C++ :: How To Pass Function Return From Switch

How to do i fix this code im not good at using function, im trying to pass a statement from switch to my double passengersR i always come out with a error

this output:
Name passenger:
Enter number passenger:
Choose from A to D
if choose A
Total Regular Passenger: 6.19624e-312

did i do something wrong im not really good at function and im trying to learn how to pass switch to void function

Code:

#include<conio.h>
#include <iostream>
#include <Iomanip>
#include <string>
using namespace std;
double passengersR ()
{ double m;

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