string s;
   getline(ifile, s, '\n');
   stringstream parser(s);
   string temp;
   while(parser>>temp)
      variables.push_back(temp);
   getline(ifile, s, '\n');
   while(parser>>temp)
      terminals.push_back(temp);//Doesn't work the second time.

parser.str("");

#include <iostream>
#include <sstream>
#include <string>
 
int main()
{
  std::stringstream s ( "This is a test" );
  std::string token;
 
  while ( s>> token )
    std::cout<< token <<'\n';
 
  // Clear the stream state
  s.clear();
  // Rewind the stream
  s.seekg ( 0, std::ios::beg );
 
  while ( s>> token )
    std::cout<< token <<'\n';
}

#include<string> 
#include<iostream> 
#include<sstream> 
#include<fstream>
#include<vector>
 
using namespace std;
 
int main()
{
	string input;
	ifstream ifile("input.txt");
 
	getline(ifile,input,'\n');	
	stringstream parser(input);
	string token;
 
	vector<string> variables;
	while(parser>>token)
		variables.push_back(token);//S, A and B have been pushed
 
	getline(ifile,input,'\n');//new content in input
	parser.clear();
	parser.seekg ( 0, std::ios::beg );//clearing and resetting the get ptr doesnt let me use the new content of string input, still the previous string
	vector<string> terminals;
	while(parser>>token)
		terminals.push_back(token);//I expect to push 0 and 1, not S, A and B again,
 
 
}

   parser.clear();
   parser.seekg ( 0, std::ios::beg );//clearing and resetting the get ptr doesnt let me use the new content of string input, still the previous string
   getline(ifile,input);//new content in input
   parser.str(input);
   vector<string> terminals;

 
 string line;
 stringstream s(line);
 string token;
 while(s>>token)
    cout<<token;
 
 s.clear();//Solved my problem after using these three lines together.
 s.seekg(0,ios::beg);
 s.str(line);
 
 while(s>>token)
    cout<<token;