I don't need to validate that the IP address is reachable or anything like that. I just want to validate that the string is in dotted-quad (xxx.xxx.xxx.xxx) IPv4 format, where xxx is between 0 and 255.
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.
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.