Why Return by Value is Idiomatic in Rust (vs. Out Parameters)

Rust is a powerful programming language known for its focus on safety, performance, and developer productivity. If you’re diving into Rust, you might wonder: Why does Rust favor returning values from functions (return by value) instead of using out parameters like some other languages? This question gets to the heart of Rust’s design philosophy and its unique ownership model.

In this guide, we’ll explore why return by value is idiomatic in Rust, how it compares to out parameters, and why this choice makes Rust code safer and clearer. Rather than giving you a direct answer, let’s reason through it together with questions, examples, and insights. By the end, you’ll understand Rust’s approach and how to apply it in your own code. Let’s get started!

What Does “Return by Value” Mean in Rust?

Before we dive in, let’s clarify what return by value means. In programming, when a function returns a value, it passes a copy (or moves ownership) of the data back to the caller. In Rust, this is the default way functions provide results. For example:

fn create_string() -> String {
    String::from("Hello, Rust!")
}

Here, the function create_string returns a String value, transferring ownership to the caller.

Contrast this with out parameters, where a function modifies a variable passed to it by reference, often seen in languages like C++ or C#:

// C++ example with out parameter
void create_string(std::string* out) {
    *out = "Hello, C++!";
}

So, why does Rust lean toward returning values instead of using out parameters? Let’s explore by asking: What makes returning a value more natural in Rust’s design?

Step 1: Understanding Rust’s Ownership Model

Rust’s ownership system is the key to answering this question. Ownership ensures memory safety without a garbage collector, using three core rules:

• Each value has a single owner.
• When the owner goes out of scope, the value is dropped.
• You can borrow values with references (& or &mut), but with strict rules.

Why Does Ownership Matter for Return by Value?

Let’s think about it: How does ownership influence how functions pass data? When a function returns a value in Rust, it transfers ownership to the caller. This is explicit and aligns with Rust’s philosophy of making data flow clear. For example:

fn get_number() -> i32 {
    42
}

fn main() {
    let num = get_number(); // Ownership of 42 moves to `num`
    println!("{}", num);
}

Returning a value makes it obvious that the caller now owns num. But what if we used an out parameter instead? Let’s imagine:

fn get_number_out(out: &mut i32) {
    *out = 42;
}

This works, but ask yourself: Does this feel as clear as returning a value? Out parameters introduce side effects, making it harder to track who owns or modifies the data. Rust’s ownership model encourages explicitness, and returning values fits this perfectly.

Step 2: Comparing Return by Value to Out Parameters

To understand why return by value is idiomatic, let’s compare it to out parameters. Consider these questions:

• Which approach is easier to read and reason about?
• Which aligns better with Rust’s safety guarantees?

Return by Value: The Rust Way

When a function returns a value, the code is straightforward. The function’s signature tells you exactly what it produces:

fn add(a: i32, b: i32) -> i32 {
    a + b
}

The caller knows it gets an i32 and owns it. There’s no hidden state or modification to track.

Out Parameters: The Alternative

With out parameters, the function modifies a mutable reference passed by the caller:

fn add_out(a: i32, b: i32, result: &mut i32) {
    *result = a + b;
}

This requires the caller to:

1. Create a mutable variable.
2. Pass it as a reference.
3. Trust the function to modify it correctly.

Here’s how it looks in use:

fn main() {
    let mut sum = 0;
    add_out(5, 3, &mut sum);
    println!("{}", sum); // Prints 8
}

Ask yourself: Does this code feel more complex than returning a value? It requires extra steps and mental overhead to track the mutable state.

Comparison Table: Return by Value vs. Out Parameters

Aspect	Return by Value	Out Parameters
Clarity	Clear: Function signature shows output	Less clear: Side effects via references
Ownership	Moves ownership explicitly	Modifies borrowed data
Safety	No risk of unintended mutation	Risk of misuse with mutable refs
Code Simplicity	Fewer lines, direct usage	Extra setup for mutable variables
Flexibility	Works for all types, including complex	Limited to mutable references

Reflect on this: Why might Rust’s designers prefer a system where data flow is explicit and predictable?

Step 3: Why Return by Value is Idiomatic in Rust

Now that we’ve compared the two approaches, let’s dig into why return by value is the idiomatic choice in Rust. Here are the key reasons, each with a question to guide your thinking.

1. Alignment with Ownership and Borrowing

Rust’s ownership model is designed to prevent bugs like dangling pointers or data races. Returning a value ensures ownership is transferred cleanly. Consider this example:

fn create_vec() -> Vec<i32> {
    vec![1, 2, 3]
}

The Vec<i32> is created, owned by the function, and then moved to the caller. No references, no ambiguity. Now, think: What happens if we use an out parameter instead?

fn create_vec_out(out: &mut Vec<i32>) {
    *out = vec![1, 2, 3];
}

This requires the caller to pre-allocate a Vec and pass a mutable reference, which feels clunky. It also raises questions: Who initialized the Vec? Is it safe to modify? Returning a value avoids these concerns.

2. Code Clarity and Readability

Rust emphasizes code that’s easy to understand. Returning a value makes the function’s purpose clear. For example:

fn parse_input(input: &str) -> Option<i32> {
    input.parse().ok()
}

The signature tells you: “This function takes a string and returns an Option<i32>.” Contrast with an out parameter:

fn parse_input_out(input: &str, out: &mut Option<i32>) {
    *out = input.parse().ok();
}

Which is easier to follow? Why might a programmer prefer the first version? The return-by-value approach reduces cognitive load and makes the code more predictable.

