Boost Program Options: ignore undeclared when reading environment variables

boost_program_options_undecl_env.cpp

/* When using Boost's program_options to load environment variables,
 * there is no way of telling boost to ignore those that were not
 * specified in the `options_description` object.
 *
 * This snippet shows how to get similar behavior to the 
 * `allow_unregistered()` option available to command line and
 * configuration file parsers.
 *
 *
 * For example, if you try to read MYLIB_ONE by specifying an option
 * called "one" and using prefix "MYLIB" in `parse_environment`,
 * it will work just fine.
 * The issue arises if there is another MYLIB_X that wasn't included
 * in the `options_description` object. Then an exception will be
 * thrown.
 *
 * If you use the program arguments (argc, argv) or a configuration
 * file, there is an option in the library to simply ignore the
 * options that weren't declared in the `options_description`.
 * For environment variables, that is not the case.
 *
 * This snippet will simply read the options you declared and will
 * ignore any others that may have been declared.
 * Thus, behaving like the `allow_unregistered()` functionality.
 */

po::options_description options("My Options");
lib_options.add_options()
  (
    "MY_VAR",
    po::value<bool>(&my_var)->default_value(false),
    "Dummy boolean test"
  )
;

po::variables_map env;
po::store(po::parse_environment(
  options,
  [options](const std::string& var) {
    return std::any_of(
      options.options().cbegin(),
      options.options().cend(),
      [var](auto opt) { return var == opt->long_name(); }) ? var : "";
}), env);
po::notify(env);