Adding to a private variable not working - c++

Question

I'm currently learning c++ and trying to make a vending machine! I know my code is really bad and I am sorry about that.
I am trying to implement a bank and have the user take a loan from it, the only problem is that the bank is unable to add money to the user. Here's my code.
void Bank::askLoan() {
//ColaMachine object
ColaMachine cola;
bool loanGranted = false;
cout << string(100, '\n');
cout << "You do not have enough money!\n\n";
cout << "Would you like to take a loan?\n\n(1)-Yes\n(2)-No\n\n\n";
int choice;
cin >> choice;
switch (choice) {
case 1:
//Print the bank menu!
printBank();
while (loanGranted != true) {
cout << "Enter the amount to lend: $";
cin >> _loanValue;
//Test if _loanValue is less than or = to bankmoney, so they would scam the bank.
if (_loanValue <= _bankMoney) {
//Supposed to add money to the user.
cola.addMoney(_loanValue);
break;
}
else {
cout << "You entered too much! Try again..." << endl;
}
}
break;
case 2:
//User does not want to take a loan! Quit the game!
//Not implemented, yet.
break;
default:
cout << "Bad input! Please retry..." << endl;
}
}
If the amount entered was within the correct range it calls the addMoney() Func from ColaMachine class.
void ColaMachine::addMoney(int money) {
//This part doesnt seem to modify the actual value
//Whenever It goes back to the main game loop it doesnt change.
_money += money;
}
From what I understand += is the same as _money = _money + money;
What am I doing wrong here?
Full source on GitHub-
https://github.com/Rohukas/-LearningCPP

Answer 1

The problem is that you are creating new cola object inside askLoan() method which gets destroyed at the end of the function, so call to addMoney() method modifies state of that temporary cola object. One option is to provide cola object by pointer to the askLoan() method.
For example, in ColaMachine::chooseDrink() you would call bo.askLoan(this). this is pointer to the object from where you call bo.askLoan().
You would need to modify your askLoan() signature:
void askLoan(ColaMachine * cola) and remove ColaMachine cola; from askLoan() itself.

Answer 2

The problem is here
void Bank::askLoan() {
//ColaMachine object
ColaMachine cola;
bool loanGranted = false;
...
everytime you call Bank::askLoad you create a new ColaMachine, that's what the code above says. But the problem is that any changes to that ColaMachine are thrown away when you exit Bank::askLoan and the ColaMachine is destroyed. That's why the changes you make to the ColaMachine don't stick.
Instead you want to use same ColaMachine each time that you call Bank::askLoad. I can't say the best way to do that without seeing the rest of your code, but one way would be to pass ColaMachine as a reference parameter to Bank::askLoad
void Bank::askLoan(ColaMachine& cola) {
bool loanGranted = false;
...
Another way would be to make ColaMachine a class member variable of Bank
class Bank
{
...
void askLoan();
private:
ColaMachine cola;
};
Which is better? No idea. I think you need to read up on class design, and how the objects in your program should relate to each other. That seems to be the bit you're not getting at the moment.
=========================================================================
Having looked at your full code I can see that you make the same mistake in more than one place. In your main function you declare a Bank and a ColaMachine.
//ColaMachine Object
ColaMachine cola;
//Bank Object
Bank bank;
Those should be the only two Bank and ColaMachine objects you create, so this is wrong
void ColaMachine::chooseDrink() {
Bank bo;
...
bo.askLoan();
That Bank bo is a completely different bank from the one you declared in main. And like the previous code this Bank bo gets created and destroyed each time you call ColaMachine::chooseDrink.
I think that what you should be doing is passing the ColaMachine and Bank variables declared in main as reference parameters to the other parts of you code that need to use them. So (for instance)
void ColaMachine::chooseDrink(Bank& bank) {
...
bank.askLoan();
You have quite a lot of rewriting of this code to do.