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
derive
d 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
enum
s, 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 }");
RunManually 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 }");
RunThere 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
,
debug_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)
}
}
RunDebug
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,
}");
RunRequired 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,
)");
Run