Struct std::fmt::Formatter

pub struct Formatter<'a> {
    flags: u32,
    fill: char,
    align: Alignment,
    width: Option<usize>,
    precision: Option<usize>,
    buf: &'a mut dyn Write,
}

Configuration for formatting.

A Formatter represents various options related to formatting. Users do not construct Formatters directly; a mutable reference to one is passed to the fmt method of all formatting traits, like Debug and Display.

To interact with a Formatter, you’ll call various methods to change the various options related to formatting. For examples, please see the documentation of the methods defined on Formatter below.

Fields

flags: u32

fill: char

align: Alignment

width: Option<usize>

precision: Option<usize>

buf: &'a mut dyn Write

Implementations

impl<'a> Formatter<'a>

pub fn new(buf: &'a mut dyn Write) -> Formatter<'a>

🔬This is a nightly-only experimental API. (fmt_internals)

Creates a new formatter with default settings.

This can be used as a micro-optimization in cases where a full Arguments structure (as created by format_args!) is not necessary; Arguments is a little more expensive to use in simple formatting scenarios.

Currently not intended for use outside of the standard library.

impl<'a> Formatter<'a>

pub fn pad_integral( &mut self, is_nonnegative: bool, prefix: &str, buf: &str ) -> Result<(), Error>

Performs the correct padding for an integer which has already been emitted into a str. The str should not contain the sign for the integer, that will be added by this method.

Arguments

• is_nonnegative - whether the original integer was either positive or zero.
• prefix - if the ‘#’ character (Alternate) is provided, this is the prefix to put in front of the number.
• buf - the byte array that the number has been formatted into

This function will correctly account for the flags provided as well as the minimum width. It will not take precision into account.

Examples

use std::fmt;

struct Foo { nb: i32 }

impl Foo {
    fn new(nb: i32) -> Foo {
        Foo {
            nb,
        }
    }
}

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        // We need to remove "-" from the number output.
        let tmp = self.nb.abs().to_string();

        formatter.pad_integral(self.nb >= 0, "Foo ", &tmp)
    }
}

assert_eq!(format!("{}", Foo::new(2)), "2");
assert_eq!(format!("{}", Foo::new(-1)), "-1");
assert_eq!(format!("{}", Foo::new(0)), "0");
assert_eq!(format!("{:#}", Foo::new(-1)), "-Foo 1");
assert_eq!(format!("{:0>#8}", Foo::new(-1)), "00-Foo 1");

pub fn pad(&mut self, s: &str) -> Result<(), Error>

This function takes a string slice and emits it to the internal buffer after applying the relevant formatting flags specified. The flags recognized for generic strings are:

• width - the minimum width of what to emit
• fill/align - what to emit and where to emit it if the string provided needs to be padded
• precision - the maximum length to emit, the string is truncated if it is longer than this length

Notably this function ignores the flag parameters.

Examples

use std::fmt;

struct Foo;

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        formatter.pad("Foo")
    }
}

assert_eq!(format!("{Foo:<4}"), "Foo ");
assert_eq!(format!("{Foo:0>4}"), "0Foo");

pub fn write_str(&mut self, data: &str) -> Result<(), Error>

Writes some data to the underlying buffer contained within this formatter.

Examples

use std::fmt;

struct Foo;

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        formatter.write_str("Foo")
        // This is equivalent to:
        // write!(formatter, "Foo")
    }
}

assert_eq!(format!("{Foo}"), "Foo");
assert_eq!(format!("{Foo:0>8}"), "Foo");

pub fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<(), Error>

Writes some formatted information into this instance.

Examples

use std::fmt;

struct Foo(i32);

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        formatter.write_fmt(format_args!("Foo {}", self.0))
    }
}

assert_eq!(format!("{}", Foo(-1)), "Foo -1");
assert_eq!(format!("{:0>8}", Foo(2)), "Foo 2");

pub fn flags(&self) -> u32

👎Deprecated since 1.24.0: use the sign_plus, sign_minus, alternate, or sign_aware_zero_pad methods instead

Flags for formatting

pub fn fill(&self) -> char

Character used as ‘fill’ whenever there is alignment.

