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

Question

I'm having trouble initializing a fixed length array. My attempts so far all result in the same "use of possibly uninitialized variable: foo_array" error:

#[derive(Debug)] struct Foo { a: u32, b: u32 }  impl Default for Foo {     fn default() -> Foo { Foo{a:1, b:2} } }  pub fn main() {     let mut foo_array: [Foo; 10];      // Do something here to in-place initialize foo_array?      for f in foo_array.iter() {         println!("{:?}", f);     } } 

error[E0381]: use of possibly uninitialized variable: `foo_array`   --> src/main.rs:13:14    | 13 |     for f in foo_array.iter() {    |              ^^^^^^^^^ use of possibly uninitialized `foo_array` 

I implemented the Default trait, but Rust does not seem to call this by default akin to a C++ constructor.

What is the proper way to initialize a fixed length array? I'd like to do an efficient in-place initialization rather than some sort of copy.

Related: Why is the Copy trait needed for default (struct valued) array initialization?

Related: Is there a way to not have to initialize arrays twice?

Answer 1

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.