Конвертация из int в string и наоборот

#include <iostream>
#include <sstream>
#include <string>
 
int main()
{
    std::istringstream in(std::string("100"));
    std::ostringstream out;
    int num = 100;
   
    in >> num;
        std::cout << num << std::endl;
 
    num = 1000;
    out << num;
    std::cout << out.str() << std::endl;
 
    return 0;
}

std::string str("100");
std::istringstream in(str);

string Int2String(int value) {
  string res;
  bool continue = true;
  do {
    switch(value % 10) {
      case 0: res = "0" + res; break;
      case 1: res = "1" + res; break;
      case 2: res = "2" + res; break;
      case 3: res = "3" + res; break;
      case 4: res = "4" + res; break;
      case 5: res = "5" + res; break;
      case 6: res = "6" + res; break;
      case 7: res = "7" + res; break;
      case 8: res = "8" + res; break;
      case 9: res = "9" + res; break;
      default: continue = false;
    }
    value /=  10;
  } while (continue && value);
  return res;
}

string Int2String(int value)
{
  string res;
  char ch = 0;
  char digit[] = "0123456789";
  do { ch = value % 10;
       res = digit[ch] + res;
    value /=  10;
  } while (value);
  return res;
}