rust-by-practice/en/src/type-conversions/others.md

Others

Convert any type to String

To convert any type to String, you can simply the ToString trait for that type. Rather than doing that directly, you should implement the fmt::Display trait which will automatically provides ToString and also allows you to print the type with println!.

1. 🌟🌟

use std::fmt;

struct Point {
    x: i32,
    y: i32,
}

impl fmt::Display for Point {
    // IMPLEMENT fmt method
}

fn main() {
    let origin = Point { x: 0, y: 0 };
    // FILL in the blanks
    assert_eq!(origin.__, "The point is (0, 0)");
    assert_eq!(format!(__), "The point is (0, 0)");

    println!("Success!")
}

Parse a String

2. 🌟🌟🌟 We can use parse method to convert a String into a i32 number, this is becuase FromStr is implemented for i32 type in standard library: impl FromStr for i32

// To use `from_str` method, you needs to introduce this trait into the current scope.
use std::str::FromStr;
fn main() {
    let parsed: i32 = "5".__.unwrap();
    let turbo_parsed = "10".__.unwrap();
    let from_str = __.unwrap();
    let sum = parsed + turbo_parsed + from_str;
    assert_eq!(sum, 35);

    println!("Success!")
}

3. 🌟🌟 We can also implement the FromStr trait for our custom types

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

#[derive(Debug, PartialEq)]
struct Point {
    x: i32,
    y: i32
}

impl FromStr for Point {
    type Err = ParseIntError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let coords: Vec<&str> = s.trim_matches(|p| p == '(' || p == ')' )
                                 .split(',')
                                 .collect();

        let x_fromstr = coords[0].parse::<i32>()?;
        let y_fromstr = coords[1].parse::<i32>()?;

        Ok(Point { x: x_fromstr, y: y_fromstr })
    }
}
fn main() {
    // FILL in the blanks in two ways
    // DON'T change code anywhere else 
    let p = __;
    assert_eq!(p.unwrap(), Point{ x: 3, y: 4} );

    println!("Success!")
}

Deref

You can find all the examples and exercises of the Deref trait here.

transmute

std::mem::transmute is a unsafe function can be used to reinterprets the bits of a value of one type as another type. Both of the orginal and the result types must have the same size and neither of them can be invalid.

transmute is semantically equivalent to a bitwise move of one type into another. It copies the bits from the source value into the destination value, then forgets the original, seems equivalent to C's memcpy under the hood.

So, transmute is incredibly unsafe ! The caller has to ensure all the safes himself!

Examples

1. transmute can be used to turn a pointer into a function pointer, this is not portable on machines where function pointer and data pointer have different sizes.

fn foo() -> i32 {
    0
}

fn main() {
    let pointer = foo as *const ();
    let function = unsafe {
    std::mem::transmute::<*const (), fn() -> i32>(pointer)
    assert_eq!(function(), 0);
}

2. Extending a lifetime or shortening the lifetime of an invariant is an advanced usage of transmute, yeah, very unsafe Rust!.

struct R<'a>(&'a i32);
unsafe fn extend_lifetime<'b>(r: R<'b>) -> R<'static> {
    std::mem::transmute::<R<'b>, R<'static>>(r)
}

unsafe fn shorten_invariant_lifetime<'b, 'c>(r: &'b mut R<'static>)
                                             -> &'b mut R<'c> {
    std::mem::transmute::<&'b mut R<'static>, &'b mut R<'c>>(r)
}

3. Rather than using transmute, you can use some alternatives instead.

fn main() {
    /*Turning raw bytes(&[u8]) to u32, f64, etc.: */
    let raw_bytes = [0x78, 0x56, 0x34, 0x12];

    let num = unsafe { std::mem::transmute::<[u8; 4], u32>(raw_bytes) };

    // use `u32::from_ne_bytes` instead
    let num = u32::from_ne_bytes(raw_bytes);
    // or use `u32::from_le_bytes` or `u32::from_be_bytes` to specify the endianness
    let num = u32::from_le_bytes(raw_bytes);
    assert_eq!(num, 0x12345678);
    let num = u32::from_be_bytes(raw_bytes);
    assert_eq!(num, 0x78563412);

    /*Turning a pointer into a usize: */
    let ptr = &0;
    let ptr_num_transmute = unsafe { std::mem::transmute::<&i32, usize>(ptr) };

    // Use an `as` cast instead
    let ptr_num_cast = ptr as *const i32 as usize;

    /*Turning an &mut T into an &mut U: */
    let ptr = &mut 0;
    let val_transmuted = unsafe { std::mem::transmute::<&mut i32, &mut u32>(ptr) };

    // Now, put together `as` and reborrowing - note the chaining of `as`
    // `as` is not transitive
    let val_casts = unsafe { &mut *(ptr as *mut i32 as *mut u32) };

    /*Turning an &str into a &[u8]: */
    // this is not a good way to do this.
    let slice = unsafe { std::mem::transmute::<&str, &[u8]>("Rust") };
    assert_eq!(slice, &[82, 117, 115, 116]);

    // You could use `str::as_bytes`
    let slice = "Rust".as_bytes();
    assert_eq!(slice, &[82, 117, 115, 116]);

    // Or, just use a byte string, if you have control over the string
    // literal
    assert_eq!(b"Rust", &[82, 117, 115, 116]);
}