wip: adding optional and variant lecture

lectures/optional_and_variant.md

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+
+**`std::optional` and `std::variant` in Modern C++**
+--
+
+<p align="center">
+  <a href="https://youtu.be/dummy_link"><img src="https://img.youtube.com/vi/dummy_link/maxresdefault.jpg" alt="Video Thumbnail" align="right" width=50% style="margin: 0.5rem"></a>
+</p>
+
+## **Introduction**
+
+When working with modern C++ (C++17 and beyond), we often need tools to handle optional values or represent data that can take one of several types. That’s where `std::optional` and `std::variant` come into play. Today, we’ll explore what these features are, why they’re useful, and how you can leverage them in your projects.
+
+
+## **What is `std::optional`?**
+
+### Why use `std::optional`?
+
+Imagine a function that searches for an item in a container. If the item is found, the function should return it. But what if it isn’t? Before C++17, you might have returned a special value (like `-1` for integers) or used a pointer, potentially introducing ambiguity or risking undefined behavior.
+
+`std::optional` solves this by explicitly representing the absence of a value. It's a type-safe mechanism that avoids the pitfalls of ad-hoc solutions.
+
+### Examples of `std::optional` in action
+
+#### A simple search function
+
+````cpp
+#include <optional>
+#include <iostream>
+#include <vector>
+
+std::optional<int> Find(const std::vector<int>& data, int value) {
+    for (int element : data) {
+        if (element == value) {
+            return element;  // Return the value if found
+        }
+    }
+    return std::nullopt;  // Explicitly indicate "no value"
+}
+
+int main() {
+    std::vector<int> numbers = {1, 2, 3, 4, 5};
+    auto result = Find(numbers, 3);
+
+    if (result) {  // Check if a value exists
+        std::cout << "Found: " << *result << '\n';
+    } else {
+        std::cout << "Not found.\n";
+    }
+}
+````
+In this example, `std::optional<int>` clearly communicates that the function may or may not return a value.
+
+#### A factory function
+
+````cpp
+std::optional<std::string> CreateString(bool should_create) {
+    if (should_create) {
+        return "Hello, World!";
+    }
+    return std::nullopt;
+}
+
+int main() {
+    auto maybe_string = CreateString(true);
+
+    if (maybe_string) {
+        std::cout << *maybe_string << '\n';
+    } else {
+        std::cout << "No string created.\n";
+    }
+}
+````
+---
+
+## **What is `std::variant`?**
+
+### Why use `std::variant`?
+
+`std::variant` is a type-safe union introduced in C++17. It allows a variable to hold one value out of a defined set of types. Think of it as a more flexible alternative to `enum` or `std::any`, but with static type checking.
+
+For instance, if a variable can hold either an integer or a string, you can use `std::variant` instead of rolling your own solution with `void*` or `boost::variant`.
+
+### Examples of `std::variant` in action
+
+#### Basic usage
+
+````cpp
+#include <variant>
+#include <iostream>
+#include <string>
+
+int main() {
+    std::variant<int, std::string> value;
+
+    value = 42;  // Assign an integer
+    std::cout << "Integer: " << std::get<int>(value) << '\n';
+
+    value = "Hello, std::variant!";  // Assign a string
+    std::cout << "String: " << std::get<std::string>(value) << '\n';
+}
+````
+#### Pattern matching with `std::visit`
+
+````cpp
+#include <variant>
+#include <iostream>
+#include <string>
+
+int main() {
+    std::variant<int, std::string> value = "Hello, Variant!";
+
+    std::visit([](auto&& arg) {
+        using T = std::decay_t<decltype(arg)>;
+        if constexpr (std::is_same_v<T, int>) {
+            std::cout << "Integer: " << arg << '\n';
+        } else if constexpr (std::is_same_v<T, std::string>) {
+            std::cout << "String: " << arg << '\n';
+        }
+    }, value);
+}
+````
+Here, `std::visit` applies a visitor (a callable object) to the value contained in the variant.
+
+---
+
+## **Key differences and common use cases**
+
+| Feature           | `std::optional`                                      | `std::variant`                                  |
+|--------------------|------------------------------------------------------|------------------------------------------------|
+| Purpose           | Represents optional values (may or may not exist).   | Represents one of several types.              |
+| Typical Use Case  | Returning a value or "nothing" from a function.       | Handling inputs or data with multiple types.  |
+| Type Safety       | Yes.                                                  | Yes.                                           |
+| Pattern Matching  | Not applicable.                                       | Supported via `std::visit`.                   |
+
+---
+
+## **Summary**
+
+`std::optional` and `std::variant` are two powerful tools in the C++ toolbox that greatly enhance type safety and code readability.
+
+- Use `std::optional` when a value might be absent.
+- Use `std::variant` when a value can be one of several types.
+
+These features enable us to write cleaner, more expressive code while avoiding common pitfalls. Experiment with them in your projects and see how they can simplify your development workflow!