c++ - Interesting mistake in string pointer -

i created function transforms number equivalent in given base , prints string. looks flawless gives absurd results. code below should translate 100 base 9 , give "121".

#include <iostream> #include <fstream> #include <string>  using namespace std; void doldur(string *s,int u,int base){     *s="";     while(u!=0){         *s=""+u%base+*s;         u/=base;     }     return; } int main() {     ofstream fout ("dualpal.out");     ifstream fin ("dualpal.in");     int i;     string hey;     doldur(&hey,100,9);     cout<<hey;     cin>>i;     return 0; } 

but ridiculously, prints dualpal.outualpal.outdualpal.out.(also gives other interesting results different bases)

where's flaw?

you're incrementing pointer empty string u%base places , using construct std::string, looks null terminator. causes undefined behaviour. use std::string off bat:

*s = std::string() + ...; 

next, there's no conversion int std::string. use function std::to_string:

*s = std::to_string(u%base) + *s; 

the first operand pointless, removed it. finally, of dereferencing bit tiresome, no? i'd make 1 , return it:

std::string doldur(const std::string &s,int u,int base){     std:string ret;     while(u!=0){         ret = std::to_string(u%base) + ret;         u/=base;     }     return ret; } 

don't worry performance loss returning. or use reference , change original if prefer:

void doldur(std::string &s,int u,int) {     s.clear();     while(u!=0){         s = std::to_string(u%base) + s;         u/=base;     } }