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<main>
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<h1 id="convert-by-as"><a class="header" href="#convert-by-as">Convert by <code>as</code></a></h1>
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<p>Rust provides no implicit type conversion(coercion) between primitive types. But explicit type conversions can be performed using the <code>as</code> keyword.</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">// FIX the errors and FILL in the blank
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// DON'T remove any code
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fn main() {
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let decimal = 97.123_f32;
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let integer: __ = decimal as u8;
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let c1: char = decimal as char;
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let c2 = integer as char;
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assert_eq!(integer, 'b' as u8);
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println!("Success!");
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}</code></pre></pre>
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<ol start="2">
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<li>🌟🌟 By default, overflow will cause compile errors, but we can add an global annotation to suppress these errors.</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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assert_eq!(u8::MAX, 255);
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// The max of `u8` is 255 as shown above.
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// so the below code will cause an overflow error: literal out of range for `u8`.
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||||
// PLEASE looking for clues within compile errors to FIX it.
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||||
// DON'T modify any code in main.
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||||
let v = 1000 as u8;
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||||
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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>🌟🌟 When casting any value to an unsigned type <code>T</code>, <code>T::MAX + 1</code> is added or subtracted until the value fits into the new type.</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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assert_eq!(1000 as u16, __);
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assert_eq!(1000 as u8, __);
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||||
// For positive numbers, this is the same as the modulus
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||||
println!("1000 mod 256 is : {}", 1000 % 256);
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assert_eq!(-1_i8 as u8, __);
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// Since Rust 1.45, the `as` keyword performs a *saturating cast*
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||||
// when casting from float to int. If the floating point value exceeds
|
||||
// the upper bound or is less than the lower bound, the returned value
|
||||
// will be equal to the bound crossed.
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||||
assert_eq!(300.1_f32 as u8, __);
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assert_eq!(-100.1_f32 as u8, __);
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// This behavior incurs a small runtime cost and can be avoided
|
||||
// with unsafe methods, however the results might overflow and
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||||
// return **unsound values**. Use these methods wisely:
|
||||
unsafe {
|
||||
// 300.0 is 44
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||||
println!("300.0 is {}", 300.0_f32.to_int_unchecked::<u8>());
|
||||
// -100.0 as u8 is 156
|
||||
println!("-100.0 as u8 is {}", (-100.0_f32).to_int_unchecked::<u8>());
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||||
// nan as u8 is 0
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||||
println!("nan as u8 is {}", f32::NAN.to_int_unchecked::<u8>());
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||||
}
|
||||
}</code></pre></pre>
|
||||
<ol start="4">
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||||
<li>🌟🌟🌟 Raw pointers can be converted to memory address (integer) and vice versa.</li>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">
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||||
// FILL in the blanks
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||||
fn main() {
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||||
let mut values: [i32; 2] = [1, 2];
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||||
let p1: *mut i32 = values.as_mut_ptr();
|
||||
let first_address: usize = p1 __;
|
||||
let second_address = first_address + 4; // 4 == std::mem::size_of::<i32>()
|
||||
let p2: *mut i32 = second_address __; // p2 points to the 2nd element in values
|
||||
unsafe {
|
||||
// Add one to the second element
|
||||
__
|
||||
}
|
||||
|
||||
assert_eq!(values[1], 3);
|
||||
|
||||
println!("Success!");
|
||||
}</code></pre></pre>
|
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<ol start="5">
|
||||
<li>🌟🌟🌟</li>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">fn main() {
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||||
let arr :[u64; 13] = [0; 13];
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||||
assert_eq!(std::mem::size_of_val(&arr), 8 * 13);
