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<main>
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<h1 id="debug-and-display"><a class="header" href="#debug-and-display">Debug and Display</a></h1>
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<p>All types which want to be printable must implement the <code>std::fmt</code> formatting trait: <code>std::fmt::Debug</code> or <code>std::fmt::Display</code>.</p>
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<p>Automatic implementations are only provided for types such as in the <code>std</code> library. All others have to be manually implemented.</p>
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<h2 id="debug"><a class="header" href="#debug">Debug</a></h2>
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<p>The implementation of <code>Debug</code> is very straightforward: All types can <code>derive</code> the <code>std::fmt::Debug</code> implementation. This is not true for <code>std::fmt::Display</code> which must be manually implemented.</p>
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<p><code>{:?}</code> must be used to print out the type which has implemented the <code>Debug</code> trait.</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// This structure cannot be printed either with `fmt::Display` or
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// with `fmt::Debug`.
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struct UnPrintable(i32);
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// To make this struct printable with `fmt::Debug`, we can derive the automatic implementations provided by Rust
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#[derive(Debug)]
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struct DebugPrintable(i32);
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<span class="boring">}</span></code></pre></pre>
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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">
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/* Fill in the blanks and Fix the errors */
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struct Structure(i32);
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fn main() {
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// Types in std and Rust have implemented the fmt::Debug trait
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println!("__ months in a year.", 12);
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println!("Now __ will print!", Structure(3));
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}</code></pre></pre>
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<ol start="2">
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<li>🌟🌟 So <code>fmt::Debug</code> definitely makes one type printable, but sacrifices some elegance. Maybe we can get more elegant by replacing <code>{:?}</code> with something else( but not <code>{}</code> !)</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">#[derive(Debug)]
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struct Person {
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name: String,
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age: u8
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}
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fn main() {
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let person = Person { name: "Sunface".to_string(), age: 18 };
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/* Make it output:
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Person {
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name: "Sunface",
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age: 18,
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}
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*/
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println!("{:?}", person);
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}</code></pre></pre>
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<ol start="3">
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<li>🌟🌟 We can also manually implement <code>Debug</code> trait for our types</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">
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#[derive(Debug)]
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struct Structure(i32);
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#[derive(Debug)]
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struct Deep(Structure);
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fn main() {
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// The problem with `derive` is there is no control over how
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// the results look. What if I want this to just show a `7`?
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/* Make it print: Now 7 will print! */
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println!("Now {:?} will print!", Deep(Structure(7)));
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}</code></pre></pre>
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<h2 id="display"><a class="header" href="#display">Display</a></h2>
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<p>Yeah, <code>Debug</code> is simple and easy to use. But sometimes we want to customize the output appearance of our type. This is where <code>Display</code> really shines.</p>
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<p>Unlike <code>Debug</code>, there is no way to derive the implementation of the <code>Display</code> trait, we have to manually implement it.</p>
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<p>Another thing to note: the placeholder for <code>Display</code> is <code>{}</code> not <code>{:?}</code>.</p>
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<ol start="4">
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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">
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/* Make it work*/
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use std::fmt;
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struct Point2D {
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x: f64,
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y: f64,
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}
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impl fmt::Display for Point2D {
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/* Implement.. */
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}
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impl fmt::Debug for Point2D {
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/* Implement.. */
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}
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fn main() {
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let point = Point2D { x: 3.3, y: 7.2 };
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assert_eq!(format!("{}",point), "Display: 3.3 + 7.2i");
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assert_eq!(format!("{:?}",point), "Debug: Complex { real: 3.3, imag: 7.2 }");
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println!("Success!");
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}</code></pre></pre>
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<h3 id="-operator"><a class="header" href="#-operator"><code>?</code> operator</a></h3>
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<p>Implementing <code>fmt::Display</code> for a structure whose elements must be handled separately is tricky. The problem is each <code>write!</code> generates a <code>fmt::Result</code> which must be handled in the same place.</p>
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<p>Fortunately, Rust provides the <code>?</code> operator to help us eliminate some unnecessary codes for dealing with <code>fmt::Result</code>.</p>
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<ol start="5">
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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">
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/* Make it work */
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use std::fmt;
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struct List(Vec<i32>);
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impl fmt::Display for List {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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// Extract the value using tuple indexing,
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// and create a reference to `vec`.
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let vec = &self.0;
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write!(f, "[")?;
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// Iterate over `v` in `vec` while enumerating the iteration
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// count in `count`.
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for (count, v) in vec.iter().enumerate() {
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// For every element except the first, add a comma.
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// Use the ? operator to return on errors.
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if count != 0 { write!(f, ", ")?; }
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write!(f, "{}", v)?;
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}
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// Close the opened bracket and return a fmt::Result value.
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write!(f, "]")
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}
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}
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fn main() {
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let v = List(vec![1, 2, 3]);
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assert_eq!(format!("{}",v), "[0: 1, 1: 2, 2: 3]");
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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/formatted-output/debug-display.md">here</a>(under the solutions path), but only use it when you need it :)</p>
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