c++ overriding default function in templated class

Question

As a learning exercise, I wanted to make my own Hash Table class (yes, I know about std::unordered_map and std::unordered set). So, I wrote this code:

using std::cout;
using std::endl;
using std::vector;
using std::unique_ptr;

template <class K, class V, class U=std::hash<K>>
class hashTable
{
    int order=0;
    vector<myNode<K, V>> nodes;
public:
    hashTable(U u = U()){}; //  : hashPtr(u) 
    size_t gethash(K key, int level=0, const U & u=U());
};

template<class K, class V, class U>
size_t hashTable<K, V, U>::gethash(K key, int level, const U & u)
{
    return u(key) % divisors[level];
}

And it compiles fine and does what I expect when in main I have:

hashTable<int,int> hast;
for(int i=0;i<40;++i)
    cout << "hash of "<<i<<" is " << hast.gethash(i, 2) << endl;

However, when I write the following function:

size_t nodeHash(myNode<int,int> node) {
    int i = node.getkey();
    int j = node.getvalue();
    std::hash<int> hash_fn;
    return hash_fn(i)+hash_fn(j);
}

and in main I write:

hashTable < myNode<int, int>, int, nodeHash> hashMyNode;

I get the compilation error: function "nodeHash" is not a type name.

I know that I don't know what I am doing because these templated functions are new to me. I seem to know just enough to be "dangerous." However, if someone were able to nudge me in the right direction or give me a full-fledged solution to include an external function into a class (like std::unordered_map or std::sort does), I certainly would appreciate it.

Edit:

auto node = myNode<int, int>(1, 3);
hashTable < myNode<int, int>, int, size_t (*)(myNode<int,int> node)> hashMyNode;
hashMyNode.gethash(node, 2, nodeHash);

I get the following errors:

Severity    Code    Description Project File    Line    Suppression State
Error (active)  E1776   function "myNode<K, V>::myNode(const myNode<int, int> &) [with K=int, V=int]" (declared implicitly) cannot be referenced -- it is a deleted function    somePrime   E:\source\repos\somePrime\somePrime.cpp 139 

and

Severity    Code    Description Project File    Line    Suppression State
Error   C2280   'myNode<int,int>::myNode(const myNode<int,int> &)': attempting to reference a deleted function  somePrime   e:\source\repos\someprime\someprime.cpp 139 

this refers to the node variable.

Answer 1

You can open the std namespace and specialize the hash template for your node type as:

namespace std {
template<>
struct hash<myNode<int, int>>
{
    std::size_t operator()(myNode<int, int> const& node) const {
        int i = node.getkey();
        int j = node.getvalue();
        std::hash<int> hash_fn;
        return hash_fn(i)+hash_fn(j);
    }
};
}

and then create a table as:

hashTable < myNode<int, int>, int> hashMyNode;

now instead of using a function as a hash, you can create hash for your node as std::hash<myNode<int,int>> hash_fn{} but you don't have to explicitly create them, because you've already providing it type as default parameter (3rd parameter) of hashTable