3. Safety and Predictability

Out parameters introduce side effects, which can lead to bugs. For example, if a function forgets to set the out parameter, the caller might use uninitialized data. Rust’s compiler prevents this with return values because the function must return something matching its signature.

Consider: How does returning a value help Rust’s compiler catch errors? By enforcing a return type, Rust ensures the function provides a valid value, reducing the risk of undefined behavior.

4. Move Semantics and Zero-Cost Abstractions

Rust’s move semantics make returning values efficient. When you return a value, Rust often avoids copying large data structures thanks to return value optimization (RVO). For example:

fn large_data() -> Vec<i32> {
    let mut v = Vec::with_capacity(1000);
    for i in 0..1000 {
        v.push(i);
    }
    v // Moved, not copied
}

The Vec is moved to the caller without expensive copying. Ask yourself: Would using an out parameter be as efficient? With an out parameter, you’d need to pre-allocate and pass a mutable reference, which doesn’t guarantee the same optimization.

5. Functional Programming Influence

Rust draws inspiration from functional programming, where functions are pure and return values rather than modifying state. Returning values aligns with this philosophy, making Rust code more composable. For example:

fn double(x: i32) -> i32 {
    x * 2
}

fn main() {
    let result = double(5).to_string(); // Chaining is natural
}

With out parameters, chaining becomes awkward. Reflect: How does returning values make Rust code more flexible for chaining or composition?

Step 4: When Might Out Parameters Make Sense?

While return by value is idiomatic, there are rare cases where out parameters are useful in Rust. Let’s explore: When might you choose an out parameter over a return value?

• Performance in Specific Cases: If a function modifies a large, pre-allocated structure, an out parameter might avoid allocation overhead. For example, appending to an existing Vec:

fn append_data(v: &mut Vec<i32>) {
    v.push(42);
}

• Multiple Outputs: If a function needs to produce multiple results, out parameters can avoid creating a tuple or custom struct. However, Rust often prefers returning a tuple:

fn split_string(s: &str) -> (String, String) {
    let parts: Vec<&str> = s.split_at(s.len() / 2);
    (parts[0].to_string(), parts[1].to_string())
}

Even here, returning a tuple is clearer than using multiple out parameters. Ask: Why might a tuple be better than multiple out parameters for clarity?

Step 5: Common Patterns in Rust Code

To see why return by value is idiomatic, let’s look at common Rust patterns that rely on it:

• Option and Result Types: Rust’s Option and Result types are returned by value to handle errors or optional data:

fn divide(a: i32, b: i32) -> Result<i32, String> {
    if b == 0 {
        Err("Division by zero".to_string())
    } else {
        Ok(a / b)
    }
}

• Chaining Operations: Returning values enables method chaining, like in iterators:

let numbers = vec![1, 2, 3]
    .into_iter()
    .map(|x| x * 2)
    .collect::<Vec<i32>>();

• Builder Patterns: Many Rust APIs return structs that can be modified and returned:

let config = Config::new()
    .set_timeout(30)
    .build();

Reflect: How do these patterns show the power of returning values in Rust?

Step 6: Addressing Common Concerns

You might be thinking: What about performance? Isn’t returning large values expensive? Let’s address some concerns:

• Performance: Thanks to move semantics and RVO, returning values is often as efficient as using out parameters. The compiler optimizes moves to avoid unnecessary copies.
• Complex Returns: For multiple outputs, tuples or structs are idiomatic and clear.
• Legacy Code: If you’re coming from C++ or C#, out parameters might feel familiar, but Rust’s ownership model makes them less necessary.

Ask yourself: Do these concerns outweigh the benefits of clarity and safety in Rust’s approach?

Step 7: Practical Example: Building a Parser

Let’s apply this knowledge with a real-world example. Suppose you’re writing a function to parse a CSV string into a vector of numbers. Here’s the return-by-value approach:

fn parse_csv(input: &str) -> Result<Vec<i32>, String> {
    let numbers: Result<Vec<i32>, _> = input
        .split(',')
        .map(|s| s.trim().parse().map_err(|e| e.to_string()))
        .collect();
    numbers
}

Now, the out-parameter version:

fn parse_csv_out(input: &str, out: &mut Vec<i32>) -> Result<(), String> {
    out.clear();
    for s in input.split(',') {
        let num = s.trim().parse().map_err(|e| e.to_string())?;
        out.push(num);
    }
    Ok(())
}

Which is better? Why does the first version feel more “Rusty”? The return-by-value version is concise, explicit, and avoids side effects. It also leverages Rust’s Result type for error handling, making the code more robust.

FAQs About Return by Value in Rust

Conclusion: Embrace Return by Value in Rust

Rust’s preference for return by value over out parameters comes from its ownership model, focus on clarity, and emphasis on safety. By returning values, you write code that’s easier to read, safer to use, and aligns with Rust’s functional-inspired design. Next time you write a function, ask: How can I make this function’s output explicit and predictable?

Start experimenting with Rust’s return-by-value approach in your projects. Try rewriting a function that uses out parameters to return a value instead. What do you notice about the code’s clarity?

Resource:

• Dive deeper into Rust’s ownership model with The Rust Programming Language Book.
• 16 KB Memory Page Sizes: Guide for Android App Developers
• How to Fix Error Code 102630