How do you validate that a string is a valid IPv4 address in C++?

The solution that I settled on was:

#include <arpa/inet.h>
// ...
bool Config::validateIpAddress(const string &ipAddress)
{
    struct sockaddr_in sa;
    int result = inet_pton(AF_INET, ipAddress.c_str(), &(sa.sin_addr));
    return result != 0;
}

This works for most cases that were mentioned in other answers. It doesn't recognize IP addresses with octal or hex formatting, but that's acceptable for my application.

You probably want the inet_pton, which returns -1 for invalid AF argument, 0 for invalid address, and +1 for valid IP address. It supports both the IPv4 and future IPv6 addresses. If you still need to write your own IP address handling, remember that a standard 32-bit hex number is a valid IP address. Not all IPv4 addresses are in dotted-decimal notation.

This function both verifies the address, and also allows you to use the same address in related socket calls.

Boost.Asio provides the class ip::address. If you just want to verify the string, you can use it like this:

std::string ipAddress = "127.0.0.1";
boost::system::error_code ec;
boost::asio::ip::address::from_string( ipAddress, ec );
if ( ec )
    std::cerr << ec.message( ) << std::endl;

This also works for hexadecimal and octal quads. This is also a much more portable solution.

This looks deceptively simple but has a few pitfalls. For example, many of the solutions posted in the previous answers assume that the quads are in base 10 - but a quad starting with a zero must be treated as a base 8 (octal) number, hence for example any quad part starting with zero and containing the digits 8 or 9 is not valid. I.e, the IP number 192.168.1.010 is not 192.168.1.10 but in reality is 192.168.1.8, and the IP number 192.168.019.14 is not valid since the third quad contains the invalid base 8 digit 9.

I emphatically encourage you to use the functions provided by the socket library included in your operating system or compiler environment.

Edit: (Thought it was implicit, but) of course, you can also have hexadecimal quads, a la 192.168.1.0x0A for 192.168.1.10, and of course you can mix and match to your sadistic content happily using upper and lower case, a la 0xC0.0xa8.1.010 for 192.168.1.8. Try some examples using ping if you want to have fun. This works just fine cross-platform (tested a while back while swearing under Linux, NetBSD, and Win32.)

Further edit in response to KaluSingh Gabbar's request: For example, you can specify 192.168.1.10 as 0xc0a8010a and it still represents a valid IP number, a la:

[mihailim@home ~]$ ping 0xc0a8010a
PING 0xc0a8010a (192.168.1.10) 56(84) bytes of data.
^C
--- 0xc0a8010a ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 2479ms