Of course! Here is the detailed solution to help you complete your task. *** Building upon our previous task, you have successfully created the class declaration for `HittableList`. This class, by inheriting from `Hittable`, cleverly follows the Composite design pattern, allowing us to treat a collection of objects in the same way we treat a single object. You've defined its interface with constructors and `add`/`clear` methods. Now, we need to implement the core of this class: the mechanism for actually storing our list of hittable objects. ### The Challenge: A List of Many Shapes Our goal is to create a single list that can hold any object that "is-a" `Hittable`. This means we want to put `Sphere` objects, and in the future, `Plane` objects, `Cube` objects, and more, all into the same container. This is a classic polymorphism problem. How do we do this in C++? Our first instinct might be to use the standard library's dynamic array, `std::vector`. So, what about `std::vector`? This seems logical, but it runs into two immediate and insurmountable problems: 1. **Abstract Classes Cannot Be Instantiated**: Our `Hittable` class is **abstract** because it contains a pure virtual function (`hit(...) = 0`). The C++ language forbids you from creating an instance of an abstract class. Since `std::vector` needs to be able to create and manage instances of the type it holds, `std::vector` is not possible. 2. **The Object Slicing Problem**: Even if `Hittable` were not abstract, this approach would fail due to a subtle but deadly C++ pitfall called **object slicing**. If you were to add a `Sphere` object to a `std::vector`, the vector would allocate space for a `Hittable` object and copy the `Hittable` part of your `Sphere` into it. All the specific data that makes it a `Sphere` (like its `radius`) would be "sliced off" and lost forever. The correct way to store polymorphic objects in a collection is to store **pointers** to them. Instead of storing the objects themselves, we store their memory addresses. This way, the full `Sphere` object lives somewhere in memory, and our vector just holds a pointer to it, preserving its complete type and data. ### The Modern C++ Solution: `std::shared_ptr` In "classic" C++, you would use raw pointers (`Hittable*`), which you would manage manually with `new` and `delete`. This is notoriously error-prone and a common source of memory leaks. Modern C++ provides a much safer, more robust solution: **smart pointers**. For our purposes, `std::shared_ptr` is the perfect tool. A `std::shared_ptr` is a smart pointer from the C++ standard library (`` header) that automatically manages the lifetime of a dynamically allocated object using a technique called **reference counting**. Here’s how it works: * When you create a `std::shared_ptr` to an object, it increments a hidden reference counter. * When you copy that `std::shared_ptr`, the counter increments again. * When a `std::shared_ptr` is destroyed (for example, when it goes out of scope), the counter decrements. * When the counter reaches zero, it means no one is using the object anymore, and the `std::shared_ptr` automatically deletes the object, freeing its memory. This gives us completely automatic memory management for our scene objects. We can add objects to our `HittableList`, and when the list is destroyed, all the objects it holds will be cleaned up automatically. ### Updating `hittable_list.h` with the Object Vector Let's modify your `src/hittable_list.h` file to include this storage mechanism. We will add the necessary headers and declare our `objects` member variable. ```cpp #ifndef HITTABLE_LIST_H #define HITTABLE_LIST_H #include "hittable.h" // --- NEWLY ADDED CODE START --- // Include the headers for smart pointers and dynamic arrays. #include #include // --- NEWLY ADDED CODE END --- class HittableList : public Hittable { public: // --- NEWLY ADDED CODE START --- // The list of objects in our scene. We use a vector of shared_ptr to Hittable. // - `std::vector`: A dynamic array to hold our collection of objects. // - `std::shared_ptr`: A smart pointer for automatic memory management. // - ``: The base class type, allowing us to store any derived class // (like Sphere) that inherits from Hittable. std::vector> objects; // --- NEWLY ADDED CODE END --- HittableList() {} HittableList(std::shared_ptr object) { add(object); } void clear() { objects.clear(); } void add(std::shared_ptr object) { objects.push_back(object); } virtual bool hit(const Ray& r, double t_min, double t_max, HitRecord& rec) const override; }; #endif ``` ### Deconstructing the Changes Let's review the critical line we just added: `std::vector> objects;` * **`std::vector<...>`**: We are using a vector, a dynamic array that can grow as we add more objects to our scene. We need to `#include ` to use it. * **`std::shared_ptr<...>`**: We are telling the vector to store `shared_ptr`s, not the objects themselves. This is our key to automatic memory management and avoiding object slicing. We need to `#include ` for this. * **``**: The `shared_ptr`s will point to objects of type `Hittable`. Because of polymorphism, a `std::shared_ptr` can point to an instance of any class that derives from `Hittable`, such as our `Sphere` class. This is what allows our list to be heterogeneous, holding many different kinds of shapes. Your `HittableList` class now has a robust, type-safe, and memory-safe way to store all the objects that will make up your 3D scenes. ### Next Steps The `HittableList` class is now fully defined, with its storage mechanism in place. However, its most important behavior—its own `hit` method—is still just an empty declaration. This method needs to be able to take a ray and test it against *every object* in its `objects` list. Your next task is to **implement the `hit` method for `HittableList`, which iterates through its objects and finds the closest hit**. This will be the final piece of the puzzle to enable rendering scenes with multiple objects. --- ### Further Reading To deepen your understanding of the powerful C++ features and concepts you've just used, explore these resources: * **`std::shared_ptr` Explained**: A deep dive into how `shared_ptr` works, including its use of reference counting and control blocks. * [Learn C++: `std::shared_ptr`](https://www.learncpp.com/cpp-tutorial/stdshared_ptr/) * **Object Slicing in C++**: A detailed explanation of this common C++ pitfall and how using pointers or references prevents it. * [Stack Overflow: What is object slicing?](https://stackoverflow.com/questions/274626/what-is-object-slicing) * **Rule of Zero**: A modern C++ guideline. By using standard library containers and smart pointers (like `std::vector` and `std::shared_ptr`), our `HittableList` class requires no manual memory management (no custom destructor, copy constructor, etc.). The compiler-generated defaults do the right thing automatically. * [A GotW post by Herb Sutter: The Rule of Zero](https://herbsutter.com/2013/05/29/gotw-90-solution-factories/)