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Step 1.4: Clone-on-write

Estimated time: 1 day

Clone-on-write

Rust has a Cow (clone-on-write) smart pointer in its standard library. Understanding how to use it is essential to write idiomatic and ergonomic Rust code.

In a nutshell:

  • it allows to combine usage of owned and borrowed data in a single abstraction, which leads to better ergonomics and minimize performance penalties asap (as much as possible);
  • it encloses and provides immutable access to borrowed data, and clones the data lazily when mutation or ownership is required.
use std::borrow::Cow;

fn describe(error: &Error) -> Cow<'static, str> {
    match *error {
        // Returning &'str - a borrowed reference to static str.
        Error::NotFound => "Error: Not found".into(),
        
        // Returning String - an owned String allocated in heap.
        Error::Custom(e) => format!("Error: {}", e).into(),
    }
}

For better understanding Cow purpose, design, limitations and use cases read through:

Alternative implementations

beef crate provides alternative Cow types, being faster and leaner.

There are two versions of Cow exposed by this crate:

  • beef::Cow is 3 words wide: pointer, length, and capacity. It stores the ownership tag in capacity.
  • beef::lean::Cow is 2 words wide, storing length, capacity, and the ownership tag all in one word.

Both versions are leaner than the std::borrow::Cow:

use std::mem::size_of;

const WORD: usize = size_of::<usize>();

assert_eq!(size_of::<std::borrow::Cow<str>>(), 4 * WORD);
assert_eq!(size_of::<beef::Cow<str>>(), 3 * WORD);

// Lean variant is two words on 64-bit architecture
#[cfg(target_pointer_width = "64")]
assert_eq!(size_of::<beef::lean::Cow<str>>(), 2 * WORD);

Read implementation details and design insights in its README.

Task

Write a simple program which prints out the path to its configuration file. The path should be detected with the following precedence:

  1. default path is /etc/app/app.conf;
  2. if APP_CONF env var is specified (and not empty) then use it with higher priority than default;
  3. if --conf command line argument is specified (error if empty) then use it with the highest priority.

If neither APP_CONF env var nor --conf command line argument is specified, then no allocation should happen for path detection.

Questions

After completing everything above, you should be able to answer (and understand why) the following questions:

  • What is Cow? How it works?
  • When Cow is useful and why? Give some meaningful examples.