Rust for C-Programmers

9.4 Default Values and the Default Trait

Often, it’s useful to create a struct instance with default values. Rust provides the Default trait for this.

9.4.1 Deriving Default

If all fields in a struct themselves implement Default, you can derive Default for your struct.

#[derive(Default, Debug)]
struct AppConfig {
    server_address: String, // Default is ""
    port: u16,              // Default is 0
    timeout_ms: u32,        // Default is 0
}

fn main() {
    let config: AppConfig = Default::default();
    // Or let config = AppConfig::default();
    println!("Default config: {:?}", config);
    // Output: AppConfig { server_address: "", port: 0, timeout_ms: 0 }

    // Combine with struct update syntax
    let custom_config = AppConfig {
        port: 8080,
        ..Default::default() // Use defaults for other fields
    };
    println!("Custom config: {:?}", custom_config);
    // Output: AppConfig { server_address: "", port: 8080, timeout_ms: 0 }
}

9.4.2 Implementing Default Manually

If deriving isn’t suitable, implement Default manually.

struct ConnectionSettings {
    retries: u8,
    use_tls: bool,
}

impl Default for ConnectionSettings {
    fn default() -> Self {
        ConnectionSettings {
            retries: 3, // Custom default
            use_tls: true, // Custom default
        }
    }
}

fn main() {
    let settings = ConnectionSettings::default();
    println!("Default retries: {}", settings.retries); // 3
}