The differences between Function Pointers and Closures — How to Rust

In Rust, both function pointers and closures allow you to assign functions to variables and pass them as arguments, but they differ in key ways that influence how and when you might choose to use one over the other. Here’s a detailed comparison:

Function Pointers

A function pointer is a type of variable that directly points to a function with a given signature. It’s similar to function pointers in C and C++. In Rust, function pointers are used when you need to pass functions around that do not capture any environment from their surrounding scope.

• Syntax: The type of a function pointer is denoted using thefnkeyword, followed by its signature. For example, fn(i32) -> i32 is a type of a function pointer that takes an argumenti32 and returns an i32.
• Usage: Function pointers are typically used in situations where you need to pass pure functions without any captured context.
• Limitations: Function pointers can’t capture state. They are best suited for cases where the function is stateless and external to the current scope.

Example

fn add_one(x: i32) -> i32 {
    x + 1
}

let fn_pointer: fn(i32) -> i32 = add_one;
println!("Function pointer output: {}", fn_pointer(2));

Closures

Closures are more flexible compared to function pointers. They can capture their surrounding environment and maintain states. In Rust, closures are implemented as anonymous structs that can implement one or more of the traits: Fn, FnMut, and FnOnce, depending on how they capture their environment.

• Syntax: Closures are defined using pipes (||) and can capture variables from their scope. The closure’s type is generally inferred by Rust.
• Capturing: Closures can capture variables by reference, by mutable reference, or by taking ownership, depending on what operations are performed on the variables inside the closure.
• Flexibility and Use Cases: Closures are particularly useful in handling tasks that require maintaining some context or state, such as filtering or mapping items in a collection, or handling callbacks that interact with the environment they’re defined in.

Example:

let x = 4;
let equal_to_x = move |z| z == x;  // The closure takes ownership of `x` using `move`
println!("Closure output: {}", equal_to_x(4));  // Output: true

Comparison

• Performance: Function pointers can be slightly more performant than closures, as they are simple pointers to functions without any overhead. However, in practice, this difference is often negligible due to Rust’s optimizations.
• Capability: Closures are more capable due to their ability to capture state. Function pointers are limited to referring to predefined functions without any external state.
• Interchangeability: Although both can be used in similar contexts, you can’t use a function pointer where a closure that captures the environment is necessary, and vice versa without adapting the approach.

Conclusion

Choose function pointers in Rust when you have a stateless function that you want to pass around, particularly when interfacing with C code or when absolute performance is crucial and the function does not require any contextual data.

Opt for closures when you need to capture and manipulate the surrounding environment or maintain state within a function, providing greater flexibility and power in most Rust applications, such as iterators and threading models.