C++ Passing a set of objects to a member functions of abstract class

Question

I would like to pass objects to a member functions of class and execute some functions from within it. When I do this using the *myCars pointers, everything works fine. But when I try to pass this to do the same for a pure virtual class, the program returns error (see Solution A). I've got a temporary solution (see Solution B), but it does not seem right to me. Any idea to fix it?

main.cpp

#include <iostream>
#include "car.h"
#include "veicles.h"
#include "traffic.h"

int main(){

        int number = 3;
        Car *myCars = new Car[number];

        Traffic mtc;
        mtc.import(myCars, number);

        Veicles *myVeicles = new Car[number];

//      Solution A
//      Traffic mtvA;
//      mtvA.import(myVeicles, number);

//      Solution B
        std::cout << "\nsolution B\n";
        Traffic mtvB;
        mtvB.import(&myVeicles, number);

        delete[] myCars;
        delete[] myVeicles;

        return 0;
}

car.h

#ifndef CAR_H
#define CAR_H

#include <iostream>
#include "veicles.h"

class Car : public Veicles{
public:
    void move();

};

#endif

car.cpp

#include "car.h"

void Car::move(){

        std::cout << "the car is moving\n";
}

traffic.h

#ifndef TRAFFIC_H
#define TRAFFIC_H

#include "car.h"

class Traffic{
public:

    void import(Car p[], int number);
    void import(Veicles *v[], int number);
private:

    Veicles *vc;

};

#endif

traffic.cpp

#include "traffic.h"
#include "car.h"

void Traffic::import(Car p[], int number){

        for(int i = 0; i < number; i++){
                p[i].move();
        }
} 

void Traffic::import(Veicles *v[], int number){

        vc = *v; 
        for(int i = 0; i < number; i++){
               // vc -> move();
               vc[i].move(); //  EDIT: R Sahu sugestion
        }
}

veicles.h

#ifndef VEICLES_H
#define VEICLES_H

class Veicles{
public:

    virtual void move() = 0;
};

#endif

veicles.cpp

#include "veicles.h"

// some stuffs to do latter..

makefile

all:    clean car veicles traffic main
        g++ car.o traffic.o main.o -o main.out
run:
        ./main.out

car:    car.h
        g++ -c car.cpp -o car.o

veicles: car.h veicles.h
        g++ -c veicles.cpp -o veicles.o

traffic: traffic.h car.h
        g++ -c traffic.cpp -o traffic.o


main: traffic.h car.h
        g++ -c main.cpp -o main.o

clean:
        rm -f *.o
        rm -f *.gch

Answer 1

  void import(Car p[], int number);

is the same as

  void import(Car* p, int number);

You may not call it with an argument of type Vehicle* since a base class pointer cannot be automatically converted to derived class pointer.

Imagine you had:

class Truck : public Veicles{
 ...
};

and

Veicles *myVeicles = new Truck[number];

If the compiler let you call that function, you would suddenly able to use a Truck where a Car was expected. That would lead to disaster.

---

The following function works

void Traffic::import(Veicles *v[], int number){
   vc = *v; 
   for(int i = 0; i < number; i++){
      vc -> move();
   }
}

However, please note that you are not using i to access the elements of vc. You are calling move() on the first object only.

The following would be a problem.

void Traffic::import(Veicles *v[], int number){
   vc = *v; 
   for(int i = 0; i < number; i++){
      vc[i].move();
   }
}

The reason for that is you may not treat an array of derived class objects as an array of base class objects.