terminate called after throwing an instance of 'std::logic_error' what(): basic_string::_S_construct null not valid

Question

I have been trying to read cfg into a multilinklist but im getting this error please someone help. It keeps on giving this error. I dont think there's any error. Please check this code and tell me where is the error which causes the program to crash with the exception attached in the picture

---

 #include <iostream>
    #include <fstream>
    #include <cstring>

    using namespace std;

    struct prod_list{

       string production;
     struct prod_list *next;

    }*head_p=NULL;

    //Node for linked list to store Non terminals

    struct rule_list{

      char non_terminal;
      struct prod_list *first;
      struct rule_list *next;

    }*head_r=NULL;

    // Global variable
       struct prod_list *temp_p,*prev_p=NULL,*first_p=NULL,*temp1_p=NULL,*temp2_p,*newTemp;
       struct rule_list *temp_r,*prev_r=NULL,*first_r=NULL ,*temp1_r=NULL;



    prod_list *adding_productions_of_the_rule(string line);
    prod_list *add_node(prod_list *root, string production);


    int main()
    {

    string line;

    ifstream file ("cfg.txt");
    if (file.is_open())
    {
      while ( getline (file,line) )
      {
            head_p = NULL;

            // Creating rule_list node
            temp_r = new rule_list;
            temp_r->non_terminal = line[0];
            head_p = adding_productions_of_the_rule(line);
            temp_r->first = head_p;
            temp_r->next = NULL;

            if(head_r == NULL)
            {
                head_r = temp_r;
                prev_r  = temp_r;
            }
            else
            {
                prev_r->next = temp_r;
                prev_r = temp_r;
            }


      }
      file.close();
    }
    else
    {
        cout << "Unable to open file";
    }

    return 0;
    }







    prod_list *adding_productions_of_the_rule(string line)
    {

            string prod = NULL;

            // Splitting line after arrow in cfg rule line.
            line = line.substr(3,line.length());
            // cout << "The cfg is "+line << endl;

            // declaring character array of line's length
            char ch[line.length()]={0};
            // memset(ch,'0',line.length());
            // cout << sizeof(ch) << endl;


            // storing line in character array and printing character array
            for(int i=0; i<line.length(); i++){
                ch[i] = line[i];
                // Displaying the characters stored in character array from string line.
                // cout << ch[i];
            }
            // cout << endl;

            char *point;
            point = strtok(ch, "|");
            while(point != NULL){

                 // cout << point << endl;

                // Assigning each production from the cfg rule to an array of strings
                // production[i] = point;
                // cout << production[i] << endl;


                // Creating prod_list node
                prod = point;
                head_p = add_node( head_p, prod );


                point = strtok(NULL, "|");
                // i++;

            }


            return head_p;
    }






    prod_list *add_node(prod_list *root, string production){


    prod_list *next_ptr = NULL;
    prod_list *prev_ptr = NULL;

    if(root == NULL){
        //list is empty
        if( (root = new prod_list) != NULL){
            next_ptr = root;
            // cin >> next_ptr->title;
            next_ptr->production = production;
            next_ptr->next = NULL;

        }else{
            cout << "Error: Unable to allocate memory. " << endl;
            return NULL;
        }
        return next_ptr;
    }else{
        //list has members.
        next_ptr = root;
        while(next_ptr->next != NULL){
            next_ptr = next_ptr->next;
        }
        prev_ptr = next_ptr;

        if((next_ptr = new prod_list)!= NULL){
            prev_ptr->next = next_ptr;
            next_ptr->production = production;
            next_ptr->next = NULL;
        }else{
            cout << "Error: Unable to allocate memory " << endl;
        }

    }
    return root;
    }

**I have been trying to read cfg into a multilinklist but im getting this error please someone help **

https://i.sstatic.net/AgNXB.png

Answer 1

Your error an be reduced to a very minimal case:

#include <string>
int main() {
    std::string prod = NULL;
}

What's happening is this is trying to make a std::string out of a NULL pointer. Obviously there is nothing there for the string to work with, so it did the only thing it could do: scream for help.

Solution:

Don't assign anything. std::string will default construct an empty string.