Изучаем Rust с помощью ИИ

Стал я пробовать grok.com вместо gemini.google.com, и надо сказать, прогресс заметный. Я в основном пристаю к ИИ на тему программирования. Здесь Grok на голову выше Gemini, как отличник супротив троечника. Поделюсь с вами одним примером. Я спросил про smart указатели в Rust, и Grok прочитал мне целую лекцию. Первые пять в списке действительно smart указатели, а остальные скорее smart контейнеры. Но всё равно хорошо.

Question: How many smart pointers like Box and Rc are present in Rust?

Rust’s standard library offers a rich set of smart pointers beyond Box<T> and Rc<T>, each tailored to specific ownership semantics, threading needs, and use cases. These smart pointers leverage Rust’s ownership model and type system to provide safe, ergonomic memory management. Let’s count and explore them, focusing on the main ones in std (and one notable exception from alloc). I’ll cover their purposes and how they fit into Rust’s ecosystem.

Smart Pointers in Rust’s Standard Library

Here are the primary smart pointers in Rust, all from std unless noted:

1. Box<T>

• Module: std::boxed
• Purpose: Unique ownership of a heap-allocated T. The simplest smart pointer, providing a single owner with automatic cleanup.
• Use Case: When you need heap allocation without sharing (e.g., recursive data structures, large objects).
• Threading: Safe to send across threads if T: Send.

2. Rc<T>

• Module: std::rc
• Purpose: Reference-counted shared ownership for single-threaded use. Multiple Rc<T> instances share the same T, deallocating when the last reference drops.
• Use Case: Shared immutable data in a single thread (e.g., graph structures).
• Threading: Not thread-safe (!Send, !Sync).
• Notes: Paired with Weak<T> for non-owning refs.

( also Arc, Weak, Cell, RefCell, Mutex, RwLock, Cow )

Comparison to C++

• C++ Equivalents:
  • Box<T> ≈ std::unique_ptr<T>
  • Rc<T> ≈ std::shared_ptr<T> (non-atomic version)
  • Arc<T> ≈ std::shared_ptr<T> (atomic)
  • Weak<T> ≈ std::weak_ptr<T>
  • Mutex<T> ≈ std::mutex + std::unique_lock
  • RwLock<T> ≈ std::shared_mutex (C++17)
  • No direct Cell/RefCell - custom runtime checks needed.
  • Cow ≈ custom variant-like type or manual ownership toggling.
• Rust Unique: Cell and RefCell leverage Rust’s borrow checker, offering safety not easily replicated in C++.

Перепишем Box<T> на Си++

Нет лучше способа понять язык программирования, чем глянуть на реализацию. В языке Rust многое напоминает Си++, но всё ж таки "трошечки не так". Разберёмся с конструкцией Box<T> для начала. Спросим grok.com. Результат смотрите в файле box.cpp.

Question: Imagine I need to re-implement Box<T> in C++. How would it look like?

Re-implementing Rust’s Box<T> in C++ is a fun challenge that lets us peek into Rust’s memory management internals and translate them into C++’s world. In Rust, Box<T> is a smart pointer that provides heap-allocated ownership with automatic cleanup, backed by the global allocator (usually malloc/free via Rust’s alloc crate). It’s simple but powerful: it allocates memory for a T, moves the value onto the heap, and deallocates it when the Box goes out of scope via Drop. In C++, we don’t have Rust’s ownership model or Drop trait, so we’ll use RAII and a custom class to mimic it.

Let’s design a C++ equivalent, breaking down the essentials of Box<T> and handling the nuances of C++’s memory model.

What Box<T> Does in Rust

• Allocation: Allocates memory on the heap for a T using the global allocator.
• Ownership: Sole owner of the heap memory; no sharing or borrowing by default.
• Move Semantics: Transfers ownership when assigned or passed (no copying unless T: Clone).
• Deallocation: Automatically frees the memory when dropped, calling T’s destructor if it has one.
• Dereferencing: Provides access to T via * or . (through Deref).

C++ Implementation

We’ll create a Box class in C++ that:

• Uses new and delete for allocation/deallocation (mirroring malloc/free).
• Implements RAII for automatic cleanup.
• Supports move semantics (C++11 and later) to mimic Rust’s ownership transfer.
• Prevents copying to enforce single ownership (like Rust).
• Provides dereference operators (* and ->).

Here’s the code: box.cpp.

template <typename T>
class Box {
private:
    T *ptr; // Raw pointer to heap-allocated memory

    // Private helper to deallocate
    void deallocate()
    {
        if (ptr) {
            delete ptr; // Calls T's destructor, then frees memory
            ptr = nullptr;
        }
    }

public:
    // Constructor: Allocate and construct T in-place
    template <typename... Args>
    explicit Box(Args &&...args) : ptr(new T(std::forward<Args>(args)...))
    {
    }

    // Destructor: Automatically clean up
    ~Box() { deallocate(); }

    // Disable copying (Rust Box doesn't allow this)
    Box(const Box &) = delete;
    Box &operator=(const Box &) = delete;

    // Move constructor: Transfer ownership
    Box(Box &&other) noexcept : ptr(other.ptr)
    {
        other.ptr = nullptr; // Leave other in a valid, empty state
    }

    // Move assignment: Transfer ownership
    Box &operator=(Box &&other) noexcept
    {
        if (this != &other) {
            deallocate();        // Free current memory
            ptr = other.ptr;     // Take ownership
            other.ptr = nullptr; // Leave other empty
        }
        return *this;
    }

    // Dereference operators
    T &operator*() { return *ptr; }
    const T &operator*() const { return *ptr; }

    T *operator->() { return ptr; }
    const T *operator->() const { return ptr; }

    // Optional: Raw pointer access (like Box::into_raw in Rust)
    T *as_raw() { return ptr; }

    // Optional: Take ownership back (like Box::from_raw, but consumes self)
    T into_inner()
    {
        T value = std::move(*ptr);
        deallocate();
        return value;
    }
};

// Factory function to simplify creation (like Box::new in Rust)
template <typename T, typename... Args>
Box<T> make_box(Args &&...args)
{
    return Box<T>(std::forward<Args>(args)...);
}

( объяснения )

Я помню время, когда байт был семибитным

Хорошая табличка, вдруг кому пригодится. В высоком разрешении для печати.