c$++,$ How can I use void and double function into the Food Cart Program? thanks

like:

void welcome$(),$

void displayMenu$(),$

void readOption$($int &option$),$

void readInt$($string prompt, int &num$),$

void readDouble$($string prompt, double &num$),$

void placeOrder$($double &cost$),$

double tipDiscount$($double &tip, double &discount, double cost$).$

$*****$

Welcome to my Food Cart Program!

What would you like to do today?$$

Pick an option from below:

Place an order

Quit

$>>9$

Invalid Option! Please choose $1-2!$

$>>1$

Enter the name of your item: Pasta

Enter the cost of your item $: $15\mathrm{.}75$

Do you want another item? $($y$/$n$)$: y

Enter the name of your item: Bowl

Enter the cost of your item $: $12\mathrm{.}75$

Do you want another item? $($y$/$n$)$: y

Enter the name of your item: Soda

Enter the cost of your item $: $3\mathrm{.}50$

Do you want another item? $($y$/$n$)$: x

Invalid Option! Please choose y$/$n$!$

$>>$ n

Your total is: $$32\mathrm{.}00$

Enter the amount of tip you want to add $: $3\mathrm{.}50$

Your total is: $$35\mathrm{.}50$

You get a $5\%$ discount!

Your discount is $$1\mathrm{.}78$

Your final total is: $$33\mathrm{.}73$

What would you like to do today?$$

Pick an option from below:

Place an order

Quit

$>>1$

Enter the name of your item: Fajita Bowl

Enter the cost of your item $: $20\mathrm{.}75$

Do you want another item? $($y$/$n$)$: y

Enter the name of your item: Vietnamese plate

Enter the cost of your item $: $22\mathrm{.}75$

Do you want another item? $($y$/$n$)$: y

Enter the name of your item: Soda

Enter the cost of your item $: $3\mathrm{.}50$

Do you want another item? $($y$/$n$)$: x

Invalid Option! Please choose y$/$n$!$

$>>$ n

Your total is: $$47\mathrm{.}00$

Enter the amount of tip you want to add $: $10\mathrm{.}00$

Your total is: $$57\mathrm{.}00$

You get a $10\%$ discount!

Your discount is $$5\mathrm{.}70$

Your final total is: $$51\mathrm{.}30$

What would you like to do today?$$

Pick an option from below:

Place an order

Quit

$>>2$

Thank you for using my program!