Stringとの型変換

Stringへの型変換

To convert any type to a String is as simple as implementing the ToString trait for the type. Rather than doing so directly, you should implement the fmt::Display trait which automatically provides ToString and also allows printing the type as discussed in the section on print!.

use std::fmt;

struct Circle {
    radius: i32
}

impl fmt::Display for Circle {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Circle of radius {}", self.radius)
    }
}

fn main() {
    let circle = Circle { radius: 6 };
    println!("{}", circle.to_string());
}

Stringの解析

文字列からの型変換において、数値への型変換はよく行われるものの一つです。これを行うイディオムはparse関数を使用することですが、このときに型を推論できるようにするか、もしくはターボフィッシュ構文（::<>）を使用して型を指定するかのいずれかを行います。以下の例では、どちらの方法も紹介しています。

This will convert the string into the type specified as long as the FromStr trait is implemented for that type. This is implemented for numerous types within the standard library.

fn main() {
    let parsed: i32 = "5".parse().unwrap();
    let turbo_parsed = "10".parse::<i32>().unwrap();

    let sum = parsed + turbo_parsed;
    println!("Sum: {:?}", sum);
}

To obtain this functionality on a user defined type simply implement the FromStr trait for that type.

use std::num::ParseIntError;
use std::str::FromStr;

#[derive(Debug)]
struct Circle {
    radius: i32,
}

impl FromStr for Circle {
    type Err = ParseIntError;
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s.trim().parse() {
            Ok(num) => Ok(Circle{ radius: num }),
            Err(e) => Err(e),
        }
    }
}

fn main() {
    let radius = "    3 ";
    let circle: Circle = radius.parse().unwrap();
    println!("{:?}", circle);
}