How to Implement Shallow Pointer Comparison in Rust

Introduction to Pointer Types in Rust

In Rust, pointer types such as Box<T>, Rc<T>, and Arc<T> behave differently compared to their C/C++ counterparts, specifically concerning equality and hashing. In Rust, the equality and hashing implementation for these pointer types utilizes deep comparisons, meaning they compare the values being referenced rather than the pointers themselves. This approach is not always aligned with what programmers coming from C/C++ might expect, as in those languages, pointer comparisons are typically shallow, relating only to the address in memory rather than the content. In this article, we will explore how to implement shallow comparisons for pointer types in Rust and address common concerns.

Why Shallow Comparison Matters

Shallow comparisons might be necessary in cases where you want to determine if two pointers reference the same memory location, not just equal content. This can be particularly useful for optimizing performance in certain scenarios, especially when working with large or complex data structures.

When implementing Eq and Hash for your custom types, the Rust language restricts you from defining orphan instances — meaning, you cannot implement traits for types you do not own. This becomes relevant when trying to apply shallow comparisons.

Implementing Shallow Comparison

Let's consider the implementation you provided for a custom type called MyType. The goal is to implement shallow equality and hashing, ensuring that Box<MyType>, Rc<MyType>, and Arc<MyType> compare based on their references. Here's how you can achieve that:

Custom Struct Definition

pub struct MyType {
    pub x: (),
}

Implementing PartialEq and Eq

To start, you must implement the PartialEq trait to allow for equality comparisons. The implementation uses the std::ptr::eq function, which checks if two pointers refer to the same memory.

impl PartialEq for MyType {
    fn eq(&self, other: &Self) -> bool {
        std::ptr::eq(self, other)
    }
}

impl Eq for MyType {}

Implementing Hash

Next, we need to implement the Hash trait. This implementation uses a raw pointer cast to perform hashing based on the pointer's address, not its content.

use std::hash::{Hash, Hasher};

impl Hash for MyType {
    fn hash<H: Hasher>(&self, state: &mut H) {
        (self as *const MyType).hash(state);
    }
}

Shallow Comparison in Practice

With these implementations, Box<MyType>, Rc<MyType>, and Arc<MyType> can now correctly utilize the shallow comparisons defined in MyType. When comparing instances of these pointer types, Rust will leverage the shallow comparison logic provided.

Addressing Common Concerns

Does This Approach Work as Expected?

Yes, the way you compare pointers using std::ptr::eq(self, other) ensures that you will get true only when both references point to the exact same object in memory. This is precisely the behavior you expect from shallow comparisons.

Is This Practice Advisable?

While the implementation is technically valid in Rust, there are some considerations:

• Code Clarity: Shallow comparisons may lead to confusion, particularly among team members who might not expect pointer behavior to differ from the default deep comparison.
• Idiomatic Rust: While you can implement such logic, it can feel less idiomatic in a language designed around ownership and borrowing. Developers accustomed to Rust's approach might prefer deep comparisons.
• Potential Pitfalls: Relying heavily on shallow comparisons could lead to bugs if mistaken identity occurs unintentionally due to pointer comparisons not mirroring the real equivalence in data.

Frequently Asked Questions

1. Can this shallow comparison implementation lead to issues?

Yes, while it works well, ensure that you handle cases appropriately in your code that might expect deep equality rather than shallow.

2. Is there a guideline on when to prefer shallow vs. deep comparison in Rust?

Generally, use shallow comparisons when performance on reference checks is critical and ensure to document your intentions clearly to prevent misunderstanding by other developers.

3. How does this shallow implementation affect the usage of equality in collections?

Shallow comparisons will not interfere with using your type in collections like HashSet and HashMap, as they will respect the hashing and equality defined.

Conclusion

In this guide, we've examined how to implement shallow comparison and hashing in a Rust struct, allowing Box, Rc, and Arc to adopt this behavior. Using std::ptr::eq provides a powerful mechanism to establish reference equality, essential in certain contexts within your Rust applications. Always consider the implications of such comparisons and aim for clarity in your implementations. The Rust community thrives on idiomatic practices, so let your coding style reflect it where possible.