Add required options helper

Fixes #44. Adds a helper function for checking required options.

include/cxxopts.hpp

@@ -397,6 +397,18 @@ namespace cxxopts
     }
   };
 
+  class option_required_exception : public OptionParseException
+  {
+    public:
+    option_required_exception(const std::string& option)
+    : OptionParseException
+      (
+        u8"Option ‘" + option + u8"’ is required but not present"
+      )
+    {
+    }
+  };
+
   namespace values
   {
     template <typename T>
@@ -841,9 +853,25 @@ namespace cxxopts
     std::string m_group;
   };
 
+  // A helper function for setting required arguments
+  void
+  check_required
+  (
+    const Options& options,
+    const std::vector<std::string>& required
+  )
+  {
+    for (auto& r : required)
+    {
+      if (options.count(r) == 0)
+      {
+        throw option_required_exception(r);
+      }
+    }
+  }
+
   namespace
   {
-
     constexpr int OPTION_LONGEST = 30;
     constexpr int OPTION_DESC_GAP = 2;
 

test/options.cpp

@@ -100,6 +100,27 @@ TEST_CASE("Short options", "[options]")
     cxxopts::invalid_option_format_error);
 }
 
+TEST_CASE("Required arguments", "[options]")
+{
+  cxxopts::Options options("required", " - test required options");
+  options.add_options()
+    ("one", "one option")
+    ("two", "second option")
+  ;
+
+  Argv argv({
+    "required",
+    "--one"
+  });
+
+  auto aargv = argv.argv();
+  auto argc = argv.argc();
+
+  options.parse(argc, aargv);
+  REQUIRE_THROWS_AS(cxxopts::check_required(options, {"two"}),
+    cxxopts::option_required_exception);
+}
+
 TEST_CASE("No positional", "[positional]")
 {
   cxxopts::Options options("test_no_positional",