Using abstract base class, invalid conversion

[SkiesBblue]

Hi experts,

Why does line parts = new Tire*[numOfParts]; generate the error invalid conversion from `Tire**' to `CarPart**' in file car.cpp? The program works if I change the variable parts to be of type Tire** instead of CarPart**.

I formatted my problem to a complete simple (though multifile) program, to make it readable and possibly useful for others having similar problems. The idea is to have a tree like structure that represents a car. A part of the car, say a tire, is connected to the car thus must get some information of the car. So, I'm passing a pointer of the car to the tire. I got that working after some trouble. But because all the parts of the car will have similar functions, I have an abstract base class CarPart. Now I'm trying to assign a pointer to Tire to a variable of pointer to CarPart. This is suppose to be whole idea of abstract base class, so why the error?

Files of the program:

carPart.h

#ifndef carPart_H
#define carPart_H
#include "vectors.h"
using namespace std;

class CarPart {                    //CAR PART (ABSTRACT BASE CLASS)
      
    public:
        CarPart* parent;           //Pointer to the parent of this part
        Vector3f vel;              //Velocity vector in the world
        void addParent(CarPart* parent) {this->parent = parent;}
        virtual void setVel(Vector3f vel) = 0;        //A pure virtual function
};
#endif

tire.h

#ifndef tire_H
#define tire_H

#include "vectors.h"
#include "carPart.h"
#include "car.h"

using namespace std;


class Tire : public CarPart{
    public:
        //These are inherited from the base class: Adding indentical redefinition here
        //wouldn't stop from compiling, but has proven to cause problems, when trying
        //access these member via pointer to instance
            /*    
            CarPart* parent;           //Pointer to the parent of this part
            Vector3f vel;              //Velocity vector in the world
            void addParent(CarPart* parent) {this->parent = parent;} */
            
        //Adding possibility to have parts:
        CarPart** parts;
        int numOfParts;
        
        void setVel(Vector3f vel);    //Adding definition for function setVel
        Tire();                        //Constructor
};
#endif

tire.cpp

#include "vectors.h"
#include "carPart.h"
#include "car.h"
#include "tire.h"

using namespace std;
 
void Tire::setVel(Vector3f vel)    //Adding definition for function setVel
{   
    this->vel = vel;
}

Tire::Tire()   //Constructor
{
    vel = Vector3f(0.0f,0.0f,0.0f);
}

car.h

#ifndef car_H
#define car_H

#include "vectors.h"
#include "carPart.h"
#include "tire.h"

using namespace std;

class Car : public CarPart{
    public:
        //These are inherited from the base class: Adding indentical redefinition here
        //wouldn't stop from compiling, but has proven to cause problems, when trying
        //access these member via pointer to instance
            /*    
            CarPart* parent;           //Pointer to the parent of this part
            Vector3f vel;              //Velocity vector in the world
            void addParent(CarPart* parent) {this->parent = parent;} */
        
        //Adding possibility to have parts:
        CarPart** parts;
        Tire** tires;
        int numOfParts;
        
        void setVel(Vector3f vel);    //Adding definition for function setVel
        Car();                        //Constructor
};
#endif

car.cpp

#include "vectors.h"
#include "carPart.h"
#include "car.h"
#include "tire.h"

using namespace std;

//THIS WORKS
Car::Car()
{
    vel = Vector3f(5.0f,5.0f,5.0f);
    
    numOfParts = 4;
    tires = new Tire*[numOfParts]; 
    for (int i = 0; i < numOfParts; i++)
    {
        tires[i] = new Tire();
        tires[i]->addParent(this);
    }
    
    tires[0]->vel = Vector3f(1.0f,1.0f,1.0f);
    tires[1]->vel = Vector3f(2.0f,1.0f,1.0f);
    tires[2]->vel = Vector3f(3.0f,1.0f,1.0f);
    tires[3]->vel = Vector3f(4.0f,1.0f,1.0f);
}
/*
Car::Car()  //DOESN'T WORK, WHY??
{
    vel = Vector3f(5.0f,5.0f,5.0f);
    
    numOfParts = 4;
    parts = new Tire*[numOfParts];       //ERROR MESSAGE: invalid conversion from `Tire**' to `CarPart**' 
    for (int i = 0; i < numOfParts; i++)
    {
        parts[i] = new Tire();
        parts[i]->addParent(this);
    }
    
    parts[0]->vel = Vector3f(1.0f,1.0f,1.0f);
    parts[1]->vel = Vector3f(2.0f,1.0f,1.0f);
    parts[2]->vel = Vector3f(3.0f,1.0f,1.0f);
    parts[3]->vel = Vector3f(4.0f,1.0f,1.0f);
}
*/
void Car::setVel(Vector3f vel)    //Adding definition for function setVel
{   
    this->vel = vel;
}

main.cpp

#include <iostream>

#include "vectors.h"
#include "car.h"
#include "carPart.h"
#include "tire.h"

using namespace std;         

int main (int argc, char *argv[])
{ 
    
    Car myCar;
    
    Tire newTire;

    char quit;  

    quit = '\0';
    while (quit != 'q')
    {  
        cout << "Hello ! Testing set of Car classes." << endl;
        cout << "" << endl;
        cout << "Press q and enter to quit." << endl;
        cout << "\n";
        cout << "\nThe car seems to have access to it's parts:";
        cout << "\n";
        cout << "\nmyCar.tires[0]->vel.x: " << myCar.tires[0]->vel.x;
        cout << "\nmyCar.tires[0]->vel.y: " << myCar.tires[0]->vel.y;
        cout << "\nmyCar.tires[0]->vel.z: " << myCar.tires[0]->vel.z;
        cout << "\n";
        cout << "\nmyCar.tires[1]->vel.x: " << myCar.tires[1]->vel.x;
        cout << "\nmyCar.tires[1]->vel.y: " << myCar.tires[1]->vel.y;
        cout << "\nmyCar.tires[1]->vel.z: " << myCar.tires[1]->vel.z;
        cout << "\n";
        cout << "\nAnd great, the tire also finds it's parent:";
        cout << "\n";
        cout << "\nmyCar.tires[0]->parent->vel.x: " << myCar.tires[0]->parent->vel.x;
        cout << "\nmyCar.tires[0]->parent->vel.y: " << myCar.tires[0]->parent->vel.y;
        cout << "\nmyCar.tires[0]->parent->vel.z: " << myCar.tires[0]->parent->vel.z;
        cout << "\n";
        cout << "\nmyCar.tires[1]->parent->vel.x: " << myCar.tires[1]->parent->vel.x;
        cout << "\nmyCar.tires[1]->parent->vel.y: " << myCar.tires[1]->parent->vel.y;
        cout << "\nmyCar.tires[1]->parent->vel.z: " << myCar.tires[1]->parent->vel.z;
        cout << "\n";
        cin >> quit;
    }

    return 0;
}

[kvprajapati]

>parts = new Tire*[numOfParts]; generate the error invalid conversion from?
Your code is about to create/allocate an array of Tire pointers which is illegal in this context. Create an array of pointers of CarParts.

parts = new CarPart*[numOfParts];   
    for (int i = 0; i < numOfParts; i++)
    {
        parts[i] = new Tire();
        parts[i]->addParent(this);
    }

[SkiesBblue]

That was fast and it works! Thank you very much! :) In the aftermath, the solution seems obvious, but it could have taken days to figure it out by myself!