Question

I am trying to write this code where there are 2 classes inherited from a parent class.But this code breaks when I try to use it in a link list.Here is the code

class airship{

public:
    airship(){};
//  virtual ~ airship();
    virtual void setData (string)=0;
    virtual void showData()=0;
    void setNext(airship* newptr);
    airship* getNext();
    void setType(int n){airshipType=n;}
    int getType(){return airshipType;}
    void setCount(int n){maxPassengerCount=n;};
    int getCount(){return maxPassengerCount;}
    void setWeight(int n){maxCargoWeight=n;}
    int getWeight(){return maxCargoWeight;}

protected:
    int maxPassengerCount;
    int maxCargoWeight;
    int airshipType;

private:
    airship* next;
};

class airplane: public airship{

protected:

    const char* airplaneName;
    int engineType;
    double range;

public:
    airplane():airship(){};
//  ~ airplane();
    void setData(string);
    void showData();
    void setEngine(int n){engineType=n;}
    int getEngine(){return engineType;}
    void setRange(int n){range=n;}
    int getRange();

};

class balloon : public airship{

protected:
    const char* balloonName;
    int gasType;
    double maxAltitude;

public:
    balloon():airship(){};
//  ~balloon();
    void setData(string);
    void showData();
    void setGas(int);
    int getGas();
    void setAlt(int);
    int getAlt();

};

class mylist{

private:
    airship* headAirship;
public:
    void createlist(fstream&);
    void setAirship(airship* Node){Node=headAirship;}
    airship* getAirship(){return headAirship;}
};

void airship::setNext(airship* Node)
{
    this->next=Node;
}

void airplane::setData(string line)
{
    string line1;
    bool flag=true;
    int npos=0,nend,count=0;
    while(line.c_str()!=NULL && flag)
    {
        nend=line.find(",",npos);

        if (nend!=-1)
        {
            line1=line.substr(npos,nend);
        }
        else
        {
            line1=line;
            flag=false;
        }
        line=line.substr(nend+1,line.length());

        if (count==0)
            this->airshipType=atoi(line1.c_str());
        else if(count==1)
            this->airplaneName=line1.c_str();
        else if(count==2)
            this->maxPassengerCount=atoi(line1.c_str());
        else if(count==3)
            this->maxCargoWeight=atoi(line1.c_str());
        else if(count==4)
            this->engineType=atoi(line1.c_str());
        else
            this->range=atoi(line1.c_str());

        count=count+1;
    }
}

void balloon::setData(string line)
{
    string line1;
        bool flag=true;
        int npos=0,nend,count=0;
        while(line.c_str()!=NULL && flag)
        {
            nend=line.find(",",npos);

            if (nend!=-1)
            {
                line1=line.substr(npos,nend);
            }
            else
            {
                line1=line;
                flag=false;
            }
            line=line.substr(nend+1,line.length());

            if (count==0)
                this->airshipType=atoi(line1.c_str());
            else if(count==1)
                this->balloonName=line1.c_str();
            else if(count==2)
                this->maxPassengerCount=atoi(line1.c_str());
            else if(count==3)
                this->maxCargoWeight=atoi(line1.c_str());
            else if(count==4)
                this->gasType=atoi(line1.c_str());
            else
                this->maxAltitude=atoi(line1.c_str());

            count=count+1;
        }

}

void mylist::createlist(fstream &myfile)
{

    string ipdata;
    int type;

    while (getline(myfile,ipdata))
    {
            airship* newNode;
            type=ipdata.find(",",0);
            string ch=ipdata.substr(type-1,type);
            if (atoi(ch.c_str())==0)
            {
                airplane* planeNode=new airplane();
                planeNode->setData(ipdata);
                newNode=planeNode;
            }
            else
            {
                balloon* balloonNode=new balloon();
                balloonNode->setData(ipdata);
                newNode=balloonNode;
            }
            newNode->setNext(headAirship);
            this->headAirship=newNode;

    }


}


int main()
{
    mylist* list=0;
    airship* firstNode=0,*otherNode=0;

    /*if(argv[1]==NULL)
    {   
        cout<<"File not Found"<<endl;
        return -1;      
    }*/

    fstream ipFile("data.txt",ios::in | ios::out);

    if(ipFile.is_open())
    {
        list->createlist(ipFile);
    }

    list->setAirship(firstNode);


    return 0;
}

The airplane::setdata(string) function does not work properly and it is not able to detect headAirship pointer in the createList function.And that is why I get a memory exception error in the createlist function when I try to do this:newNode->setNext(headAirship);

Unhandled exception at 0x00912ae0 in Assignment4Solution2.exe: 0xC0000005:
Access violation reading location 0x00000000.

Answer 1

I think the problem is here:

mylist* list=0;

In that line, you create a pointer to a myList object, but you don’t make it point to any such object; it’s pointing to address 0

Then you do

list->createlist(ipFile);

Since list is not pointing an address containing a myList object, invoking any of the myList methods is supposed to give the error you observed: Access violation reading location 0x00000000.

Instead, when you create the list pointer, invoke the constructor for myList so that the object gets created and the pointer is initialized to point to it

myList * list = new myList();

Hope that helps 🙂