|
||||
let a: *const [u64] = &arr;
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||||
let b = a as *const [u8];
|
||||
unsafe {
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||||
assert_eq!(std::mem::size_of_val(&*b), __)
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||||
}
|
||||
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||||
println!("Success!");
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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/as.md">here</a>(under the solutions path), but only use it when you need it</p>
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<h1 id="frominto"><a class="header" href="#frominto">From/Into</a></h1>
|
||||
<p>The <code>From</code> 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.</p>
|
||||
<p>The <code>From</code> and <code>Into</code> traits are inherently linked, and this is actually part of its implementation. It means if we write something like this: <code>impl From<T> for U</code>, then we can use
|
||||
<code>let u: U = U::from(T)</code> or <code>let u:U = T.into()</code>.</p>
|
||||
<p>The <code>Into</code> trait is simply the reciprocal of the <code>From</code> trait. That is, if you have implemented the <code>From</code> trait for your type, then the <code>Into</code> trait will be automatically implemented for the same type.</p>
|
||||
<p>Using the <code>Into</code> trait will typically require the type annotations as the compiler is unable to determine this most of the time.</p>
|
||||
<p>For example, we can easily convert <code>&str</code> into <code>String</code> :</p>
|
||||
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
||||
let my_str = "hello";
|
||||
|
||||
// three conversions below all depends on the fact: String implements From<&str>:
|
||||
let string1 = String::from(my_str);
|
||||
let string2 = my_str.to_string();
|
||||
// Explicit type annotation is required here
|
||||
let string3: String = my_str.into();
|
||||
}</code></pre></pre>
|
||||
<p>Because the standard library has already implemented this for us : <code>impl From<&'_ str> for String</code> .</p>
|
||||
<p>Some implementations of <code>From</code> trait can be found <a href="https://doc.rust-lang.org/stable/std/convert/trait.From.html#implementors">here</a>.</p>
|
||||
<ol>
|
||||
<li>🌟🌟🌟</li>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">fn main() {
|
||||
// impl From<bool> for i32
|
||||
let i1: i32 = false.into();
|
||||
let i2: i32 = i32::from(false);
|
||||
assert_eq!(i1, i2);
|
||||
assert_eq!(i1, 0);
|
||||
|
||||
// FIX the error in two ways
|
||||
/* 1. use a similar type which `impl From<char>`, maybe you
|
||||
should check the docs mentioned above to find the answer */
|
||||
// 2. a keyword from the last chapter
|
||||
let i3: i32 = 'a'.into();
|
||||
|
||||
// FIX the error in two ways
|
||||
let s: String = 'a' as String;
|
||||
|
||||
println!("Success!");
|
||||
}</code></pre></pre>
|
||||
<h3 id="implement-from-for-custom-types"><a class="header" href="#implement-from-for-custom-types">Implement <code>From</code> for custom types</a></h3>
|
||||
<ol start="2">
|
||||
<li>🌟🌟</li>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">// From is now included in `std::prelude`, so there is no need to introduce it into the current scope
|
||||
// use std::convert::From;
|
||||
|
||||
#[derive(Debug)]
|
||||
struct Number {
|
||||
value: i32,
|
||||
}
|
||||
|
||||
impl From<i32> for Number {
|
||||
// IMPLEMENT `from` method
|
||||
}
|
||||
|
||||
// FILL in the blanks
|
||||
fn main() {
|
||||
let num = __(30);
|
||||
assert_eq!(num.value, 30);
|
||||
|
||||
let num: Number = __;
|
||||
assert_eq!(num.value, 30);
|
||||
|
||||
println!("Success!");
|
||||
}</code></pre></pre>
|
||||
<ol start="3">
|
||||
<li>🌟🌟🌟 When performing error handling it is often useful to implement <code>From</code> trait for our own error type. Then we can use <code>?</code> to automatically convert the underlying error type to our own error type.</li>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">use std::fs;
|
||||
use std::io;
|
||||
use std::num;
|
||||
|
||||
enum CliError {
|
||||
IoError(io::Error),
|
||||
ParseError(num::ParseIntError),
|
||||
}
|
||||
|
||||
impl From<io::Error> for CliError {
|
||||
// IMPLEMENT from method
|
||||
}
|
||||
|
||||
impl From<num::ParseIntError> for CliError {
|
||||
// IMPLEMENT from method
|
||||
}
|
||||
|
||||
fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
|
||||
// ? automatically converts io::Error to CliError
|
||||
let contents = fs::read_to_string(&file_name)?;
|
||||
// num::ParseIntError -> CliError
|
||||
let num: i32 = contents.trim().parse()?;
|
||||
Ok(num)
|
||||
}
|
||||
|
||||
fn main() {
|
||||
println!("Success!");
|
||||
}</code></pre></pre>
|
||||
<h3 id="tryfromtryinto"><a class="header" href="#tryfromtryinto">TryFrom/TryInto</a></h3>
|
||||
<p>Similar to <code>From</code> and <code>Into</code>, <code>TryFrom</code> and <code>TryInto</code> are generic traits for converting between types.</p>
|
||||
<p>Unlike <code>From/Into</code>, <code>TryFrom</code> and <code>TryInto</code> are used for fallible conversions and return a <code>Result</code> instead of a plain value.</p>
|
||||
<ol start="4">
|
||||
<li>🌟🌟</li>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">// TryFrom and TryInto are included in `std::prelude`, so there is no need to introduce it into the current scope
|
||||
// use std::convert::TryInto;
|
||||
|
||||
fn main() {
|
||||
let n: i16 = 256;
|
||||
|
||||
// Into trait has a method `into`,
|
||||
// hence TryInto has a method ?
