Accessing and changing parent values from subclass instances

Question

I am a beginner and tryed to find an answer on this website but I was not able to figure out how to solve my specific C++ OOP problem.

Short: I want to access and change values of a parent class from subclass instances, but somehow my approach seems not to be working.

Example: There are many Car instances in my program (created with new construct). If one of the Car objects detects a collision, all Car instances should inverse their movement. The Car that registered the collision should call the parents' ChangeSpeed method or change the speed value directly.

Problem: the speed variable seems not to be updated. Is there something wrong with this particular code/ approach or do I have to search for my problem somewhere else?

// SpeedControl.h ------------------------------
class SpeedControl
{
public:
    void ChangeSpeed(int);
protected:
    int speed;
};

class Car:
        public SpeedControl
{
public:
    void MoveCar();
    void DetectCollision();

private:
    int position;
};


// SpeedControl.cpp ------------------------------
#include SpeedControl.h

SpeedControl::SpeedControl(void)
{
    speed = 10;
}

SpeedControl::~SpeedControl(void)
{
}

SpeedControl::ChangeSpeed(int _value)
{
    speed *= _value;
}


// Car.cpp ------------------------------
#include SpeedControl.h

Car::Car(void)
{
    position = 100;
}

Car::~Car(void)
{
}

Car::MoveCar()
{
    position += speed; // speed should be accessible?
}

Car::DetectCollision()
{
    speed *= (-1); // inverse speed variable in parent class to inverse direction of ALL cars
    // alternative:
    // ChangeSpeed(-1); // call parent function to inverse speed

}

change *class Car:SpeedControl* to *class Car: public SpeedControl* (http://stackoverflow.com/questions/3811424/default-class-inheritance-access) — thang, Feb 06 '13 at 12:47
You should also stay *what exactly is not working*. In this case, it's as thang suggested, private inheritance. — Bartek Banachewicz, Feb 06 '13 at 12:48
you've changed the question now. speed is now accessible. remove that comment. inheritance is **the wrong construct to use here**. you want aggregation. create SpeedControl as a separate object. delare a (smart)pointer to it in Car. have all your cars share the same SpeedControl. — thang, Feb 06 '13 at 13:00
specified my problem in the text, sorry. also my "SpeedControl" was already public, I only forgot to type it in. — user2046619, Feb 06 '13 at 13:00
so then speed is accessible. what is with that comment that implies that speed is not accessible. — thang, Feb 06 '13 at 13:01

score 0 · Answer 1 · answered Feb 06 '13 at 12:48

0

This looks like a problem where the observer pattern is the right solution.

answered Feb 06 '13 at 12:48

Marius Bancila

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score 0 · Answer 2 · answered Feb 06 '13 at 12:49

0

You should implement a GetSpeed() in your SpeedControl class, which returns the current speed.

Then you can use this in MoveCar function:

ChangeSpeed(GetSpeed() + GetSpeed());  // Assuming you want to double the speed.

and in your collision:

ChangeSpeed(-GetSpeed());

Alternatively, you could make "ChangeSpeed" a multiplication rather than an addition.

answered Feb 06 '13 at 12:49

Mats Petersson

126,704
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this wouldn't work... because the car class won't have access to GetSpeed. – thang Feb 06 '13 at 12:57

score 0 · Answer 3 · answered Feb 06 '13 at 12:49

You didn't state what exactly is wrong with your code, but from glancing on it, you're using private inheritance, which hides all parent's members from derived classes.

class Child : public Parent { };

The line above will ensure that the Parent members are accessible from Child.

score 0 · Answer 4 · answered Feb 06 '13 at 12:52

0

You seem to misunderstand inheritance. What you need is a static variable.

Define static int speed in Car class. You can drop SpeedControl class.

answered Feb 06 '13 at 12:52

palindrom

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Accessing and changing parent values from subclass instances

4 Answers4