C++ dec2hex string function

#include <iostream>
#include <sstream>
#include <cstdio>
 
using namespace std;
 
string dec2hex(unsigned long long i);
string dec2hex_c(unsigned int i);
 
int main(int argc, char *argv[])
{
    int i1 = 255;
    int i2 = 2147483647;//2.14 billion
    unsigned long i3 = 4294967295UL;//4.29 billion
    unsigned long long i4 = 4300000000ULL;//4.3 billion >more than 32 bits
 
    cout << i1 << "," << dec2hex(i1)<< "," << dec2hex_c(i1) << endl;
    cout << i2 << "," << dec2hex(i2)<< "," << dec2hex_c(i2) << endl;
    cout << i3 << "," << dec2hex(i3)<< "," << dec2hex_c(i3) << endl;
    cout << i4 << "," << dec2hex(i4)<< "," << dec2hex_c(i4) << endl;
 
    return 0;
}
 
string dec2hex(unsigned long long i)
{
    stringstream ss;
    ss << hex << uppercase << i;
    //ss << hex << lowercase << i;
    //ss << showbase << hex << lowercase << i; //prepends 0x
    //string s; ss >> s; return s; //alternate way to extract string
    return ss.str();
}
 
string dec2hex_c(unsigned int i) //has limit of 32 bit integer
{
    char s[20];
    sprintf(s, "%X", i);//uppercase
    //sprintf(s, "%x", i);//lowercase
    return string(s);
}

255,FF,FF
2147483647,7FFFFFFF,7FFFFFFF
4294967295,FFFFFFFF,FFFFFFFF
4300000000,1004CCB00,4CCB00