Sprimo/skills/m01-ownership/comparison.md

Ownership: Comparison with Other Languages

Rust vs C++

Memory Management

Aspect	Rust	C++
Default	Move semantics	Copy semantics (pre-C++11)
Move	let b = a; (a invalidated)	auto b = std::move(a); (a valid but unspecified)
Copy	let b = a.clone();	auto b = a;
Safety	Compile-time enforcement	Runtime responsibility

Rust Move vs C++ Move

// Rust: after move, 'a' is INVALID
let a = String::from("hello");
let b = a;  // a moved
// println!("{}", a);  // COMPILE ERROR

// Equivalent in C++:
// std::string a = "hello";
// std::string b = std::move(a);
// std::cout << a;  // UNDEFINED (compiles but buggy)

Smart Pointers

Rust	C++	Purpose
Box<T>	std::unique_ptr<T>	Unique ownership
Rc<T>	std::shared_ptr<T>	Shared ownership
Arc<T>	std::shared_ptr<T> + atomic	Thread-safe shared
RefCell<T>	(manual runtime checks)	Interior mutability

---

Rust vs Go

Memory Model

Aspect	Rust	Go
Memory	Stack + heap, explicit	GC manages all
Ownership	Enforced at compile-time	None (GC handles)
Null	Option<T>	nil for pointers
Concurrency	Send/Sync traits	Channels (less strict)

Sharing Data

// Rust: explicit about sharing
use std::sync::Arc;
let data = Arc::new(vec![1, 2, 3]);
let data_clone = Arc::clone(&data);
std::thread::spawn(move || {
    println!("{:?}", data_clone);
});

// Go: implicit sharing
// data := []int{1, 2, 3}
// go func() {
//     fmt.Println(data)  // potential race condition
// }()

Why No GC in Rust

1. Deterministic destruction: Resources freed exactly when scope ends
2. Zero-cost: No GC pauses or overhead
3. Embeddable: Works in OS kernels, embedded systems
4. Predictable latency: Critical for real-time systems

---

Rust vs Java/C#

Reference Semantics

Aspect	Rust	Java/C#
Objects	Owned by default	Reference by default
Null	Option<T>	null (nullable)
Immutability	Default	Must use final/readonly
Copy	Explicit .clone()	Reference copy (shallow)

Comparison

// Rust: clear ownership
fn process(data: Vec<i32>) {  // takes ownership
    // data is ours, will be freed at end
}

let numbers = vec![1, 2, 3];
process(numbers);
// numbers is invalid here

// Java: ambiguous ownership
// void process(List<Integer> data) {
//     // Who owns data? Caller? Callee? Both?
//     // Can caller still use it?
// }

---

Rust vs Python

Memory Model

Aspect	Rust	Python
Typing	Static, compile-time	Dynamic, runtime
Memory	Ownership-based	Reference counting + GC
Mutability	Default immutable	Default mutable
Performance	Native, zero-cost	Interpreted, higher overhead

Common Pattern Translation

// Rust: borrowing iteration
let items = vec!["a", "b", "c"];
for item in &items {
    println!("{}", item);
}
// items still usable

// Python: iteration doesn't consume
// items = ["a", "b", "c"]
// for item in items:
//     print(item)
// items still usable (different reason - ref counting)

---

Unique Rust Concepts

Concepts Other Languages Lack

1. Borrow Checker: No other mainstream language has compile-time borrow checking
2. Lifetimes: Explicit annotation of reference validity
3. Move by Default: Values move, not copy
4. No Null: Option<T> instead of null pointers
5. Affine Types: Values can be used at most once

Learning Curve Areas

Concept	Coming From	Key Insight
Ownership	GC languages	Think about who "owns" data
Borrowing	C/C++	Like references but checked
Lifetimes	Any	Explicit scope of validity
Move	C++	Move is default, not copy

---

Mental Model Shifts

From GC Languages (Java, Go, Python)

Before: "Memory just works, GC handles it"
After:  "I explicitly decide who owns data and when it's freed"

Key shifts:

• Think about ownership at design time
• Returning references requires lifetime thinking
• No more null - use Option<T>

From C/C++

Before: "I manually manage memory and hope I get it right"
After:  "Compiler enforces correctness, I fight the borrow checker"

Key shifts:

• Trust the compiler's errors
• Move is the default (unlike C++ copy)
• Smart pointers are idiomatic, not overhead

From Functional Languages (Haskell, ML)

Before: "Everything is immutable, copying is fine"
After:  "Mutability is explicit, ownership prevents aliasing"

Key shifts:

• Mutability is safe because of ownership rules
• No persistent data structures needed (usually)
• Performance characteristics are explicit

---

Performance Trade-offs

Language	Memory Overhead	Latency	Throughput
Rust	Minimal (no GC)	Predictable	Excellent
C++	Minimal	Predictable	Excellent
Go	GC overhead	GC pauses	Good
Java	GC overhead	GC pauses	Good
Python	High (ref counting + GC)	Variable	Lower

When Rust Ownership Wins

1. Real-time systems: No GC pauses
2. Embedded: No runtime overhead
3. High-performance: Zero-cost abstractions
4. Concurrent: Data races prevented at compile time

When GC Might Be Preferable

1. Rapid prototyping: Less mental overhead
2. Complex object graphs: Cycles are tricky in Rust
3. GUI applications: Object lifetimes are dynamic
4. Small programs: Overhead doesn't matter