"Special hash! ";
        void *v = (void *)&d;
        size_t j = sizeof(d);
        std::string s = "";
        for (size_t k = 0; k < j; k++)
        {
            s += (((char *)v)[k]);
        }
        std::cout<<"Pre-hash: "<<s<<std::endl;
        return operator()(s);
    }

(it's inside the class declaration while the other 3 are in a different file) So when I call operator() of the Hash class on a int, double or std::string it works fine, but the moment I try to call it on a different class of mine i get this error:

error: cannot convert ‘std::string’ {aka ‘std::__cxx11::basic_string<char>’} to ‘const lasd::BST<int>&’
   35 |                         return operator()(s);
      |                                           ^
      |                                           |
      |                                           std::string {aka std::__cxx11::basic_string<char>}

So it seems to me that it's trying to call operator(BST) again, even though i've passed a string to it. Is there a way I can solve this? I don't care about the hash function being incorrect, I have a uni assignment to do it this way, I only need to understand if it is even possible to call a specialized template function from a normal template one. I also tried using

return operator()<std::string>(s);

as I've seen in some other answers of a similiar question, but I get the following error:

error: expected primary-expression before ‘>’ token
   35 |                         return operator()<std::string>(s);
      |                                                      ^

https://gcc.godbolt.org/z/9oz8Gf9aM

Answer 1

If I understood correctly, your operator() needs a Hash instance to call its specialized operator()

auto operator()(const Data& d) const noexcept
{
    // ....
    std::string s = "";
    return Hash<std::string>{}.operator()(s);
    //     ^^^^^^^^^^^^^^^^^^^ 
    // OR simply
    // return Hash<std::string>{}(s);
  
}

---

Update

From the updated post/ error message, you are using the specialization Hash<std::string> before it has been declared as well as defined. That is at the line:

return Hash<std::string>{}(s);

compiler does not know about the Hash specialization for std::string . Hence, the error!

(One way) to fix it, you need to split the declaration and the definition as follows:

template<typename Data> class Hash
{
public:
    // declare the operator()
    auto operator()(const Data& d) const noexcept;
};
template<> class Hash<int> { /* ...code... */ };
template<> class Hash<double> { /* ...code... */ };
template<> class Hash<std::string> { /* ...code... */ };

// define the operator()
template<typename Data>
auto Hash<Data>::operator()(const Data& d) const noexcept
{
    // code!
    return Hash<std::string>{}(s);
}

See a demo