c++ - Segmentation fault in reversing string program -


i trying reverse string. can explain me why giving me segmentation fault? 

#include <iostream> #include <string> using namespace std; int main(){   string str,rstr;   int len=str.length(),i=0;   cin>>str;   while(str[i]!='\0'){     rstr[--len]=str[i++];   }   rstr[str.length()]='\0';   cout<<rstr;   return 0; }

p.s.: need reverse without using library functions.

i want go way doing it, practice purposes, try changes , start there

#include <iostream> #include <string> using namespace std; int main(){   string str,rstr;   cin>>str;                  // --- moved line   rstr = str;                // --- added line   int len=str.length(),i=0;   while(str[i]!='\0'){    rstr[--len]=str[i++];   }   rstr[str.length()]='\0';   cout<<rstr;   return 0; }

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