|
||||
let n: u8 = match n.__() {
|
||||
Ok(n) => n,
|
||||
Err(e) => {
|
||||
println!("there is an error when converting: {:?}, but we catch it", e.to_string());
|
||||
0
|
||||
}
|
||||
};
|
||||
|
||||
assert_eq!(n, __);
|
||||
|
||||
println!("Success!");
|
||||
}</code></pre></pre>
|
||||
<ol start="5">
|
||||
<li>🌟🌟🌟</li>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">#[derive(Debug, PartialEq)]
|
||||
struct EvenNum(i32);
|
||||
|
||||
impl TryFrom<i32> for EvenNum {
|
||||
type Error = ();
|
||||
|
||||
// IMPLEMENT `try_from`
|
||||
fn try_from(value: i32) -> Result<Self, Self::Error> {
|
||||
if value % 2 == 0 {
|
||||
Ok(EvenNum(value))
|
||||
} else {
|
||||
Err(())
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
fn main() {
|
||||
assert_eq!(EvenNum::try_from(8), Ok(EvenNum(8)));
|
||||
assert_eq!(EvenNum::try_from(5), Err(()));
|
||||
|
||||
// FILL in the blanks
|
||||
let result: Result<EvenNum, ()> = 8i32.try_into();
|
||||
assert_eq!(result, __);
|
||||
let result: Result<EvenNum, ()> = 5i32.try_into();
|
||||
assert_eq!(result, __);
|
||||
|
||||
println!("Success!");
|
||||
}</code></pre></pre>
|
||||
<blockquote>
|
||||
<p>You can find the solutions <a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/type-conversions/from-into.md">here</a>(under the solutions path), but only use it when you need it</p>
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<li>English: <a href="https://std.rs">Standary library</a></li>
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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>
|
||||
<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>
|
||||
<li>🌟🌟</li>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">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!");
|
||||
}</code></pre></pre>
|
||||
<h3 id="parse-a-string"><a class="header" href="#parse-a-string">Parse a String</a></h3>
|
||||
<ol start="2">
|
||||
<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>
|
||||
</ol>
|
||||
<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.
|
||||
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!");
|
||||
}</code></pre></pre>
|
||||
<ol start="3">
|
||||
<li>🌟🌟 We can also implement the <code>FromStr</code> trait for our custom types</li>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">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(',')
|
||||
.map(|x| x.trim())
|
||||
.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!");
|
||||
}</code></pre></pre>
|
||||
<h3 id="deref"><a class="header" href="#deref">Deref</a></h3>
|
||||
<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>
|
||||
<h3 id="transmute"><a class="header" href="#transmute">Transmute</a></h3>
|
||||
<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>
|
||||
<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>
|
||||
<p>So, <strong><code>transmute</code> is incredibly unsafe !</strong> The caller has to ensure all the safes himself!</p>
|
||||
<h4 id="examples"><a class="header" href="#examples">Examples</a></h4>
|
||||
<ol>
|
||||
<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>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">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);
|
||||
}</code></pre></pre>
|
||||
<ol start="2">
|
||||
<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>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">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)
|
||||
}</code></pre></pre>
|
||||
<ol start="3">
|
||||
<li>Rather than using <code>transmute</code>, you can use some alternatives instead.</li>
|
||||
</ol>
|
||||
<pre><pre class="playground"><code class="language-rust editable edition2021">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]);
|
||||
}</code></pre></pre>
|
||||
<blockquote>
|
||||
<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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