From and Into

The From and Into traits are inherently linked, and this is actually part of its implementation:

  • From: If you are able to convert type A from type B
  • Into: then it should be easy to believe that we should be able to convert type B to type A.

From

The From trait allows for a type to define how to create itself from another type, hence providing a very simple mechanism for converting between several types.

There are numerous implementations of this trait within the standard library for conversion of primitive and common types.

For example we can easily convert a str into a String

 
#![allow(unused)]
fn main() {
let my_str = "hello";
let my_string = String::from(my_str);
}

We can do similar for defining a conversion for our own type.

 
use std::convert::From;

#[derive(Debug)]
struct Number {
    value: i32,
}

impl From<i32> for Number {
    fn from(item: i32) -> Self {
        Number { value: item }
    }
}

fn main() {
    let num = Number::from(30);
    println!("My number is {:?}", num);
}

Into

The Into trait is simply the reciprocal of the From trait:

  • That is, if you have implemented the From trait for your type, Into will call it when necessary.

  • Using the Into trait will typically require specification of the type to convert into as the compiler is unable to determine this most of the time.

  • However this is a small trade-off considering we get the functionality for free.

Into Example

 
use std::convert::From;

#[derive(Debug)]
struct Number {
    value: i32,
}

impl From<i32> for Number {
    fn from(item: i32) -> Self {
        Number { value: item }
    }
}

fn main() {
    let int = 5;
    // Try removing the type annotation
    let num: Number = int.into();
    println!("My number is {:?}", num);
}

Compare with as keyword