CODEWITHSUNDEEP
c++classoop
Andrei Albu
Andrei Albu

Reputation: 11

Objects of same class having varying selection of the class members (C++)

Let's say class A has members "int B" and "int C". When I create an object of this class, I would like to choose whether it will contain only "int B" or only "int C" (or maybe if it has them both).

(I know that an alternative is declaring B and C as std::vectors and then resizing them to 1, but that means I'll have to add [0] after them each time I use them, and that also means I'll have to waste 20 bytes per member, because it seems that std::vector uses 20B by default, whether you assign anything to it or not.)

Upvotes: 0

Views: 232

Answers (2)

Travis Pessetto
Travis Pessetto

Reputation: 3298

Use inheritance. You may need some virtual methods too.

class Settlement
{
   public:
      virtual void updateMarketCount()=0;//pure virtual function
};

class Town : public Settlement
{
    public:
       virtual void updateMarketCount();
    private:
       int m_markets;
       int m_industries;
};

class Village : public Settlement
{
     public:
       virtual void updateMarketCount();
     private:
       int m_markets;
};

This way you can assign all of these objects to a generic settlement pointer. For example:

 std::shared_ptr<Settlement> mysettlementPtr(new Town());//C++ 11 only. Found in memory header
 Settlement* mysettlementPtr = new Town();//C++ < 11

Here is how to do a vector:

std::vector<std::shared_ptr<Settlement>> mySettlements;
mySettlements.push_back(std::shared_ptr<Town>(new Town()));
mySettlements.push_back(std::shared_ptr<Village>(new Village()));

Here is a very simple working example:

Settlement.h:

#pragma once
class Settlement
{
    public:
        virtual void increaseMarkets()=0;
        virtual int getMarketCount()=0;
};

Town.h

#pragma once
#include "Settlement.h"
class Town : public Settlement
{
    public:
        Town():m_markets(0){}
        virtual void increaseMarkets(){++m_markets;}
        virtual int getMarketCount(){return m_markets;}
    private:
       int m_markets;
       int m_industries;
};

Village.h

#pragma once
#include "Settlement.h"

class Village : public Settlement
{
    public:
        virtual void increaseMarkets(){++m_markets;}
        virtual int getMarketCount(){return m_markets;}
     private:
       int m_markets;
};

SimpleExample.cpp:

#include<iostream>
#include<memory>
#include "Settlement.h"
#include "Village.h"
#include "Town.h"

int main()
{
    std::shared_ptr<Settlement> mysettlement(new Town());
    std::cout << "I HAVE: " << mysettlement->getMarketCount() << " MARKETS" <<  std::endl;
    mysettlement->increaseMarkets();
    std::cout << "NOW I HAVE " << mysettlement->getMarketCount() << " MARKETS" << std::endl;
    std::cout << "PRESS ENTER TO CONTINUE." << std::endl;
    system("pause");
    return 0;
}

NOTE: It is best to extract functions into .cpp files related to the headers.

Upvotes: 3

ruben2020
ruben2020

Reputation: 1549

How about this?

First, define 3 container classes that contain A, B or both.

class withA
{
  int varA;
};

class withB
{
  int varB;
};

class withAB
{
  int varA;
  int varB;
};

Next, define a template class, which is the class you want, and it contains this member variable, inside of which, there's A, B or A and B.

template<class T>
class theThing
{
   T m_thingMember;
};

Next, this is how you declare it.

theThing<withA> thingObjA;
theThing<withB> thingObjB;
theThing<withAB> thingObjAB;

Upvotes: 0

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