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<main>
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<h1 id="others"><a class="header" href="#others">Others</a></h1>
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<h3 id="convert-any-type-to-string"><a class="header" href="#convert-any-type-to-string">Convert any type to String</a></h3>
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<p>To convert any type to <code>String</code>, you can simply use the <code>ToString</code> trait for that type. Rather than doing that directly, you should implement the <code>fmt::Display</code> trait which will automatically provides <code>ToString</code> and also allows you to print the type with <code>println!</code>.</p>
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<ol>
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<li>🌟🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">use std::fmt;
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struct Point {
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x: i32,
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y: i32,
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}
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impl fmt::Display for Point {
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// IMPLEMENT fmt method
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}
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fn main() {
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let origin = Point { x: 0, y: 0 };
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// FILL in the blanks
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assert_eq!(origin.__, "The point is (0, 0)");
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assert_eq!(format!(__), "The point is (0, 0)");
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println!("Success!");
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}</code></pre></pre>
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<h3 id="parse-a-string"><a class="header" href="#parse-a-string">Parse a String</a></h3>
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<ol start="2">
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<li>🌟🌟🌟 We can use <code>parse</code> method to convert a <code>String</code> into a <code>i32</code> number, this is because <code>FromStr</code> is implemented for <code>i32</code> type in standard library: <code>impl FromStr for i32</code></li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">// To use `from_str` method, you need to introduce this trait into the current scope.
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use std::str::FromStr;
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fn main() {
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let parsed: i32 = "5".__.unwrap();
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let turbo_parsed = "10".__.unwrap();
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let from_str = __.unwrap();
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let sum = parsed + turbo_parsed + from_str;
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assert_eq!(sum, 35);
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println!("Success!");
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}</code></pre></pre>
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<ol start="3">
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<li>🌟🌟 We can also implement the <code>FromStr</code> trait for our custom types</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">use std::str::FromStr;
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use std::num::ParseIntError;
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#[derive(Debug, PartialEq)]
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struct Point {
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x: i32,
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y: i32
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}
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impl FromStr for Point {
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type Err = ParseIntError;
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fn from_str(s: &str) -> Result<Self, Self::Err> {
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let coords: Vec<&str> = s.trim_matches(|p| p == '(' || p == ')' )
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.split(',')
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.map(|x| x.trim())
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.collect();
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let x_fromstr = coords[0].parse::<i32>()?;
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let y_fromstr = coords[1].parse::<i32>()?;
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Ok(Point { x: x_fromstr, y: y_fromstr })
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}
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}
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fn main() {
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// FILL in the blanks in two ways
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// DON'T change code anywhere else
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let p = __;
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assert_eq!(p.unwrap(), Point{ x: 3, y: 4} );
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println!("Success!");
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}</code></pre></pre>
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<h3 id="deref"><a class="header" href="#deref">Deref</a></h3>
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<p>You can find all the examples and exercises of the <code>Deref</code> trait <a href="https://practice.rs/smart-pointers/deref.html">here</a>.</p>
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<h3 id="transmute"><a class="header" href="#transmute">Transmute</a></h3>
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<p><code>std::mem::transmute</code> is a <strong>unsafe function</strong> can be used to reinterprets the bits of a value of one type as another type. Both of the original and the result types must have the same size and neither of them can be invalid.</p>
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<p><code>transmute</code> 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 <code>memcpy</code> under the hood.</p>
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<p>So, <strong><code>transmute</code> is incredibly unsafe !</strong> The caller has to ensure all the safes himself!</p>
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<h4 id="examples"><a class="header" href="#examples">Examples</a></h4>
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<ol>
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<li><code>transmute</code> 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.</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">fn foo() -> i32 {
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0
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}
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fn main() {
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let pointer = foo as *const ();
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let function = unsafe {
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std::mem::transmute::<*const (), fn() -> i32>(pointer)
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};
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assert_eq!(function(), 0);
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}</code></pre></pre>
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<ol start="2">
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<li>Extending a lifetime or shortening the lifetime of an invariant is an advanced usage of <code>transmute</code>, yeah, <strong>very unsafe Rust!</strong>.</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">struct R<'a>(&'a i32);
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unsafe fn extend_lifetime<'b>(r: R<'b>) -> R<'static> {
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std::mem::transmute::<R<'b>, R<'static>>(r)
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}
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unsafe fn shorten_invariant_lifetime<'b, 'c>(r: &'b mut R<'static>)
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-> &'b mut R<'c> {
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std::mem::transmute::<&'b mut R<'static>, &'b mut R<'c>>(r)
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}</code></pre></pre>
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<ol start="3">
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<li>Rather than using <code>transmute</code>, you can use some alternatives instead.</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">fn main() {
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/*Turning raw bytes(&[u8]) to u32, f64, etc.: */
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let raw_bytes = [0x78, 0x56, 0x34, 0x12];
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let num = unsafe { std::mem::transmute::<[u8; 4], u32>(raw_bytes) };
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// Use `u32::from_ne_bytes` instead
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let num = u32::from_ne_bytes(raw_bytes);
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// Or use `u32::from_le_bytes` or `u32::from_be_bytes` to specify the endianness
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let num = u32::from_le_bytes(raw_bytes);
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assert_eq!(num, 0x12345678);
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let num = u32::from_be_bytes(raw_bytes);
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assert_eq!(num, 0x78563412);
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/*Turning a pointer into a usize: */
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let ptr = &0;
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let ptr_num_transmute = unsafe { std::mem::transmute::<&i32, usize>(ptr) };
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// Use an `as` cast instead
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let ptr_num_cast = ptr as *const i32 as usize;
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/*Turning an &mut T into an &mut U: */
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let ptr = &mut 0;
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let val_transmuted = unsafe { std::mem::transmute::<&mut i32, &mut u32>(ptr) };
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// Now, put together `as` and reborrowing - note the chaining of `as`
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// `as` is not transitive
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let val_casts = unsafe { &mut *(ptr as *mut i32 as *mut u32) };
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/*Turning an &str into a &[u8]: */
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// This is not a good way to do this.
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let slice = unsafe { std::mem::transmute::<&str, &[u8]>("Rust") };
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assert_eq!(slice, &[82, 117, 115, 116]);
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// You could use `str::as_bytes`
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let slice = "Rust".as_bytes();
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assert_eq!(slice, &[82, 117, 115, 116]);
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// Or, just use a byte string, if you have control over the string
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// literal
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assert_eq!(b"Rust", &[82, 117, 115, 116]);
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}</code></pre></pre>
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<blockquote>
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<p>You can find the solutions <a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/type-conversions/others.md">here</a>(under the solutions path), but only use it when you need it</p>
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