How to accept empty value in boost::program_options

c++ boost boost-program-options
cppalex picture cppalex · Nov 26, 2009 · Viewed 7k times · Source

I'm using boost::program_options library to process command line params. I need to accept a file name via -r option, in case if it is empty (-r given without params) I need to use stdin.

desc.add_options()
 ("replay,r", boost::program_options::value<std::string>(), "bla bla bla")

In this case boost wouldn't accept -r without params and throw an exception. default_value () option does not work as well as it would make library return value even if user didn't give -r option.

Any ideas how to work around?

Answer

Vladimir Prus picture Vladimir Prus · Nov 27, 2009

Please use the implicit_value method, e.g

desc.add_options()
 ("replay,r", po::value<std::string>()->implicit_value("stdin"), "bla bla bla")

This makes the option accept either 0 or 1 token, and if no tokens are provided, it will act as if 'stdin' was provided. Of course, you can pick any other implicit value -- including empty string and '-' as suggested by mch.

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