Attached is a patch to program_options that implements optional argument support, in the style of GNU getopt_long. It adds one new method to 'value_type': 'implicit_value()'. The idea is that, if the option is given without an argument, it is assigned an "implicit" value. Otherwise, if the long-option is given an explicit argument via the equals sign (--arg=value), or the short-option is given an adjacent argument (-avalue), the explicit value will override the implicit one. For example: desc.add_options() ("proxy,p", value<int>(&proxy_port)->implicit_value(DefaultProxyPort), "run proxy service") ; ... if (vm.count("proxy")) enable_the_proxy(proxy_port); ./prog --proxy --opt2 --opt3 -> enables proxy on default port. ./prog --proxy=9000 --opt2 --opt3 -> enables proxy on port 9000. ./prog --opt2 --opt3 -> does not enable the proxy. -Bryan