Programming Forums - problem with user defined class mixed with functions

Hi, I am a beginner in C++ programming. Not only am I new to the language, I only recently registered with this website; so please bear with me.

My question is rather lengthy:
I am trying to create a program for my college C++ class that keeps an inventory for a very simple convenience store. It only sells 5 things: gas, beer, soda pop, candy and chips. Each item has its own price. My job is to write an inventory tracking program so that as a customer comes through, I can track how much a customer owes and track home much inventory I have.

I need to:
1. Print the initial inventory. Printing of the inventory should be done in tabular format, with all items lining up in columns.
2. Prompt the user for the quantity of a product to buy. The program will print the price of the item to the user.
3. Continue prompting until the user finishes. The program will print the updated inventory.

I also need to:
create a class "inventory", which contains the quantity of each of the five items (as an integer) and the price of each of the five items (as a double).

The requirements are basically to have a user defined class, use a switch statement somewhere in the code, and to have 7 functions.

The functions purposes and their declarations are:
1. Inventory int(int,double,int,double,int,double,int,double,int,double). This function takes in the initial quantities and the price for each of the 5 items. It returns an Inventory.
2. void report (Inventory). This function provides a report of the present inventory. This includes the quantity and price of each item and the total worth of the present inventory.
3. Inventory gas (Inventory, int). This function takes in an Inventory and a quantity of gas. It then provides a price message to the customer and updates the Inventory. It returns the updated Inventory.
4. Inventory pop (Inventory, int). Same as above but for soda pop.
5. Inventory beer (Inventory, int). Same as above but for beer.
6. Inventory candy (Inventory, int). Same as above but for candy.
7. Inventory chips (Inventory, int). Same as above but for chips

Classes and function are new things to me. I don't completely understand them.
Now here's my problem: Although I'm not exactly sure what the problem is, I know the problem lies in either the class definition, the function call, or the function definition. Everytime I run, the errors are:

'gas': cannot convert parameter 1 from 'int' to 'inventory'. ---- regarding the function call
'gas': local function definitions are illegal.----regarding the function definition.

I know my question isn't specific, and I'm basically asking you to "make it work somehow". But if I know how to do just the first function, (gas function takes in an inventory and a quantity of gas. It then provides a price message to the customer and updates the Inventory. It returns the updated Inventory. ) I could probably do the rest.

#include<iostream>

using namespace std;





class inventory

{

public:

        int gas;

        int pop;

        int beer;

        int candy;

        int chips;

        double pgas;

        double ppop;

        double pbeer;

        double pcandy;

        double pchips;

};









        





inventory gas(inventory, int);



int main()

{

        cout<<"Starting Inventory\n"

                <<"============================================\n"

                <<"     Gas     1000   Price: $3.00\n"

                <<"     Pop     1000   Price: $1.00\n"

                <<"    Beer     1000   Price: $2.50\n"

                <<"   Candy     1000   Price: $0.75\n"

                <<"   Chips     1000   Price: $1.00\n"

                <<"============================================\n"

                <<"   Inventory Value:     $8250.00\n\n";



        

        inventory a;

        a.gas=1000;

        a.beer=1000;

        a.pop=1000;

        a.candy=1000;

        a.chips=1000;





        char choice;

        cout.setf(ios::fixed);

        cout.setf(ios::showpoint);

        cout.precision(2);

        

        cout<<"\nWhat would you like to buy?\n"

                <<"Choices are: g(gas), s(sodapop), b(beer), c(candy), p(potato chips), q(quit):\n";

        cin>>choice;

        

        inventory b;

        switch (choice)

        {

        case 'g':

                int gallons;

                

                cout<<"how many gallons?: ";

                cin>>gallons;

                cout<<gas(b.gas, gallons);        //want to create function call

                cout<<"Ok, "<<gallons<<" gallons. That'll be: $"<<endl;

                break;

        case 's':

                cout<<"how many bottles?: ";

                cin>>b.pop;

                break;



        case 'b':

                cout<<"how many bottles?: ";

                cin>>b.beer;

                break;



        case 'c':

                cout<<"how many bars?: ";

                cin>>b.candy;

                break;



        case 'p':

                cout<<"how many bags?: ";

                cin>>b.chips;

                break;



        case 'q':

        cout<<"Present Inventory\n"

                <<"============================================\n"

                <<"     Gas     1000   Price: $3.00\n"

                <<"     Pop     1000   Price: $1.00\n"                        //insert function calls

                <<"    Beer     1000   Price: $2.50\n"

                <<"   Candy     1000   Price: $0.75\n"

                <<"   Chips     1000   Price: $1.00\n"

                <<"============================================\n"

                <<"   Inventory Value:     $8250.00\n\n";

                break;



        default:

                cout<<"That's not a valid choice, choose again.";

        }

        

        inventory gas(inventory, int)    //sad attempt at function definition

        {

                inventory::gas=1000-int;

                inventory::pgas=3*int;

                return (pgas);

        

        }



        











}