C++ Programming Homework, Creating a simple cash register

[Ogre]

The goal of this program is to create a cash register for the sale of three candies. We are suppose to use call functions to perform a sale, show how many customers there were, total revenue and how many candies were sold. We are not allowed to use any global variables besides the constants that I have declared. I am getting an error that says that every value that I am returning is not initialized, I am not understanding how to fix this error. Hopefully this all makes sense, thanks for the help!

#include <iostream>
#include <iomanip>


using namespace std;


const double  mm = 3.50;


const double mandi = 4.0;


const double raisn = 2.5;


double displayCount(double cmm, double cmandi, double craisn);


double makeSale();


void displayRevenue(double rev);


void displayCustomers(double customers);


int menu()
{
    int choice;
    cout << "Enter 1 to make a sale" << "/n" << "Enter 2 to see number of boxes of each type of candy sold"
        << "\n" << "Enter 3 for total revenue" << "\n" << "Enter 4 to see the number of customers " <<
        "\n" << "Enter 5 to close the program" << endl;
    cin >> choice;


    while (choice == 1 || choice == 2 || choice == 3 || choice == 4 || choice == 5)
    {
        if (choice == 1)
        {
        makeSale();
        }
        else if (choice == 2)
        {
            double cmm, cmandi, craisn;
        displayCount(cmm, cmandi, craisn);
        }
        else if (choice == 3)
        {
         displayRevenue(choice);
        }
        else if (choice == 4)
        {
         double customers;
         displayCustomers(customers);
        }
        else if (choice == 5)
        {
            break;
        }
        else
        {
            cout << "please read the options again and choose one on the list" << endl;
        }
    }
    cout << "please try again" << endl;


    return choice;
}


double makeSale()
{
    int a;
    double cmm = 0, cmandi = 0, craisn = 0, customers = 0;
    cout << "Enter 1 for Mike and Ike" << endl << "Enter 2 for Rasiennettes" << endl << "Enter 3 for M & M's " << endl;
    cin >> a;
        double y,rev;
        if (a == 1)
        {
            cout << "What is the quantity? " << endl;
            cin >> y;
            double rev = rev + (y * mandi);
            cmandi = cmandi + y;
            customers++;


        }
        else if (a == 2)
        {
            cout << "What is the quantity? " << endl;
            cin >> y;
            double rev = rev + (y * raisn);
            craisn = craisn + y;
            customers++;
        }
        else if (a == 3)
        {
            cout << "What is the quantity? " << endl;
            cin >> y;
            double rev = rev + (y * mm);
            cmm = cmm + y;
            customers++;
        }
        else
        {
            cout << "please try again" << endl;
        }
    return rev;
    return cmm;
    return cmandi;
    return craisn;
    return customers;
}


double displayCount(double & cmm, double & cmandi, double & craisn)
{
        cout << "You sold " << cmm << " M&Ms and " << cmandi << " Mike and Ikes and " << craisn << " Raisennettes" << endl;
    return 0;
}


void displayRevenue(double & rev)
{
        cout << "The total revenue for the day is " << rev << endl;
}




void displayCustomers(double customers) 
{
        cout << " The number of customers is: " << customers << endl;
}