构造器

说明

Rust 没有语言层面的构造器。 取而代之的是常用一个[关联函数][] new 创建对象：

示例

#![allow(unused)]
fn main() {
/// Time in seconds.
///
/// # Example
///
/// ```
/// let s = Second::new(42);
/// assert_eq!(42, s.value());
/// ```
pub struct Second {
    value: u64
}
impl Second {
    // Constructs a new instance of [`Second`].
    // Note this is an associated function - no self.
    pub fn new(value: u64) -> Self {
        Self { value }
    }
    /// Returns the value in seconds.
    pub fn value(&self) -> u64 {
        self.value
    }
}
}

Default Constructors

Rust supports default constructors with the Default trait:

// A Rust vector, see liballoc/vec.rs
pub struct Vec<T> {
    buf: RawVec<T>,
    len: usize,
```rust
/// Time in seconds.
///
/// # Example
///
/// ```
/// let s = Second::default();
/// assert_eq!(0, s.value());
/// ```
pub struct Second {
    value: u64
}
impl Second {
    /// Returns the value in seconds.
    pub fn value(&self) -> u64 {
        self.value
    }
}
impl<T> Vec<T> {
    // Constructs a new, empty `Vec<T>`.
    // Note this is a static method - no self.
    // This constructor doesn't take any arguments, but some might in order to
    // properly initialise an object
    pub fn new() -> Vec<T> {
        // Create a new Vec with fields properly initialised.
        Vec {
            // Note that here we are calling RawVec's constructor.
            buf: RawVec::new(),
            len: 0,
        }
impl Default for Second {
    fn default() -> Self {
        Self { value: 0 }
    }
}

Default can also be derived if all types of all fields implement Default, like they do with Second:

#![allow(unused)]
fn main() {
/// Time in seconds.
///
/// # Example
///
/// ```
/// let s = Second::default();
/// assert_eq!(0, s.value());
/// ```
#[derive(Default)]
pub struct Second {
    value: u64
}
impl Second {
    /// Returns the value in seconds.
    pub fn value(&self) -> u64 {
        self.value
    }
}
}

Note: When implementing Default for a type, it is neither required nor recommended to also provide an associated function new without arguments.

Hint: The advantage of implementing or deriving Default is that your type can now be used where a Default implementation is required, most prominently, any of the *or_default functions in the standard library.

参阅