Expand description
? formatting.
Debug should format the output in a programmer-facing, debugging context.
Generally speaking, you should just derive a Debug implementation.
When used with the alternate format specifier #?, the output is pretty-printed.
For more information on formatters, see the module-level documentation.
This trait can be used with #[derive] if all fields implement Debug. When
derived for structs, it will use the name of the struct, then {, then a
comma-separated list of each field’s name and Debug value, then }. For
enums, it will use the name of the variant and, if applicable, (, then the
Debug values of the fields, then ).
Stability
Derived Debug formats are not stable, and so may change with future Rust
versions. Additionally, Debug implementations of types provided by the
standard library (libstd, libcore, liballoc, etc.) are not stable, and
may also change with future Rust versions.
Examples
Deriving an implementation:
#[derive(Debug)]
struct Point {
x: i32,
y: i32,
}
let origin = Point { x: 0, y: 0 };
assert_eq!(format!("The origin is: {:?}", origin), "The origin is: Point { x: 0, y: 0 }");Manually implementing:
use std::fmt;
struct Point {
x: i32,
y: i32,
}
impl fmt::Debug for Point {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Point")
.field("x", &self.x)
.field("y", &self.y)
.finish()
}
}
let origin = Point { x: 0, y: 0 };
assert_eq!(format!("The origin is: {:?}", origin), "The origin is: Point { x: 0, y: 0 }");There are a number of helper methods on the Formatter struct to help you with manual
implementations, such as debug_struct.
Types that do not wish to use the standard suite of debug representations
provided by the Formatter trait (debug_struct, debug_tuple,
debut_list, debug_set, debug_map) can do something totally custom by
manually writing an arbitrary representation to the Formatter.
impl fmt::Debug for Point {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "Point [{} {}]", self.x, self.y)
}
}Debug implementations using either derive or the debug builder API
on Formatter support pretty-printing using the alternate flag: {:#?}.
Pretty-printing with #?:
#[derive(Debug)]
struct Point {
x: i32,
y: i32,
}
let origin = Point { x: 0, y: 0 };
assert_eq!(format!("The origin is: {:#?}", origin),
"The origin is: Point {
x: 0,
y: 0,
}");Required methods
Formats the value using the given formatter.
Examples
use std::fmt;
struct Position {
longitude: f32,
latitude: f32,
}
impl fmt::Debug for Position {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_tuple("")
.field(&self.longitude)
.field(&self.latitude)
.finish()
}
}
let position = Position { longitude: 1.987, latitude: 2.983 };
assert_eq!(format!("{:?}", position), "(1.987, 2.983)");
assert_eq!(format!("{:#?}", position), "(
1.987,
2.983,
)");