Constructors

Description

Rust does not have constructors as a language construct. Instead, the convention is to use an associated function new to create an object:

#![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:

#![allow(unused)]
fn main() {
/// 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 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.

See also

• The default idiom for a more in-depth description of the Default trait.

• The builder pattern for constructing objects where there are multiple configurations.