How do I increment an IP address represented as a string?

Quick/Dirty!

void increment(std::string& ip)
{
    std::string::size_type dot = ip.find_last_of('.');
    std::stringstream stream(ip.substr(dot+1));
    int part = 0;

    stream >> part;

    part++;

    stream.str(""); stream.clear();

    stream << part;

    ip.replace(dot+1, std::string::npos, stream.str());
}

int a,b,c,d;
sscanf(str, "%d.%d.%d.%d", &a,&b,&c,&d);
sprintf(str, "%d.%d.%d.%d\0", a,b,c,d+1);

I would write a method that accepts a string in that format.
Convert it to 4 integers. increment. (Important Check range)
Then convert back to a string.

If you want somthing more long term and robust a class representing the IP address. Then you maintain the class an manipulate as appropraite and convert to string when needed.

#include <iostream>
#include <istream>
#include <sstream>
#include <string>
#include <stdexcept>



class MyIp
{
    struct Dot
    {};
    struct Byte
    {
        Byte(unsigned char& val)
            :m_val(val)
        {}
        unsigned char&  m_val;
    };
    friend std::istream& operator>>(std::istream& str,MyIp::Dot const& d);
    friend std::istream& operator>>(std::istream& str,MyIp::Byte const& b);
    friend std::ostream& operator<<(std::ostream& str,MyIp const& ip);
    public:
        MyIp(std::string const& ip)
        {
            std::stringstream str(ip);
            str >> Byte(ad[0]) >> Dot() >> Byte(ad[1]) >> Dot() >> Byte(ad[2]) >> Dot() >> Byte(ad[3]);
            std::string leftover;
            if (str >> leftover)
            {   throw std::runtime_error("InvalidIP: Long");
            }
        }
        void inc(int index)
        {
            if ((index >= 0) && (index <=3))
            {
                ++ad[index];
                if (ad[index] == 0)
                {
                    inc(index-1);
                }
            }
        }
    private:
        unsigned char   ad[4];
};
std::istream& operator>>(std::istream& str,MyIp::Dot const& d)
{
    char x  = str.get();
    if (x != '.')
    {   throw std::runtime_error("Invalid IP: Dot");
    }
    return str;
}
std::istream& operator>>(std::istream& str,MyIp::Byte const& b)
{
    unsigned int val;
    str >> val;
    if (!str || val > 255)
    {   throw std::runtime_error("Invalid IP: Val");
    }
    b.m_val = static_cast<unsigned char>(val);
    return str;
}
std::ostream& operator<<(std::ostream& str,MyIp const& ip)
{
    return str  << static_cast<unsigned int>(ip.ad[0])
                << "." << static_cast<unsigned int>(ip.ad[1])
                << "." << static_cast<unsigned int>(ip.ad[2])
                << "." << static_cast<unsigned int>(ip.ad[3]);
}

int main()
{
    try
    {
        std::string ip("127.0.0.1");

        MyIp    addr(ip);

        std::cout << addr << "\n";
        addr.inc(3);
        std::cout << addr << "\n";
    }
    catch(std::exception const& e)
    {
        std::cout << "What: " << e.what() << "\n";
    }
}

You can also use the location of the last "." and take from there to the end to convert to an int, bump it up 1, check boundaries and then convert it to a string and append to the base part.

This probably isn't very sane but it was fun to think about.

Since the IP address space is 32-bits you could write a function to convert IP addresses into unsigned 32-bit integers. Then you can add or subtract 1 or as much as you want, and convert back into an IP address. You wouldn't have to worry about range checking.

In pseduo-code for 192.123.34.134 you'd do:

int i = (192 << 24) + (123 << 16) + (34 << 8) + 134

More generally, for a.b.c.d:

int i = (a << 24) + (b << 16) + (c << 8) + d

Now change i as much as you want (i++, i+=10000) and convert back:

String ip = (i >> 24) + "." + 
            ((i >> 16) mod 256) + "." + 
            ((i >> 8) mod 256) + "." + 
            (i mod 256);

Excuse the syntax - I couldn't write C++ to save myself.

MK