How to pad an IP address with leading zeroes

Question

I have seen solutions written online in C, but I want a C++ way to pad an IPv4 address with zeroes.

C code found online

using namespace std;

#include<iostream>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void padZeroIP(char *str)
{
    int oct1=0;
    int oct2=0;
    int oct3=0;
    int oct4=0;

    int i = 0;

    const char s[2] = ".";
    char *token;
    
    /* get the first token */
    token = strtok(str, s);

    oct1 = atoi(token);
    
    /* walk through other tokens */
    while( token != NULL ) 
    {
        token = strtok(NULL, s);
        
        if(i==0)
            oct2 = atoi(token);
        else if(i==1)
            oct3 = atoi(token);
        else if(i==2)
            oct4 = atoi(token);
        i++;
    }

    sprintf(str,"%03d.%03d.%03d.%03d",oct1,oct2,oct3,oct4);
}

Answer 1

Once again, with feeling:

std::string padZeroIP(const std::string& str)
{
    using boost::asio::ip::address_v4;
    auto ip = address_v4::from_string(str).to_bytes();

    std::string result(16, '\0');

    std::snprintf(result.data(), result.size(), //
                  "%03d.%03d.%03d.%03d",        //
                  ip[0], ip[1], ip[2], ip[3]);

    result.resize(15);

    return result;
}

This doesn't assume input is valid and doesn't reinvent the wheel. See it Live On Coliru:

   127.0.0.1 -> 127.000.000.001
     1.1.1.1 -> 001.001.001.001

OVERKILL

Keep in mind that IPv4 addresses can take many forms, E.g. 127.1 is valid for 127.0.0.1. So is 0177.1 or 0x7f.1