链表中的分段错误

Question

I am trying to make a programme for polynomials. I add an element to polynomial using the addTerm function.There appears to be a segfault in the first "else" part of the function.

#include <iostream>
#include <math.h>
using namespace std;
class Polynomial{
 protected:
  class Term{
  public:
    int exponent;
    int coefficient;
    Term *next;
    Term(int exp, int coeff,Term *n){
      exponent = exp;
      coefficient = coeff;
      next=n;
    };
    friend class Polynomial;
   };
   Term *head;
 public:
 Polynomial(){
  head=nullptr;
  head->next=nullptr;
 }
 Polynomial(const Polynomial &p){
  head=nullptr;
  Term* curr=p.head;
  while(curr!=nullptr){
    addTerm(curr->exponent,curr->coefficient);
    curr=curr->next;
  }
   }
 ~Polynomial(){
  Term* curr=head;
  while(curr!=nullptr){
    Term* next=curr->next;
    delete curr;
    curr=next;
  }
  head=nullptr;
  };

  Polynomial & operator = (const Polynomial &p){
  Term* curr=head;
  while(curr!=nullptr){
    Term* next=curr->next;
    delete curr;
    curr=next;
  }
  head=nullptr;
  Term* current=p.head;
  while(current!=nullptr){
    addTerm(current->exponent,current->coefficient);
    current=current->next;
  }
  return *this;

};

void addTerm(int expon, int coeff){
  if(head==nullptr){                              //adeia lista
    Term* t=new Term(expon,coeff,nullptr);
    head=t;
    // cout<<t->exponent<<t->coefficient;
  }
  else{
    if(expon>head->exponent){                  
      Term* temp= new Term(expon,coeff,head);
      temp->next=head;
      head=temp;
    }
    else{

     Term* current=head;
      for(current=head;expon!=current->exponent;current=current->next){
        if(current==nullptr) break;
      }
      if(current->exponent==expon){
        current->coefficient=current->coefficient+coeff;
      }
      else{
        current=head;
        Term* prev=head;
        while(expon<current->exponent){

         prev=current;
          current=current->next;
       }
        Term* temp=new Term(expon,coeff,current);
        prev->next=temp;
     }
}
}

The code I run it on is:

int main(){
 Polynomial p;
 p.addTerm(3,1);
 p.addTerm(2,3);
 }

Answer 1

Your program crashes in the Polynomial constructor which looks like this:

Polynomial() {
  head=nullptr;
  head->next=nullptr;
}

That does not work because you have set "head" to null and then after that you try to access head->next.

You cannot use head->next when head is null. Trying to do so will lead to a "segmentation fault" crash.