CODEWITHSUNDEEP
c++templateshashmapparameter-passinghashtable
Damian Pereira
Damian Pereira

Reputation: 513

Passing function to template object when initializing template in C++

I'm trying to write an implementation for hash map, I'm not allowed to use anything from stdlib except for iostream, string and cassert.

It needs to be generic, so the values that populate the buckets can be of any type. I need templates for this, but can't manage to pass the hash function in any way. This would be the header file:

template<typename Value, typename hashFunction>
class hashTable{
    public:
      hashTable(int size){
        //Creates an empty vector of size on the table
      }
      define(Value v){
        loads value in Vector[hashFunction(v)];
      }
      ...
    private:
      Vector with all the elements
}

Note: I guess I don't need templates for the keys, do I?

I can't define the hash function inside my class because I'd have to make one that works with all types (string to int, int to int, double to int, etc). So I guess the only solution is to pass the function as argument in my main. This would be the main.

int hashF(int v){return v}
int main(){
  hashTable<int,int,hashF> table(5);
}

But this doesn't work, g++ tells me "expected type but got hashF". I guess I could pass a pointer to a function, but that seems like a hack rather than a real solution. Is there a better way?

Upvotes: 4

Views: 249

Answers (2)

Damian Pereira
Damian Pereira

Reputation: 513

Managed to get it working with Neil's advice. My hash.h:

template<typename C, typename D, typename H>
class Tabla {
public:
Tabla(int s){
    cout << hashF(3) << endl;
    size=s;
}
private:
    H hashF;
    int size;
};

My hash.cpp

struct KeyHash {
unsigned long operator()(const int& k) const
{
    return k % 10;
}
};
int main(){
    Tabla<int,int,KeyHash> tab(3);
    return 0;
}

This example is just to show I'm able to use the function inside the template, then I'd have to code the define and delete functions that use that KeyHash.

Dunno why I have to wrap it like this, but it works. Found the specifics of it here

Upvotes: 0

Dimitrios Bouzas
Dimitrios Bouzas

Reputation: 42929

template<typename Value, int(*fun)(Value)>
class hashTable {
  std::vector<Value> v;
public:
  hashTable(std::size_t size) : v(size) { }
  void define(Value &&val) { v[fun(val)]  = val; }
};

Live Demo

Non function pointer way:

template<typename Value, typename F>
class hashTable {
  std::vector<Value> v;
  F fun;
public:
  hashTable(std::size_t size, F fun_) : v(size), fun(fun_) { }
  void define(Value &&val) { v[fun(val)]  = val; }
};

Live Demo

Upvotes: 1

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