Displays nothing when debugged

Question

Possible Duplicate:
How to pass objects to functions in C++?

Main class

#include "List.h"
#include "Car.h"
#include "Worker.h"
#include "Queue.h"
#include <iostream>
#include <string>

using namespace std;
void initWorkerList(List<Worker>);
void initCarList(List<Car>, Queue, Queue);

int main() {
    List<Worker> WorkerList;
    List<Car> CarList;
    Queue q1, q2;
    initWorkerList(WorkerList);
    initCarList(CarList, q1, q2); // Error here
        //..... e.g cout << "Successful!"; but it does not displays it...
}

void initWorkerList(List<Worker> WorkerList) {
    Worker w1 = Worker("Ben Ang", "Ben123", "pass123", 'M');
    WorkerList.add(w1);
    Worker w2 = Worker("Grace Eng", "Gr4ce", "loveGrace", 'W');
    WorkerList.add(w2);
    Worker w3 = Worker("Rebecca Xuan", "Xuanz", "Rebecca Xuan", 'W');
    WorkerList.add(w3);
}

void initCarList(List<Car> CarList, Queue q1, Queue q2) {
    Car c1 = Car("SJS1006Z","Toyota", "Saloon car");
    Car c2 = Car("SFW6666E", "hyundai", "Taxi (Saloon)");
    Car c3 = Car("SCF1006G","Mercedes", "Large Van");
    Car c4 = Car("SBQ1006Z", "Puma", "Saloon Car");

    q1.enqueue(c1);
    q2.enqueue(c1);
    q2.enqueue(c3);
    q1.enqueue(c4);
    q1.enqueue(c1);
    q1.enqueue(c1);
    q1.enqueue(c1);
    q2.enqueue(c2);
    q2.enqueue(c2);
}

There is no error at all. But nothing is displayed when being debugged...I have tried and my guess is there is something wrong with initCarList(CarList,q1,q2); cause after that code, other codes can work at all. Is there anything wrong with it? Thanks

Umm...considering you are not printing anything it seems like reasonable behavior to me... You're also taking your function arguments by value, so you're modifying a copy that ceases to exist when the function returns. — Ed S., Jan 13 '13 at 06:49
Try reading: [How to pass objects to functions in C++?](http://stackoverflow.com/questions/2139224/how-to-pass-objects-to-functions-in-c) — WhozCraig, Jan 13 '13 at 06:53
@Sandeep compiled it with -g? is that a website or application? — unknown, Jan 13 '13 at 06:55

Abhijit · Accepted Answer · 2013-01-13T08:10:49.443

You are passing the Queue Variables by value rather than by reference.

initCarList(CarList, q1, q2); // Error here

So any change in initCarList wont get reflected back to caller

Change your function signature to

void initCarList(List<Car> CarList, Queue& q1, Queue& q2) {

and the declaration to

void initCarList(List<Car>, Queue&, Queue&);

If you pass parameter by value, any change within initCarList is local to the function scope and does not get reflected back.

Pass by Value
Caller              Callee

|------|            |------|
workedList          workedList
| ___  |            | ___  |
||   | |-------->   ||   | | <------
||___| |            ||___| |       |
|      |            |      |       |
|q1    |            |q1    |     (Changing any of these variables 
| ___  |            | ___  |      won't be reflected back)
||   | |-------->   ||   | |       |
||___| |            ||___| |       |
|      |            |      |       |
|q2    |            |q2    |       |
| ___  |            | ___  |       |
||   | |-------->   ||   | |       |
||___| |            ||___| | <------
|______|            |______|


Pass by reference

Caller              Callee

--------            --------
|wList |            |wList |
| ___  |            | ____ |
||   | |-------->   ||    || <------
||___|<|------------||-*__||       |
|   _  |            |      |       |
|q1    |            |q1    |     (Changing any of these variables 
| ___  |            | ____ |       will be reflected back)
||   | |-------->   ||    ||       |
||___|<|------------||-*__||       |
|      |            |      |       |
|q2    |            |q2    |       |
| ___  |            | ____ |       |
||   | |-------->   ||    ||       |
||___|<|------------||-*__|| <------
|______|            |______|

thanks! I understand now thanks to your drawing! :D – unknown Jan 13 '13 at 07:15 — unknown, Jan 13 '13 at 07:15
Wow, that's some nice ASCII art :D – chris Jan 13 '13 at 07:18 — chris, Jan 13 '13 at 07:18

score 2 · Answer 2 · answered Jan 13 '13 at 06:50

2

You're passing your variables in by value, which means that the function's parameters hold a copy of them, which you modify and discard when the function ends. Pass by reference instead to modify the original variable. For example, initCarList would become:

void initCarList(List<Car> CarList, Queue &q1, Queue &q2)

You also don't use the CarList parameter, so you might as well take it out if this is how it is in your code.

answered Jan 13 '13 at 06:50

chris

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OK. I have removed the carlist alr. It works too! Thanks! :D – unknown Jan 13 '13 at 07:16

score 2 · Answer 3 · answered Jan 13 '13 at 06:53

Your functions pass by value which means functions make a copy of passed in variables and manipulate on the copied ones. To modify on original ones you need to pass parameter by reference

Change:

void initWorkerList(List<Worker>);
void initCarList(List<Car>, Queue, Queue);

To:

void initWorkerList(List<Worker> &);
void initCarList(List<Car>&, Queue&, Queue&);

Displays nothing when debugged

3 Answers3