String & int conversion (c++)

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int main(void){

string s= "12345678";

/*string to int:: c_str() version*/
cout<<atoi(s.c_str())+123<<endl;

/*string to int :: stringstream version*/
int tmp;
stringstream ss1(s);
ss1 >> tmp;
cout<< tmp+4<<endl;

int ints = 123;
/*int to string :: sprintf version*/
char str_2[10];
sprintf(str_2,"%d",ints);
cout<<str_2[2]<<endl;
/*int to string :: stringstream version*/
stringstream ss;
ss <<ints;
string str = ss.str();
cout<<str<<endl;
cout<<str[2]<<endl;

return 0;
}