Trait Debug

1.0.0 · Source

pub trait Debug {
    // Required method
    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>;
}

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 (std, core, alloc, 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, 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)
    }
}

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 };

let expected = "The origin is: Point {
    x: 0,
    y: 0,
}";
assert_eq!(format!("The origin is: {origin:#?}"), expected);

Required Methods§

1.0.0 · Source

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

§Errors

This function should return Err if, and only if, the provided Formatter returns Err. String formatting is considered an infallible operation; this function only returns a Result because writing to the underlying stream might fail and it must provide a way to propagate the fact that an error has occurred back up the stack.

§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,
)");

Debug

Trait Debug Copy item path

§Stability

§Examples

Required Methods§

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

§Errors

§Examples

Implementors§

impl Debug for Click

impl Debug for grafix_toolbox::GL::event::Event

impl Debug for EventReply

impl Debug for grafix_toolbox::lib::Ordering

impl Debug for ErrorKind

impl Debug for SeekFrom

impl Debug for grafix_toolbox::stdlib::ord

impl Debug for TryReserveErrorKind

impl Debug for AsciiChar

impl Debug for Infallible

impl Debug for FromBytesWithNulError

impl Debug for c_void

impl Debug for core::fmt::Alignment

impl Debug for DebugAsHex

impl Debug for Sign

impl Debug for Locality

impl Debug for AtomicOrdering

impl Debug for SimdAlign

impl Debug for IpAddr

impl Debug for Ipv6MulticastScope

impl Debug for core::net::socket_addr::SocketAddr

impl Debug for FpCategory

impl Debug for IntErrorKind

impl Debug for GetDisjointMutError

impl Debug for SearchStep

impl Debug for BacktraceStatus

impl Debug for VarError

impl Debug for std::fs::TryLockError

impl Debug for Shutdown

impl Debug for AncillaryError

impl Debug for BacktraceStyle

impl Debug for std::sync::mpsc::RecvTimeoutError

impl Debug for std::sync::mpsc::TryRecvError

impl Debug for CheckedCastError

impl Debug for PodCastError

impl Debug for byteorder_lite::BigEndian

impl Debug for byteorder_lite::LittleEndian

impl Debug for ColorType

impl Debug for ExtendedColorType

impl Debug for ImageError

impl Debug for ImageFormatHint

impl Debug for LimitErrorKind

impl Debug for ParameterErrorKind

impl Debug for UnsupportedErrorKind

impl Debug for FilterType

impl Debug for DynamicImage

impl Debug for image::images::flat::Error

impl Debug for NormalForm

impl Debug for ImageFormat

impl Debug for image::metadata::cicp::CicpColorPrimaries

impl Debug for CicpMatrixCoefficients

impl Debug for CicpTransferCharacteristics

impl Debug for CicpVideoFullRangeFlag

impl Debug for image::metadata::Orientation

impl Debug for DIR

impl Debug for FILE

impl Debug for timezone

impl Debug for tpacket_versions

impl Debug for moxcms::cicp::CicpColorPrimaries

impl Debug for MatrixCoefficients

impl Debug for TransferCharacteristics

impl Debug for CmsError

impl Debug for DataColorSpace

impl Debug for LutStore

impl Debug for LutType

impl Debug for LutWarehouse

impl Debug for MeasurementGeometry

impl Debug for ProfileClass

impl Debug for ProfileSignature

impl Debug for ProfileText

impl Debug for ProfileVersion

Trait Debug