[Boost-users] [program_options] patch - optional argument support (gnu getopt_long style)

From: Bryan Green (bgreen_at_[hidden])
Date: 2007-07-23 18:26:25

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