Get arguments in optional<T> with .present<T>()

fixes: p-ranav/argparse#66

README.md

@@ -140,6 +140,18 @@ program.add_argument("-o", "--output")
   .help("specify the output file.");
 ```
 
+#### Accessing optional arguments without default values
+
+If you do not require an optional argument to present but has no good default value for it, you can combine testing and accessing the argument as following:
+
+```cpp
+if (auto fn = program.present("-o")) {
+    do_something_with(*fn);
+}
+```
+
+Similar to `get`, the `present` method also accepts a template argument.  But rather than returning `T`, `parser.present<T>(key)` returns `std::optional<T>`, so that when the user does not provide a value to this parameter, the return value compares equal to `std::nullopt`.
+
 ### Negative Numbers
 
 Optional arguments start with ```-```. Can ```argparse``` handle negative numbers? The answer is yes!

include/argparse.hpp

@@ -702,6 +702,23 @@ private:
     throw std::logic_error("No value provided");
   }
 
+  /*
+   * Get argument value given a type.
+   * @pre The object has no default value.
+   * @returns The stored value if any, std::nullopt otherwise.
+   */
+  template <typename T> auto present() const -> std::optional<T> {
+    if (mDefaultValue.has_value())
+      throw std::logic_error("Argument with default value always presents");
+
+    if (mValues.empty())
+      return std::nullopt;
+    else if constexpr (details::is_container_v<T>)
+      return any_cast_container<T>(mValues);
+    else
+      return std::any_cast<T>(mValues.front());
+  }
+
   template <typename T>
   static auto any_cast_container(const std::vector<std::any> &aOperand) -> T {
     using ValueType = typename T::value_type;
@@ -811,14 +828,25 @@ public:
     parse_args(arguments);
   }
 
-  /* Getter enabled for all template types other than std::vector and std::list
-   * @throws std::logic_error in case of an invalid argument name
-   * @throws std::logic_error in case of incompatible types
+  /* Getter for options with default values.
+   * @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) {
     return (*this)[aArgumentName].get<T>();
   }
 
+  /* Getter for options without default values.
+   * @pre The option has no default value.
+   * @throws std::logic_error if there is no such option
+   * @throws std::bad_any_cast if the option is not of type T
+   */
+  template <typename T = std::string>
+  auto present(std::string_view aArgumentName) -> std::optional<T> {
+    return (*this)[aArgumentName].present<T>();
+  }
+
   /* Indexing operator. Return a reference to an Argument object
    * Used in conjuction with Argument.operator== e.g., parser["foo"] == true
    * @throws std::logic_error in case of an invalid argument name

test/test_parse_args.cpp

@@ -19,6 +19,32 @@ TEST_CASE("Parse a string argument with default value" *
   REQUIRE(program.get("--config") == "foo.yml");
 }
 
+TEST_CASE("Parse a string argument without default value" *
+          test_suite("parse_args")) {
+  argparse::ArgumentParser program("test");
+  program.add_argument("--config");
+
+  WHEN("no value provided") {
+    program.parse_args({"test"});
+
+    THEN("the option is nullopt") {
+      auto opt = program.present("--config");
+      REQUIRE_FALSE(opt);
+      REQUIRE(opt == std::nullopt);
+    }
+  }
+
+  WHEN("a value is provided") {
+    program.parse_args({"test", "--config", ""});
+
+    THEN("the option has a value") {
+      auto opt = program.present("--config");
+      REQUIRE(opt);
+      REQUIRE(opt->empty());
+    }
+  }
+}
+
 TEST_CASE("Parse an int argument with value" * test_suite("parse_args")) {
   argparse::ArgumentParser program("test");
   program.add_argument("--count")
@@ -103,6 +129,37 @@ TEST_CASE("Parse a vector of float arguments" * test_suite("parse_args")) {
   REQUIRE(vector[4] == 5.5f);
 }
 
+TEST_CASE("Parse a vector of float without default value" *
+          test_suite("parse_args")) {
+  argparse::ArgumentParser program("test");
+  program.add_argument("--vector").scan<'f', float>().nargs(3);
+
+  WHEN("no value is provided") {
+    program.parse_args({"test"});
+
+    THEN("the option is nullopt") {
+      auto opt = program.present<std::vector<float>>("--vector");
+      REQUIRE_FALSE(opt.has_value());
+      REQUIRE(opt == std::nullopt);
+    }
+  }
+
+  WHEN("a value is provided") {
+    program.parse_args({"test", "--vector", ".3", "1.3", "6"});
+
+    THEN("the option has a value") {
+      auto opt = program.present<std::vector<float>>("--vector");
+      REQUIRE(opt.has_value());
+
+      auto &&vec = opt.value();
+      REQUIRE(vec.size() == 3);
+      REQUIRE(vec[0] == .3f);
+      REQUIRE(vec[1] == 1.3f);
+      REQUIRE(vec[2] == 6.f);
+    }
+  }
+}
+
 TEST_CASE("Parse a vector of double arguments" * test_suite("parse_args")) {
   argparse::ArgumentParser program("test");
   program.add_argument("--vector")