Paint Job Estimator C++ - c++

Question

Hi I am working on a project for school and cannot for the life of me figure out how to get the totalJobCost function to work. The other functions work without a problem but i don't think they are passing the var back to main for totalJobCost to grab as the totalJobCost outputs 0. here is the code that i am using:
#include "stdafx.h"
#include <iostream>
using namespace std;
void space(double paintarea, double paintcost, double paintneeded, double totalpaint);
void cost(double hrs, double hrcost, double spacetopaint);
void totalJobCost(double allTheirPaintCost, double allTheirWages, double theirTotalJobCost);
const double AREA_FORMULA = 220.00;
const double AREAFORMULA_PAINT = 1.00;
const double AREAFORMULA_HOURS = 8.00;
const double AREAFORMULAHOURS_WAGES = 35.00;
int main()
{
double areaTP;
double paintCST = 0;
double paintNeeded = 0;
double allPaintCost = 0;
double hoursNeeded = 0;
double hoursWages = 0;
double allWages = 0;
double allJobCost = 0;
cout << "Enter the square footage you need to paint, then press enter" << endl;
cin >> areaTP;
cout << "Enter the price by gallons of paint you will use, then press enter" << endl;
cin >> paintCST;
while (paintCST < 10)
{
cout << "Enter the price by gallons of paint you will use, then press enter. cannot be less than 10 :";
cin >> paintCST;
}
space(areaTP, paintCST, paintNeeded, allPaintCost);
cost(hoursNeeded, hoursWages, areaTP);
totalJobCost(allPaintCost, hoursWages, allJobCost);
system("Pause");
return 0;
}
void space(double paintarea, double paintcost, double paintneeded, double totalpaint)
{
paintneeded = paintarea / AREA_FORMULA * AREAFORMULA_PAINT;
totalpaint = paintneeded * paintcost;
cout << "How many gallons of paint you will need: " << paintneeded << endl;
cout << "Your total paint cost will be: " << totalpaint << endl;
}
void cost(double hrs, double hrcost, double spacetopaint)
{
hrs = (spacetopaint / AREA_FORMULA) * AREAFORMULA_HOURS;
hrcost = hrs * AREAFORMULAHOURS_WAGES;
cout << "The number of hours for the job will be: " << hrs << endl;
cout << "The total amount of wages will be: " << hrcost << endl;
}
void totalJobCost(double totalpaint, double hrcost, double theirTotalJobCost)
{
theirTotalJobCost = totalpaint + hrcost;
cout << "The total price of your paint job will be: " << theirTotalJobCost << endl;
}

Answer 1

You need to declare your arguments (totalpaint and hrcost) as references.
Currently, functions space() and cost() just make copies of totalpaint and hrcost when called, update them, then print them. But when the functions return, the values stored in totalpaint and hrcost are lost.
To fix this, you should declare those functions as follows:
void space(double paintarea, double paintcost, double paintneeded, double& totalpaint)
void cost(double hrs, double& hrcost, double spacetopaint)
Now whatever variable you pass in as totalpaint or hrcost will be updated when space() or cost() operates on it.

Answer 2

This is a pass by value vs. pass by reference issue.
In C++, booleans, characters, integer numbers, floating-point numbers,
arrays, classes—including strings, lists, dictionaries, sets, stacks,
queues—and enumerations are value types, while references and pointers
are reference types.
CPP reference
The variables you are using are doubles (double precision floating point), so they are value types. When you pass value type variables to functions as parameters, the current value of the variables is copied to the calling stack of the function you called. Once inside the function, the parameter names are just names you use to access the copied values. Whatever you do to these copied values will not affect the value of the original variables you passed to the function. Read up on function scope and the calling stack architecture of C/C++ to understand more.
To change the value of a variable across function calls, you need to pass a reference to its location in memory. If you declare a variable in the first few lines of a function, its location in memory will be part of that function’s call stack, and you can safely access that memory in any function calls that are called within the original function. So you can do this:
int main() {
double variable = 0;
function(&variable);
cout << variable;
}
void function(double* variable_address) {
*variable_address = 1.5;
}
This involves the dereference operator. Sorry if this is too much info, but pass by reference and pass by value are easier to understand if you know what’s happening in the underlying function call and memory architecture of C/C++.