Inheriting a method from a base class (vehicle) and value from a derived class (car) to implement in another derived class (lane) using C++

Question

I have a problem regarding inheritance on C++. The Vehicle class has SetStartingPosition() and GetStartingPosition() functions. The Car class inherits these functions and sets the starting position to 5 in the constructor with SetStartingPosition(5).

What I want is that when a new vehicle (new Car) is added to a lane (in the Lane class) with the use of the AddVehicleToALane(Vehicle* vehicle) function, the position of this Car is assigned the value of 5.

Vehicle.h:

Class Vehicle{
public:
    Vehicle();
    Vehicle(double position);
    virtual ~Vehicle();
    void SetStartingPosition(double startingpos){
        fStartingPos = startingpos
    }

    double GetStartingPosition(){
        return fStartingPos;
    }

private:
    double fStartingPos;
}

Car.h:

#include "vehicle.h"
Class Car: public Vehicle{
public:
    Car();
    Car(double position);
}

Car.cpp:

Car::Car(double position): Vehicle(position){
    SetStartingPosition(5);
}

Lane.h:

#include "vehicle.h"
#include "car.h"
Class Lane{
public:
    void AddRandomVehicle();

    void AddVehicleToALane(Vehicle* vehicle){
     fVehicles.push_back(vehicle);
    }

private:
    std::vector<Vehicle*> fVehicles;
    Vehicle *v;
}

Lane.cpp:

void Lane::AddRandomVehicle(){
    int i = 0;
    AddVehicleToALane(new Car(fVehicles[i]->GetStartingPosition()));
}

The final command v->GetStartingPosition() doesn't work. Basically, when a new instance of Car (new Car) is created, I want this car to obtain the starting position = 5 (from car.cpp), but the final command doesn't work.

Could anyone give me some tips as how to solve this? I don't think it should be too difficult, but I've spent hours trying to figure out and I can't solve it! Any help would be greatly appreciated.

You use the phrase "_doesn't work_" multiple times, but fail to explain _how_ it doesn't work. — Algirdas Preidžius, Jan 02 '18 at 12:33
Welcome to stackoverflow.com. Please take some time to read [the help pages](http://stackoverflow.com/help), especially the sections named ["What topics can I ask about here?"](http://stackoverflow.com/help/on-topic) and ["What types of questions should I avoid asking?"](http://stackoverflow.com/help/dont-ask). Also please [take the tour](http://stackoverflow.com/tour) and [read about how to ask good questions](http://stackoverflow.com/help/how-to-ask). Lastly please learn how to create a [Minimal, Complete, and Verifiable Example](http://stackoverflow.com/help/mcve). — Some programmer dude, Jan 02 '18 at 12:34
What about `AddVehicleToALane(new Car(0));`? Your code passes a position to the constructor and ignores it, so you can pass anything. Not an answer because I could not understand what you are trying to achieve here... — Serge Ballesta, Jan 02 '18 at 12:39
Should using fVehicles[int i]->GetStartingPosition() work? I tried running it, but it crashes every time I run it, but I'm not sure why. — L. Howard, Jan 02 '18 at 12:40
`v` doesn't point to a `Vehicle` (or anything, really), so you cannot call *any* functions using that pointer. — Bo Persson, Jan 02 '18 at 12:41
This line: Car::Car(double position) : Vehicle(position) ... How do you call the Vehicle constructor from the Car constructor and pass it "position"? When Vehicle has no such constructor taking the argument? — Zebrafish, Jan 02 '18 at 12:44
@AlgirdasPreidžius, when i replace v->GetStartingPosition() with 5, the programme runs and the number 5 is stored as the starting position, but i don't know how to inherit the starting position from the car class. — L. Howard, Jan 02 '18 at 12:44
@BoPersson I will edit the code to show what I think another solution could be, replacing v->GetStartingPosition() with fVehicles[i]->GetStartingPosition(). Should this work? — L. Howard, Jan 02 '18 at 12:47
@L.Howard - You will have a similar problem with `fVehicle`. It is initially empty, so when you create the first car there is nothing to read from. — Bo Persson, Jan 02 '18 at 12:51
@BoPersson do you have any idea how I can solve that? Can i just initialise i = 0, then use fVehicles[i]->GetStartingPosition()? — L. Howard, Jan 02 '18 at 12:55
@L.Howard The only thing I could suggest is learning from a [good C++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). There are plenty of things wrong with the code you shared, in addition to, your comments suggesting that you are mixing up _instances_ of the class, with the _class_ itself. — Algirdas Preidžius, Jan 02 '18 at 12:55
@AlgirdasPreidžius i want to set the position (equal to 5) of an instance of Car (new Car) instantiated in the Lane.cpp file, inheriting this value of 5 from the Car class. Do you know how i could do that? — L. Howard, Jan 02 '18 at 13:18

Caleth · Answer 1 · 2018-01-02T13:00:31.227

3

There are many things wrong with your code.

Lane::v is never assigned, so when you v->GetStartingPosition(), you have undefined behaviour.

Vehicle doesn't have a constructor that accepts a double, so Car::Car(double position): Vehicle(position) is a syntax error.

Car::Car(double position) is private, so you can only new Car(...) from within Car methods

Why does car take a double position and then overwrite it? why not Car::Car() { SetStartingPosition(5); }?

Why do you have a Car class at all? You don't have anything to distinguish it from Vehicle after it is constructed, and none of the methods of Vehicle are virtual (particularly the destructor!)

edited Jan 02 '18 at 13:00

answered Jan 02 '18 at 12:46

Caleth

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Also, *Car(double position)* is private so he won't be able to create a Car at all. – dsp_user Jan 02 '18 at 12:58
These are helpful tips. @Caleth are you saying that my SetStartingPosition(5) function can be placed in the Car::Car() {} constructor? – L. Howard Jan 02 '18 at 13:07
@dsp_user could you perhaps explain how i could fix that? – L. Howard Jan 02 '18 at 13:08
1

just add public: Car(); Car(double); A class has, unlike a struct, all members private unless explicitly declared public (or protected) – dsp_user Jan 02 '18 at 13:10
@dsp_user and Caleth the main problem i have is that I want to be able to extract the value of 5 from the SetStartingPosition(5) function in the Car.cpp class and put this value of 5 as the position of the instance of the Car "new Car" in the lane.cpp file. Do you understand my problem? – L. Howard Jan 02 '18 at 13:14
does `AddVehicleToALane(new Car(5))` not do what you want? – Caleth Jan 02 '18 at 13:17
1

Note that you still need to define the body for your constructor (*Vehicle(double position) {} * ( in this case you defined an empty body ( {} ), which doesn't make sense because you passed an argument to the constructor so it's better to use something like *Vehicle(double position) { fStartingPos = position; }* or even *Vehicle(double position) : fStartingPos(position) {} * – dsp_user Jan 02 '18 at 13:41
@Caleth you're quite right that does what i want it to do, but doing AddVehicleToALane(new Car(5)) is not inheriting the Starting Position from Car.cpp. Do you see what I mean? – L. Howard Jan 02 '18 at 14:17
@Caleth i want to obtain the value of 5 from the function SetStartingPosition(5) in car.cpp. – L. Howard Jan 02 '18 at 14:19
@Caleth your explanation worked!!! my SetStartingPosition() function was in the wrong constructor!!!!!!! – L. Howard Jan 02 '18 at 14:33
Thank you all for the help, I appreciate it greatly! Keep helping others, it's great. – L. Howard Jan 02 '18 at 14:34

Inheriting a method from a base class (vehicle) and value from a derived class (car) to implement in another derived class (lane) using C++

1 Answers1