Boost program options allowed set of input values

Question

Is there a way to set an allowed set of input variables for parameters? For example parameter "arg" can have only string values like "cat" and "dog".

Answer 1

You can use the custom validator feature. Define a distinct type for your option, and then overload the validate function on that type.

struct catdog {
  catdog(std::string const& val):
    value(val)
  { }
  std::string value;
};

void validate(boost::any& v, 
              std::vector<std::string> const& values,
              catdog* /* target_type */,
              int)
{
  using namespace boost::program_options;

  // Make sure no previous assignment to 'v' was made.
  validators::check_first_occurrence(v);

  // Extract the first string from 'values'. If there is more than
  // one string, it's an error, and exception will be thrown.
  std::string const& s = validators::get_single_string(values);

  if (s == "cat" || s == "dog") {
    v = boost::any(catdog(s));
  } else {
    throw validation_error(validation_error::invalid_option_value);
  }
}

The exceptions thrown from that code are no different from the exceptions thrown for any other invalid option value, so you should already be prepared to handle them.

Use the special option type instead of just string when you define your options:

desc.add_options()
  ("help", "produce help message")
  ("arg", po::value<catdog>(), "set animal type")
;

I've composed a live example demonstrating use of this code.