Default Macro

A convenient macro to implement Default for structs using field initializers.

Usage

Add this to your Cargo.toml:

[dependencies]
default2 = "2"

Example

Use the default2::default! macro to define a struct and its default values in one place.

default2::default! {
    pub struct Process {
        id: i32 = 10,
        name: String = "main".into(),
        cpus: usize = num_cpus::get(),
        payload: u64,
    }
}

The macro will generate the standard struct definition along with a Default implementation:

pub struct Process {
    id: i32,
    name: String,
    cpus: usize,
    payload: u64,
}

impl Default for Process {
    fn default() -> Self {
        Process {
            id: 10,
            name: "main".into(),
            cpus: num_cpus::get(),
            payload: std::default::Default::default(),
        }
    }
}

Const Defaults

You can also generate a const default function by adding the #[const_default] attribute to your struct. This will generate an inherent method const_default().

You are responsible for ensuring that all default value expressions are valid in a const context.

use default2::default;

default! {
    #[const_default]
    pub struct MyConfig {
        timeout: u32 = 500,
        name: &'static str = "default_config",
    }
}

// You can now create a const instance:
const MY_CONFIG: MyConfig = MyConfig::const_default();
assert_eq!(MY_CONFIG.timeout, 500);

// The standard non-const Default trait is still implemented:
let other_config = MyConfig::default();
assert_eq!(other_config.name, "default_config");