Labeeb Maqsood Labeeb Maqsood - 8 months ago 83
C++ Question

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

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;


//Node for linked list to store Non terminals

struct rule_list{

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


// 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;
prev_r->next = temp_r;
prev_r = temp_r;

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;

cout << "Error: Unable to allocate memory. " << endl;
return NULL;
return next_ptr;
//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;
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 **

Answer Source

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.


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