Testing a Clap arg with default value

main.rs

extern crate clap;

use clap::{Arg, App};

fn main() {
    let matches = App::new("A test")
        .arg(Arg::with_name("has-default")
             .short("d")
             .takes_value(true))
        .get_matches();

    let value: u32 = matches.value_of("has-default").and_then(|s| s.parse().ok()).unwrap_or(42);
    println!("The flag's value was {}", value);
}