how do I change an array inside a structure inside a private class?(c++)

Question

This has just turned into a flame war. I will ask the question again later if I dont figure it out.

The values are changed inside the function, but once the function ends the values go back to what they were set to originally.

class inventory
 {
private:
struct instock {

    double everything[4]; //Gets set to 0-3. Then user changes values.

};

public:
instock stock;
void changegas(double userinput);
void stocking(); //populates the array
void output();  //outputs the array
};

.

void inventory::stocking(){
for(int i=0; i<4; i++){
    stock.everything[i]=i;
 }
}

void inventory::changegas(double input){
double a;
a = input;
stock.everything[0] += a;
std::cout << "Gas remaining: " << stock.everything[0];
}

void inventory::output(){
std::cout << std::endl;
    for(int i=0; i<4; i++){
 std::cout << stock.everything[i];
 }
}

void inventory::changegas(double input){
double a;
a = input;
stock.everything[0] += a;
std::cout << "Gas remaining: " << stock.everything[0];
}

How can I make it so this function changes the value of the array stored in everything permanently?

   int main()
{


backroom.stocking();

int choice;

cout << "What would you like to change? \n1.)Gas\n2.)Tires\n3.)Soda\n4.)Snacks"<<endl;
cin >> choice;
menu(choice, backroom);
backroom.output();




return 0;
}

void menu(int zed, inventory a){
 int z = zed;
 int c = 0;
switch (z){//start switch

case 1:
    cout << "Enter the amount of Gas you want to change: ";
    cin >> c;
    a.changegas(c);
    break;
case 2:
    cout << "Enter the amount of Tires you want to change: ";
    cin >> c;
    a.changetires(c);
    break;
case 3:
    cout << "Enter the amount of Soda you want to change: ";
    cin >> c;
    a.changesoda(c);
    break;
case 4:
    cout << "Enter the amount of Snacks you want to change: ";
    cin >> c;
    a.changesnacks(c);
    break;

    break;
}//endswitch

}// end funct

I know arrays are passed by reference so I thought this would work as is. But clearly I am not understanding something correctly. Still currently learning the language. Maybe I am not passing the array properly?

Answer 1

You're passing backroom by value. You need to alter menu() to take in either an inventory reference or an inventory pointer, preferably the former. This is because when you pass a class object by value, the entire object is copied to a new instance local to the function being called and any changes made to that instance will not effect the original copy.

This should work:

void menu(int zed, inventory& a) {
    int z = zed; //you don't need this by the way
    int c = 0;

    switch (z){//start switch
    case 1:
        cout << "Enter the amount of Gas you want to change: ";
        cin >> c;
        a.changegas(c);
        break;
    case 2:
        cout << "Enter the amount of Tires you want to change: ";
        cin >> c;
        a.changetires(c);
        break;
    case 3:
        cout << "Enter the amount of Soda you want to change: ";
        cin >> c;
        a.changesoda(c);
        break;
    case 4:
        cout << "Enter the amount of Snacks you want to change: ";
        cin >> c;
        a.changesnacks(c);
        break;
    }
}