Confirm arguments are parsed before allowing ArgumentParser::get

If the developer forgot to call ArgumentParser::parse_args<>, attempts to
use ::get, ::present, etc., would raise "No value provided...".  With this
change, the error better describes what went wrong.

Signed-off-by: Sean Robinson <sean.robinson@scottsdalecc.edu>

include/argparse/argparse.hpp

@@ -911,12 +911,16 @@ public:
   }
 
   /* Getter for options with default values.
+   * @throws std::logic_error if parse_args() has not been previously called
    * @throws std::logic_error if there is no such option
    * @throws std::logic_error if the option has no value
    * @throws std::bad_any_cast if the option is not of type T
    */
   template <typename T = std::string>
   T get(std::string_view aArgumentName) const {
+    if (!mIsParsed) {
+      throw std::logic_error("Nothing parsed, no arguments are available.");
+    }
     return (*this)[aArgumentName].get<T>();
   }
 
@@ -1076,6 +1080,7 @@ private:
         throw std::runtime_error("Unknown argument");
       }
     }
+    mIsParsed = true;
   }
 
   /*
@@ -1115,6 +1120,7 @@ private:
   std::string mVersion;
   std::string mDescription;
   std::string mEpilog;
+  bool mIsParsed = false;
   std::list<Argument> mPositionalArguments;
   std::list<Argument> mOptionalArguments;
   std::map<std::string_view, list_iterator, std::less<>> mArgumentMap;

test/test_get.cpp

@@ -10,6 +10,14 @@ TEST_CASE("Getting a simple argument" * test_suite("ArgumentParser::get")) {
   REQUIRE(program.get("--stuff") == "./src");
 }
 
+TEST_CASE("Skipped call to parse_args" * test_suite("ArgumentParser::get")) {
+  argparse::ArgumentParser program("test");
+  program.add_argument("stuff");
+  REQUIRE_THROWS_WITH_AS(program.get("stuff"),
+                         "Nothing parsed, no arguments are available.",
+                         std::logic_error);
+}
+
 TEST_CASE("Missing argument" * test_suite("ArgumentParser::get")) {
   argparse::ArgumentParser program("test");
   program.add_argument("-s", "--stuff");