[SOLVED] C++, virtual data? (different hash table for derived classes)

ngoonee · 2009-07-27 02:35:15

If I have a base class Base and a derived class Derived, I can override Base's functions with new implementations in Derived.

It does not seem I can do the same with data members. Please correct me if I'm wrong. In my situation, I want to write a Base class which performs some calculations based on a certain hash table (256 by 256 char). I plan to have a great many derived classes, each with the same code but different hash tables. The hash table contains pre-calculated static values which are populated on initialization.

A workaround I can think of is to have a virtual function which returns the values, something like:-

char Derived::gethashvalue(int x,int y) {
return MyOwnHash[x,y];
}

Would this work? Is there a better way to implement this (templates, maybe?)?

Last edited by ngoonee (2009-07-27 06:01:20)

the_isz · 2009-07-27 05:50:35

You should first make clear in which way the hash tables of the derived classes differ from
the Base class' hash table. If they differ in type, use templates:

#include <hash>

template <class HashTableEntry>
class Base
{
// ...
private:
  std::hash<HashTableEntry> MyOwnHash;
// ...
};

If they differ in size and not in type, you can either use templates or just parametrize your
base class accordingly.

The template way:

#include <hash>

template <size_t HashSize>
class Base
{
// ...
private:
  std::hash<HashTableEntry> MyOwnHash;
// ...
};

Base<HashSize>::Base()
{
  MyOwnHash.resize(HashSize);
}

The parameter way:

#include <hash>

class Base
{
public:
  Base( const size_t hashSize );
// ...
private:
  std::hash<HashTableEntry> MyOwnHash;
// ...
};

Base::Base( const size_t hashSize );
{
  MyOwnHash.resize(hashSize);
}

If only the values differ, then I have written all of this in vain because you're already
done. Just write the code to fill the hash in each of your derived classes' initialization
function (e.g. the constructor) and - like you suggested - offer an access method for
it in the base class.

ngoonee · 2009-07-27 06:00:37

Ah, I realize now that its actually quite simple. Thank you for your help!

Arch Linux

#1 2009-07-27 02:35:15

[SOLVED] C++, virtual data? (different hash table for derived classes)

#2 2009-07-27 05:50:35

Re: [SOLVED] C++, virtual data? (different hash table for derived classes)

#3 2009-07-27 06:00:37

Re: [SOLVED] C++, virtual data? (different hash table for derived classes)

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