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<h1 id="convert-by-as"><a class="header" href="#convert-by-as">Convert by <code>as</code></a></h1>
<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>
<ol>
<li>🌟</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">// FIX the errors and FILL in the blank
// DON'T remove any code
fn main() {
let decimal = 97.123_f32;
let integer: __ = decimal as u8;
let c1: char = decimal as char;
let c2 = integer as char;
assert_eq!(integer, 'b' as u8);
println!("Success!");
}</code></pre></pre>
<ol start="2">
<li>🌟🌟 By default, overflow will cause compile errors, but we can add an global annotation to suppress these errors.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">fn main() {
assert_eq!(u8::MAX, 255);
// The max of `u8` is 255 as shown above.
// so the below code will cause an overflow error: literal out of range for `u8`.
// PLEASE looking for clues within compile errors to FIX it.
// DON'T modify any code in main.
let v = 1000 as u8;
println!("Success!");
}</code></pre></pre>
<ol start="3">
<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>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">fn main() {
assert_eq!(1000 as u16, __);
assert_eq!(1000 as u8, __);
// For positive numbers, this is the same as the modulus
println!("1000 mod 256 is : {}", 1000 % 256);
assert_eq!(-1_i8 as u8, __);
// Since Rust 1.45, the `as` keyword performs a *saturating cast*
// 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.
assert_eq!(300.1_f32 as u8, __);
assert_eq!(-100.1_f32 as u8, __);
// This behavior incurs a small runtime cost and can be avoided
// with unsafe methods, however the results might overflow and
// return **unsound values**. Use these methods wisely:
unsafe {
// 300.0 is 44
println!("300.0 is {}", 300.0_f32.to_int_unchecked::&lt;u8&gt;());
// -100.0 as u8 is 156
println!("-100.0 as u8 is {}", (-100.0_f32).to_int_unchecked::&lt;u8&gt;());
// nan as u8 is 0
println!("nan as u8 is {}", f32::NAN.to_int_unchecked::&lt;u8&gt;());
}
}</code></pre></pre>
<ol start="4">
<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">
// FILL in the blanks
fn main() {
let mut values: [i32; 2] = [1, 2];
let p1: *mut i32 = values.as_mut_ptr();
let first_address: usize = p1 __;
let second_address = first_address + 4; // 4 == std::mem::size_of::&lt;i32&gt;()
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>
<ol start="5">
<li>🌟🌟🌟</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">fn main() {
let arr :[u64; 13] = [0; 13];
assert_eq!(std::mem::size_of_val(&amp;arr), 8 * 13);
let a: *const [u64] = &amp;arr;
let b = a as *const [u8];
unsafe {
assert_eq!(std::mem::size_of_val(&amp;*b), __)
}
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/as.md">here</a>(under the solutions path), but only use it when you need it</p>
</blockquote>
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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&lt;T&gt; 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>&amp;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&lt;&amp;str&gt;:
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&lt;&amp;'_ str&gt; 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&lt;bool&gt; 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&lt;char&gt;`, 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&lt;i32&gt; 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&lt;io::Error&gt; for CliError {
// IMPLEMENT from method
}
impl From&lt;num::ParseIntError&gt; for CliError {
// IMPLEMENT from method
}
fn open_and_parse_file(file_name: &amp;str) -&gt; Result&lt;i32, CliError&gt; {
// ? automatically converts io::Error to CliError
let contents = fs::read_to_string(&amp;file_name)?;
// num::ParseIntError -&gt; 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) =&gt; n,
Err(e) =&gt; {
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&lt;i32&gt; for EvenNum {
type Error = ();
// IMPLEMENT `try_from`
fn try_from(value: i32) -&gt; Result&lt;Self, Self::Error&gt; {
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&lt;EvenNum, ()&gt; = 8i32.try_into();
assert_eq!(result, __);
let result: Result&lt;EvenNum, ()&gt; = 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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<h1 id="type-conversion"><a class="header" href="#type-conversion">Type Conversion</a></h1>
<p>Learning resources:</p>
<ul>
<li>English: <a href="https://std.rs">Standary library</a></li>
<li>简体中文: <a href="https://course.rs/basic/converse.html">Rust语言圣经 - 所有权与借用</a></li>
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<h1 id="others"><a class="header" href="#others">Others</a></h1>
<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>
<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: &amp;str) -&gt; Result&lt;Self, Self::Err&gt; {
let coords: Vec&lt;&amp;str&gt; = s.trim_matches(|p| p == '(' || p == ')' )
.split(',')
.map(|x| x.trim())
.collect();
let x_fromstr = coords[0].parse::&lt;i32&gt;()?;
let y_fromstr = coords[1].parse::&lt;i32&gt;()?;
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() -&gt; i32 {
0
}
fn main() {
let pointer = foo as *const ();
let function = unsafe {
std::mem::transmute::&lt;*const (), fn() -&gt; i32&gt;(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&lt;'a&gt;(&amp;'a i32);
unsafe fn extend_lifetime&lt;'b&gt;(r: R&lt;'b&gt;) -&gt; R&lt;'static&gt; {
std::mem::transmute::&lt;R&lt;'b&gt;, R&lt;'static&gt;&gt;(r)
}
unsafe fn shorten_invariant_lifetime&lt;'b, 'c&gt;(r: &amp;'b mut R&lt;'static&gt;)
-&gt; &amp;'b mut R&lt;'c&gt; {
std::mem::transmute::&lt;&amp;'b mut R&lt;'static&gt;, &amp;'b mut R&lt;'c&gt;&gt;(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(&amp;[u8]) to u32, f64, etc.: */
let raw_bytes = [0x78, 0x56, 0x34, 0x12];
let num = unsafe { std::mem::transmute::&lt;[u8; 4], u32&gt;(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 = &amp;0;
let ptr_num_transmute = unsafe { std::mem::transmute::&lt;&amp;i32, usize&gt;(ptr) };
// Use an `as` cast instead
let ptr_num_cast = ptr as *const i32 as usize;
/*Turning an &amp;mut T into an &amp;mut U: */
let ptr = &amp;mut 0;
let val_transmuted = unsafe { std::mem::transmute::&lt;&amp;mut i32, &amp;mut u32&gt;(ptr) };
// Now, put together `as` and reborrowing - note the chaining of `as`
// `as` is not transitive
let val_casts = unsafe { &amp;mut *(ptr as *mut i32 as *mut u32) };
/*Turning an &amp;str into a &amp;[u8]: */
// This is not a good way to do this.
let slice = unsafe { std::mem::transmute::&lt;&amp;str, &amp;[u8]&gt;("Rust") };
assert_eq!(slice, &amp;[82, 117, 115, 116]);
// You could use `str::as_bytes`
let slice = "Rust".as_bytes();
assert_eq!(slice, &amp;[82, 117, 115, 116]);
// Or, just use a byte string, if you have control over the string
// literal
assert_eq!(b"Rust", &amp;[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>
</blockquote>
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