I've been using STL's unordered_map recently and while it seems to work nicely I don't quite understand how the hashing function works given that the data type is given as a template parameter. In an effort to understand this data structure more thoroughly, I implemented my own little Hashmap class in C++ :
Hashmap interface:
#ifndef _HASHMAP_H_
#define _HASHMAP_H_
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <iostream>
#include <vector.h>
//Beginning of Hashmap class definition
template <class Key, class Value>
class Hashmap{
private:
int mappedElementCount;
public:
explicit Hashmap();
virtual ~Hashmap();
virtual void test();
virtual int hash(Key*);
int* getSize();
void putKVPair(Key*,Value*);
void clearMap();
//When we use these methods, we'll want a linear vector of keys and values to
//iterate over, so vector is good here
std::vector<Key>* getKeys();
std::vector<Value>* getValues();
}; //end hashmap class definition
#endif /*_HASHMAP_H_*/
Hashmap implementation:
#include "Hashmap.h"
template<class Key,class Value> Hashmap<Key,Value>::Hashmap(){
mappedElementCount = 0;
}
template<class Key,class Value> Hashmap<Key,Value>::~Hashmap(){
printf("\nDestroying the base Hashmap object!\n");
}
template<class Key,class Value> void Hashmap<Key,Value>::test(){
printf("The size of our Key is %i and the size of our Value is
%i\n",sizeof(Key),sizeof(Value));
}
template<class Key,class Value> int Hashmap<Key,Value>::hash(Key* k_ptr){
unsigned int hashval;
/* we start our hash out at 0 */
hashval = 0;
//TODO: How do we generate a hash signature when we don't know what data type
//we're going to be working with?
return hashval % mappedElementCount;
}
template<class Key,class Value> std::vector<Key>* Hashmap<Key,Value>::getKeys(){
//TODO: prepare a vector initialized with all Key objects and return it here
return keys;
}
template<class Key,class Value> std::vector<Value>* Hashmap<Key,Value>::getValues(){
//TODO: prepare a vector initialized with all Value objects and return it here
return values;
}
template<class Key,class Value> int* Hashmap<Key,Value>::getSize(){
return &mappedElementCount;
}
template<class Key,class Value> void Hashmap<Key,Value>::putKVPair(Key* k, Value* v){
//TODO: implement hashing of the key object k to determine
//the address of the value object v
//first step, generate a hash from our key
int tempHash = hash(k);
//TODO: store the Value at an address given by or influenced by tempHash
//If all was successfully completed, increment the mapped records counter
mappedElementCount++;
}
template<class Key,class Value> void Hashmap<Key,Value>::clearMap(){
//TODO: implement a cascading chain of deallocation of stored objects within the
//hashmap
//MAYBE-- only if we create new objects rather than just mapping reference
//associations,
//which is really the goal here... In the latter case, just empty the Hashmap
//itself
}
One possible OOP method of addressing this problem is to use Hashmap as a base class and provide derived classes that have known Key data types, such as the following Stringmap:
Stringmap interface:
#ifndef _STRINGMAP_H_
#define _STRINGMAP_H_
#include "Hashmap.h"
template <class Value>
class Stringmap:public Hashmap<std::string,Value>{
private:
public:
//Con/de 'structors
explicit Stringmap();
~Stringmap();
//Here we know our Key will be of type std::string
//so we can generate our hash sig by char values
//Override hash from the base class
int hash(std::string*);
//override test from base class
void test();
};
#endif /*_STRINGMAP_H_ def*/
Stringmap implementation:
#include "Stringmap.h"
template<class Value> Stringmap<Value>::Stringmap():Hashmap<std::string,Value>(){
}
template<class Value> Stringmap<Value>::~Stringmap(){
printf("\nDestroying the derived stringmap object!\n");
}
template<class Value> void Stringmap<Value>::test(){
printf("The size of our Value is %i\n",sizeof values[0]);
}
template<class Value> int Stringmap<Value>::hash(std::string* str_ptr){
unsigned int hashval;
/* we start our hash out at 0 */
hashval = 0;
/* for each character, we multiply the old hash by 31 and add the current
* character. Remember that shifting a number left is equivalent to
* multiplying it by 2 raised to the number of places shifted. So we
* are in effect multiplying hashval by 32 and then subtracting hashval.
* Why do we do this? Because shifting and subtraction are much more
* efficient operations than multiplication.
*/
for(int i=0;i<str_ptr->length();i++) {
hashval = (*(str_ptr))[i] + ((hashval << 5) - hashval);
}
/* we then return the hash value mod the hashmap size so that it will
* fit into the necessary range
*/
return hashval % (*(Hashmap<std::string,Value>::getSize()));
}
So the question is as follows: is it possible to create a hash signature when the data type to be hashed is currently unknown? If so, how? Looking at the std::hash docs, it appears that the C++ standard simply defines a hash function for each primitive data type and also for T* (for any type T)... What's missing is how this hashing is implemented for the given primitive data types and, more to the point, how it is implemented for the generic T*. I suppose I could just call hash(Key) and hope for the best, but it would be nice to understand what's going on behind the scenes.
thanks, CCJ
