What is the proper way to initialize a fixed length array?

by

The safe but somewhat inefficient solution:

#[derive(Copy, Clone, Debug)]
struct Foo {
    a: u32,
    b: u32,
}

fn main() {
    let mut foo_array = [Foo { a: 10, b: 10 }; 10];
}

Since you’re specifically asking for a solution without copies:

use std::mem::MaybeUninit;

#[derive(Debug)]
struct Foo {
    a: u32,
    b: u32,
}

// We're just implementing Drop to prove there are no unnecessary copies.
impl Drop for Foo {
    fn drop(&mut self) {
        println!("Destructor running for a Foo");
    }
}

pub fn main() {
    let array = {
        // Create an array of uninitialized values.
        let mut array: [MaybeUninit<Foo>; 10] = unsafe { MaybeUninit::uninit().assume_init() };

        for (i, element) in array.iter_mut().enumerate() {
            let foo = Foo { a: i as u32, b: 0 };
            *element = MaybeUninit::new(foo);
        }

        unsafe { std::mem::transmute::<_, [Foo; 10]>(array) }
    };

    for element in array.iter() {
        println!("{:?}", element);
    }
}

This is recommended by the documentation of MaybeUninit.

More Related Contents:

  1. How to get a slice as an array in Rust?
  2. How to convert a slice into an array reference?
  3. What is the difference between a slice and an array?
  4. How can I convert a buffer of a slice of bytes (&[u8]) to an integer?
  5. How to allocate arrays on the heap in Rust 1.0?
  6. How to set a Rust array length dynamically?
  7. Why does println! work only for arrays with a length less than 33?
  8. Temporarily transmute [u8] to [u16]
  9. Is there a way to not have to initialize arrays twice?
  10. How can I initialize an array using a function? [duplicate]
  11. How can I store function pointers in an array? [duplicate]
  12. Is there any way to allocate a standard Rust array directly on the heap, skipping the stack entirely?
  13. How do you copy between arrays of different sizes in Rust?
  14. What expressions are allowed as the array length N in [_; N]?
  15. listing directories and sub-directories to an array with path;
  16. VBA array sort function?
  17. How do I do multiple assignment in MATLAB?
  18. How to find if an array contains a string [duplicate]
  19. Array versus linked-list
  20. Difference between List and Array types in Kotlin
  21. How to automatically input an array formula as string with more than 255 characters in length into an excel cell using VBA?
  22. How can I interleave two arrays?
  23. Defining an array of differing generic types in TypeScript
  24. Why does `Array(0,1,2) == Array(0,1,2)` not return the expected result?
  25. How can I convert an array to a tuple?
  26. Querying MongoDB to match in the first item in an array
  27. Why does the reverse() function in the Swift standard library return ReverseRandomAccessCollection?
  28. Compare 2 arrays and list the differences – Swift
  29. calculating the number of “inversions” in a permutation
  30. Find the first element in a sorted array that is greater than the target

Leave a Comment