HashMap of Function pointers (Class Member Functions) - c++

Question

I have to create a very BASIC hash map of Functions pointers. My requirement is just add values in it and then get it based on key. For some political reason, I can not use any standard librabry. I have a code which works fine. But if I want a functions pointers to my CLASS MEMBER FUNCTIONS then this do not work. Any suggestion what should be the modification in below code.
In this PING and REFRESH are independent functions. So this code works. But if I move these functions to HashMap class then it fails.
Code:--
#include <iostream>
#include <cstdlib>
#include <cstring>
#include <iomanip>
using namespace std;
typedef void (*FunctionPtr)();
void ping(){
cout<<"ping";
}
void refresh(){
cout<<"refresh";
}
class HashEntry {
private:
int key;
FunctionPtr func_ptr1;;
public:
HashEntry(int key, FunctionPtr fptr) {
this->key = key;
this->func_ptr1 = fptr;
}
int getKey() {
return key;
}
FunctionPtr getValue() {
return this->func_ptr1;
}
};
const int TABLE_SIZE = 128;
class HashMap {
private:
HashEntry **table;
public:
HashMap() {
table = new HashEntry*[TABLE_SIZE];
for (int i = 0; i < TABLE_SIZE; i++)
table[i] = NULL;
}
FunctionPtr get(int key) {
int hash = (key % TABLE_SIZE);
while (table[hash] != NULL && table[hash]->getKey() != key)
hash = (hash + 1) % TABLE_SIZE;
if (table[hash] == NULL)
return NULL;
else
return table[hash]->getValue();
}
void put(int key, FunctionPtr fptr) {
int hash = (key % TABLE_SIZE);
while (table[hash] != NULL && table[hash]->getKey() != key)
hash = (hash + 1) % TABLE_SIZE;
if (table[hash] != NULL)
delete table[hash];
table[hash] = new HashEntry(key, fptr);
}
~HashMap() {
for (int i = 0; i < TABLE_SIZE; i++)
if (table[i] != NULL)
delete table[i];
delete[] table;
}
};
void main(){
HashMap* pHashsMap = new HashMap();
pHashsMap->put(1,ping);
pHashsMap->put(2,refresh);
pHashsMap->put(3,ping);
pHashsMap->put(4,refresh);
pHashsMap->put(5,ping);
pHashsMap->put(6,refresh);
cout<<" Key 1---"<<pHashsMap->get(1)<<endl;
pHashsMap->get(1)();
cout<<" Key 5---"<<pHashsMap->get(5)<<endl;
pHashsMap->get(5)();
cout<<" Key 3---"<<pHashsMap->get(3)<<endl;
pHashsMap->get(3)();
cout<<" Key 6---"<<pHashsMap->get(6)<<endl;
pHashsMap->get(6)();
delete pHashsMap;
}

Answer 1

The smart-alec answer: inspect the code for std::bind, learn from it, and create your own (though tbh, not using STL/boost isn't smart...).
the simpler answer: you need to create a union type to hold your normal function pointer and a class member function pointer, then store a bool to indicate if it is a class pointer:
class funcbind_t
{
union
{
void (*pf)();
void (SomeClass::*mfp)();
};
bool member;
funcbind_t(void (*_pf)()) : pf(_pf), member(false)
{
}
funcbind_t(void (SomeClass::*_mpf)()) : mpf(_mpf), member(true)
{
}
void operator ()()
{
if(member)
mfp();
else
fp();
}
};
as you can see, this is going to get messy when you start needing differing parameters to the functions.