No errors in code, but when i run the program I get the "Polynomial.exe has stopped working" error

Question

I have this assignment where i am supposed to use a linked list to store polynomials and perform basic mathematical functions on them. I wrote the code to accept and print the polynomial. Eclipse doesnt show any error, the programs runs and accept the coefficients properly, but after that it simply stops working and a dialog box pops up saying "polynomial.exe has stopped working" Here is the code:

#include<iostream>
using namespace std;

    class term
    {
    private:
        int coef;
        int pow;
        term *next;
        friend class polynomial;
    public:
        term(int a,int b)
        {
            coef=a;
            pow=b;
            next=NULL;
        }
    };
    class polynomial
    {
    private:
        term *head;
    public:
        polynomial()
        {
            head=NULL;
        }
        void ini()
        {
            int deg;
            cout<<"Enter Degree of Polynomial";
            cin>>deg;
            head=create(deg);
        }
        term *create(int deg)
        {
            int coeff;
            cout<<"Enter Coefficient for the term x^"<<deg;
            cin>>coeff;
            term q=term(coeff,deg);
            if(deg==0)
            {
            q.next=NULL;
            }
            else
            {
            q.next=create(deg-1);
            }
            return &q;
        }
        void pri()
        {
            term *temp;
            for(temp=head;temp->next!=NULL;temp=temp->next)
            {
                cout<<temp->coef;
            }
        }
    };
    int main()
    {
        polynomial a=polynomial();
        a.ini();
        a.pri();
        return 0;
    }

Can someone tell me why this is happening?

score 1 · Accepted Answer · edited May 23 '17 at 12:20

Each time you call term *create(int deg), you are obtaining a dangling pointer, because it returns the address of a local variable. The local variable being address ceases to exist straight after the function returns. Any de-reference of the returned pointer invokes undefined behviour.

Here is a stripped down version of the member function in question:

term *create(int deg)
{
  term q=term(coeff,deg); // local variable
  return &q;              // Oops!
}

See Can a local variable's memory be accessed outside its scope?

You could greatly simplify your code by de-coupling allocation/storage from program logic. For example, you can use an std::vector to store terms in the polynomial class. You should also de-couple the reading in and printing out of coefficient values from the polynomial class. For example,

class polinomial
{
  // construct polinomial of a certain order
  explicit polinomial(size_t order) : coeffs(order) {}

  // set coefficient of term of degree deg
  // return false if deg beyond order
  bool set_coeff(size_t deg, int coeff)
  {
    if (coeffs.size() < deg) return false;
    coeffs[deg] = coeff;
  }

  // obtain the coefficient of a term of degree deg
  int coeff(size_t deg) const { return coeffs[deg]; }

 private:
  std::vector<int> coeffs;
};

Then implement separate functions or operators to read in and print out coefficients.

@user3430238 I would probably have `polinomial` have an `std::vector` as a storage for terms. `term` would be a simple class, with no pointers. — juanchopanza, Aug 15 '14 at 09:04
@user3430238 I added an example. Basically, keep the classes simple, give them no more than one responsibility, and add functions/helper classes to implement extended functionality. — juanchopanza, Aug 15 '14 at 09:28

score 0 · Answer 2 · answered Aug 15 '14 at 08:41

you can try dynamic creation of variable, something like this one:

class term
{
private:
   int coef;
   int pow;
   term *next;
   friend class polynomial;
public:
   term(int a,int b)
   {
      coef=a;
      pow=b;
      next=NULL;
   }
};
class polynomial
{
private:
   term* head;
public:
   polynomial()
   {
      head=NULL;
   }
   ~polynomial()
   {
      if(!!head)
      {
         delete head;
         head=NULL;
      }
   }
   void ini()
   {
      int deg;
      cout<<"Enter Degree of Polynomial";
      cin>>deg;
      head=create(deg);
   }
   term* create(int deg)
   {
      int coeff;
      cout<<"Enter Coefficient for the term x^"<<deg;
      cin>>coeff;
      term *q= new term(coeff,deg);
      if(deg==0)
      {
         q->next=NULL;
      }
      else
      {
         q->next=create(deg-1);
      }
      return q;
   }
   void pri()
   {
      term *temp;
      for(temp=head;temp->next!=NULL;temp=temp->next)
      {
         cout<<temp->coef;
      }
   }
};
int main()
{
   polynomial a=polynomial();
   a.ini();
   a.pri();
   return 0;
}

No errors in code, but when i run the program I get the "Polynomial.exe has stopped working" error

2 Answers2