Add program_options example with a generic custom validator for enums

example/program_options.cpp

@@ -0,0 +1,96 @@
+
+// Copyright 2021 Samuel Debionne
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+// g++ -std=c++14 -Iinclude example/program_options.cpp -o example/program_options /usr/lib/libboost_program_options.a
+
+#define _CRT_SECURE_NO_WARNINGS
+
+#include <boost/describe.hpp>
+#include <boost/mp11.hpp>
+#include <boost/program_options.hpp>
+
+#include <type_traits>
+#include <iostream>
+
+namespace app
+{
+
+enum class log_level {
+   fatal,
+   error,
+   warning,
+   trace
+};
+
+BOOST_DESCRIBE_ENUM(log_level, fatal, error, warning, trace)
+
+using boost::describe::operators::operator<<;
+
+// Generic program_options custom validator for (described) enums
+template <
+    typename E,
+    typename Ed = boost::describe::describe_enumerators<E>,
+    typename = std::enable_if_t<
+        std::is_enum<E>::value && !boost::mp11::mp_empty<Ed>::value
+    >
+>
+void validate(boost::any& v,
+              const std::vector<std::string>& values,
+              E* target_type, int)
+{
+    using namespace boost::program_options;
+
+    // Make sure no previous assignment to 'a' 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.
+    const std::string& name = validators::get_single_string(values);
+
+    // Convert string to enum
+    E r = {};
+    bool found = boost::describe::enum_from_string(name.c_str(), r);
+
+    if (found)
+      v = boost::any(r);
+    else
+      throw validation_error(validation_error::invalid_option_value);
+}
+
+} //namespace app
+
+int main(int argc, char *argv[])
+{
+  namespace po = boost::program_options;
+
+  app::log_level lvl;
+
+  // Declare the supported options.
+  po::options_description desc("Allowed options");
+  desc.add_options()
+      ("help", "produce help message")
+      ("log-level", po::value<app::log_level>(&lvl), "set log severity level")
+  ;
+
+  try {
+      po::variables_map vm;
+      po::store(po::parse_command_line(argc, argv, desc), vm);
+      po::notify(vm);
+
+      if (vm.count("help")) {
+          std::cout << desc << "\n";
+          return 1;
+      }
+
+      if (vm.count("log-level")) {
+          std::cout << "Log level was set to " << lvl << ".\n";
+      } else {
+          std::cout << "Log level was not set.\n";
+      }
+  }
+  catch (std::exception& ex)
+  {
+      std::cerr << "ERROR: " << ex.what() << std::endl;
+  }  
+}

include/boost/describe/operators.hpp

@@ -12,6 +12,7 @@
 #include <boost/describe/bases.hpp>
 #include <boost/describe/members.hpp>
 #include <boost/describe/modifiers.hpp>
+#include <boost/describe/enumerators.hpp>
 #include <boost/mp11/algorithm.hpp>
 #include <type_traits>
 #include <iosfwd>
@@ -160,6 +161,18 @@ template<class T, class Ch, class Tr> std::enable_if_t<
     return os;
 }
 
+template <typename E, class Ch, class Tr> std::enable_if_t<
+    has_describe_enumerators<E>::value,
+    std::basic_ostream<Ch, Tr>&>
+    operator<<( std::basic_ostream<Ch, Tr>& os, E e)
+{
+  boost::mp11::mp_for_each< boost::describe::describe_enumerators<E> >([&](auto D) {
+      if( e == D.value ) os << D.name;
+  });
+
+  return os;
+}
+
 } // namespace operators
 
 } // namespace describe