Something that I do not understand about member functions in Classes.

[MisterTams]

I need someone to explain this to me because I am slightly confused.
When using public member functions I thought that all the member functions would in a way share information with each other. Take this header file I made as an example for what I am talking about.

#include <string>
using namespace std;

#ifndef INVENTORY_H
#define INVENTORY_H

class Inventory
{
private:
	int itemNumber, quantity;
	double cost, totalCost;
public:
	Inventory ()
	{
		itemNumber = 0, quantity = 0, cost = 0, totalCost = 0;
	}

	Inventory (int ItemNum, int Quantity, double COST)
	{
		totalCost = 0;
		itemNumber = ItemNum;
		quantity = Quantity;
		cost = COST;
	}

	void setItemNumber (int ItemNum)
	{ itemNumber = ItemNum; }

	void setQuantity (int Quantity)
	{ quantity = Quantity;  } 

	void setCost (double COST)
	{ cost = COST; }

	void setTotalCost ()
	{	totalCost = cost * quantity; }

	int getItemNumber ()
	{ return itemNumber; }

	int getQuantity ()
	{ return quantity; }

	double getCost ()
	{ return cost; }

	double getTotalCost ()
	{ return totalCost; }

};

#endif 

When I use getTotalCost in my main function I get a result of 0.
I am supposed to use setTotalCost to calculate the totalCost which is...
totalCost = cost * quantity;
Then I am supposed to return that value back intto main when accessing my instance of a class using the getTotalCost function.
I thought the member functions communicate with each using the private variables, so that I would be able to call getTotalCost and it would return the value that was calculated in setTotalCost.

#include <iostream>
#include <string>
#include <iomanip>
#include "Inventory.h"
using namespace std;

int main ()
{
	int NewProducts;
	Inventory *stock = nullptr;
	int ItemNum, quantity;
	double cost;

	cout << "This program was written by:    MisterTams ©" << endl;
	cout << "____________________________________________" << endl;
	cout << "At the end of the daily warehouse operations" << endl;
	cout << "enter any new items that need to be added to" << endl;
	cout << "our in-stock inventory if they were out-of- " << endl;
	cout << "stock as of earlier today or before then....\n" << endl;

	cout << "How many new different product types were added" << endl;
	cout << "during receiving operations?" << endl;

	cout << "\nEnter: ";
	cin >> NewProducts;

	stock = new Inventory [NewProducts]; //dynamically allocating an array using a pointer
	
	for (int x = 0; x < NewProducts; x++)
	{
		cout << "\nNewly Stocked Item #" << x + 1 << ":" << endl;
		cout << "Enter Item Stock Number: ";
		cin >> ItemNum;
		cout << "Enter Item Quantity: ";
		cin >> quantity;
		cout << "Enter Sale Price: $";
		cin >> cost;
		stock[x] = Inventory (ItemNum, quantity, cost);
	}

	cout << "\nNewly Stock Inventory in Warehouse" << endl;
	cout << "___________________________________________________________________" << endl;
	cout << "ITEM STOCK #" << setw(16) << "QUANTITY" << setw(19) << "COST/EA" << setw(20) << "TOTAL COST" << endl;

	for (int y = 0; y < NewProducts; y++)
	{
		cout << setw(15) << left << stock[y].getItemNumber() << ' ';
		cout << setw(12) << right << stock[y].getQuantity() << ' ';
		cout << fixed << setprecision(2);
		cout <<  setw(10) << "$";
		cout << setw(8) << right  << stock[y].getCost() << ' ';
		cout << setw(6) << "$";
		cout << setw(13) << right  << stock[y].getTotalCost() << endl;
		
	}

	delete [] stock;
	cout << "\nMemory Address of instance of stock object before nullptr operator: " << stock << endl; 
	stock = nullptr;
	cout << "Memory Address of instance of stock object after nullptr operator: " << stock << endl; //experimenting with memory address to see what nullptr does

	system("pause");
	return 0;
}

if I were to change the getTotalCost member function in my header file for the class to this...

double getTotalCost ()
{ return totalCost = cost * quantity; }

Then all is fine... But my program challenge requirements, specifically state to make a member function that stores the totalCost in a function called
setTotalCost. Then to return that resulting value I would have to call getTotalCost in main. This simply doesn't work for me. What gives? Any help would be appreciated.
Thanks.

[jlb]

Since you're using the constructor to construct stock items with the proper values you could call the set total cost function in the constructor, after you have filled in the dependent values. You could also call the set total cost function in quantity and cost setters as well, then the total cost would be updated whenever the quantity or cost changes. Lastly you can call the set total cost function after you construct your stock item to force a recalculation of the total cost.

[jonnin]

I think your understanding is good -- the class variables are shared inside the member functions and retain their values. They are basically globals to anything in the class, if you want to think that way. I didnt dig deep but if this is not happening for you, I would look for a bug.

more generally..

class x
{
int z;
void foo();
void bar();
}

if you changed z in foo and print it in bar, you should see the changes.

[MisterTams]

Thanks guys,

That helped :)