edit: on second thought I probably misread your question but I'll share this anyway
Code:
#include <iostream>
#include <sstream>
#include <string>
int main()
{
std::string num("100");
std::istringstream iss_dec(num);
std::istringstream iss_hex(num);
std::istringstream iss_oct(num);
int dec;
iss_dec >> std::dec >> dec;
int hex;
iss_hex >> std::hex >> hex;
int oct;
iss_oct >> std::oct >> oct;
std::cout << num << " interpreted as decimal: " << dec << std::endl;
std::cout << num << " interpreted as hexidecimal: " << hex << std::endl;
std::cout << num << " interpreted as octal: " << oct << std::endl;
return 0;
}