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One can manually implement fmt::Display to control the display.
attributes, derive, std::fmt, and struct
fmt::Debug hardly looks compact and clean, so it is often advantageous to customize the output appearance. This is done by manually implementing fmt::Display, which uses the {} print marker. Implementing it looks like this:
#![allow(unused)]
fn main() {
// Import (via `use`) the `fmt` module to make it available.
use std::fmt;
// Define a structure for which `fmt::Display` will be implemented. This is
// a tuple struct named `Structure` that contains an `i32`.
struct Structure(i32);
// To use the `{}` marker, the trait `fmt::Display` must be implemented
// manually for the type.
impl fmt::Display for Structure {
// This trait requires `fmt` with this exact signature.
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
// Write strictly the first element into the supplied output
// stream: `f`. Returns `fmt::Result` which indicates whether the
// operation succeeded or failed. Note that `write!` uses syntax which
// is very similar to `println!`.
write!(f, "{}", self.0)
}
}
}
fmt::Display may be cleaner than fmt::Debug but this presents a problem for the std library. How should ambiguous types be displayed? For example, if the std library implemented a single style for all Vec<T>, what style should it be? Would it be either of these two?
• Vec<path>: /:/etc:/home/username:/bin (split on :)
• Vec<number>: 1,2,3 (split on ,)
No, because there is no ideal style for all types and the std library doesn't presume to dictate one. fmt::Display is not implemented for Vec<T> or for any other generic containers. fmt::Debug must then be used for these generic cases.
This is not a problem though because for any new container type which is not generic,fmt::Display can be implemented.
use std::fmt; // Import `fmt`
// A structure holding two numbers. `Debug` will be derived so the results can
// be contrasted with `Display`.
#[derive(Debug)]
struct MinMax(i64, i64);
// Implement `Display` for `MinMax`.
impl fmt::Display for MinMax {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
// Use `self.number` to refer to each positional data point.
write!(f, "({}, {})", self.0, self.1)
}
}
// Define a structure where the fields are nameable for comparison.
#[derive(Debug)]
struct Point2D {
x: f64,
y: f64,
}
// Similarly, implement `Display` for `Point2D`
impl fmt::Display for Point2D {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
// Customize so only `x` and `y` are denoted.
write!(f, "x: {}, y: {}", self.x, self.y)
}
}
fn main() {
let minmax = MinMax(0, 14);
println!("Compare structures:");
println!("Display: {}", minmax);
println!("Debug: {:?}", minmax);
let big_range = MinMax(-300, 300);
let small_range = MinMax(-3, 3);
println!("The big range is {big} and the small is {small}",
small = small_range,
big = big_range);
let point = Point2D { x: 3.3, y: 7.2 };
println!("Compare points:");
println!("Display: {}", point);
println!("Debug: {:?}", point);
// Error. Both `Debug` and `Display` were implemented, but `{:b}`
// requires `fmt::Binary` to be implemented. This will not work.
// println!("What does Point2D look like in binary: {:b}?", point);
}
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So, fmt::Display has been implemented but fmt::Binary has not, and therefore cannot be used. std::fmt has many such traits and each requires its own implementation. This is detailed further in std::fmt.
After checking the output of the above example, use the Point2D struct as a guide to add a Complex struct to the example. When printed in the same way, the output should be:
Display: 3.3 + 7.2i
Debug: Complex { reaclass="underline" 3.3, imag: 7.2 }