Examples

use std::fmt;

struct Foo;

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        let c = formatter.fill();
        if let Some(width) = formatter.width() {
            for _ in 0..width {
                write!(formatter, "{c}")?;
            }
            Ok(())
        } else {
            write!(formatter, "{c}")
        }
    }
}

// We set alignment to the right with ">".
assert_eq!(format!("{Foo:G>3}"), "GGG");
assert_eq!(format!("{Foo:t>6}"), "tttttt");

pub fn align(&self) -> Option<Alignment>

Flag indicating what form of alignment was requested.

Examples

use std::fmt::{self, Alignment};

struct Foo;

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        let s = if let Some(s) = formatter.align() {
            match s {
                Alignment::Left    => "left",
                Alignment::Right   => "right",
                Alignment::Center  => "center",
            }
        } else {
            "into the void"
        };
        write!(formatter, "{s}")
    }
}

assert_eq!(format!("{Foo:<}"), "left");
assert_eq!(format!("{Foo:>}"), "right");
assert_eq!(format!("{Foo:^}"), "center");
assert_eq!(format!("{Foo}"), "into the void");

pub fn width(&self) -> Option<usize>

Optionally specified integer width that the output should be.

Examples

use std::fmt;

struct Foo(i32);

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        if let Some(width) = formatter.width() {
            // If we received a width, we use it
            write!(formatter, "{:width$}", format!("Foo({})", self.0), width = width)
        } else {
            // Otherwise we do nothing special
            write!(formatter, "Foo({})", self.0)
        }
    }
}

assert_eq!(format!("{:10}", Foo(23)), "Foo(23)   ");
assert_eq!(format!("{}", Foo(23)), "Foo(23)");

pub fn precision(&self) -> Option<usize>

Optionally specified precision for numeric types. Alternatively, the maximum width for string types.

Examples

use std::fmt;

struct Foo(f32);

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        if let Some(precision) = formatter.precision() {
            // If we received a precision, we use it.
            write!(formatter, "Foo({1:.*})", precision, self.0)
        } else {
            // Otherwise we default to 2.
            write!(formatter, "Foo({:.2})", self.0)
        }
    }
}

assert_eq!(format!("{:.4}", Foo(23.2)), "Foo(23.2000)");
assert_eq!(format!("{}", Foo(23.2)), "Foo(23.20)");

pub fn sign_plus(&self) -> bool

Determines if the + flag was specified.

Examples

use std::fmt;

struct Foo(i32);

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        if formatter.sign_plus() {
            write!(formatter,
                   "Foo({}{})",
                   if self.0 < 0 { '-' } else { '+' },
                   self.0.abs())
        } else {
            write!(formatter, "Foo({})", self.0)
        }
    }
}

assert_eq!(format!("{:+}", Foo(23)), "Foo(+23)");
assert_eq!(format!("{:+}", Foo(-23)), "Foo(-23)");
assert_eq!(format!("{}", Foo(23)), "Foo(23)");

pub fn sign_minus(&self) -> bool

Determines if the - flag was specified.

Examples

use std::fmt;

struct Foo(i32);

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        if formatter.sign_minus() {
            // You want a minus sign? Have one!
            write!(formatter, "-Foo({})", self.0)
        } else {
            write!(formatter, "Foo({})", self.0)
        }
    }
}

assert_eq!(format!("{:-}", Foo(23)), "-Foo(23)");
assert_eq!(format!("{}", Foo(23)), "Foo(23)");

pub fn alternate(&self) -> bool

Determines if the # flag was specified.

Examples

use std::fmt;

struct Foo(i32);

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        if formatter.alternate() {
            write!(formatter, "Foo({})", self.0)
        } else {
            write!(formatter, "{}", self.0)
        }
    }
}

assert_eq!(format!("{:#}", Foo(23)), "Foo(23)");
assert_eq!(format!("{}", Foo(23)), "23");

pub fn sign_aware_zero_pad(&self) -> bool

Determines if the 0 flag was specified.

Examples

use std::fmt;

struct Foo(i32);

impl fmt::Display for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        assert!(formatter.sign_aware_zero_pad());
        assert_eq!(formatter.width(), Some(4));
        // We ignore the formatter's options.
        write!(formatter, "{}", self.0)
    }
}

assert_eq!(format!("{:04}", Foo(23)), "23");

pub fn debug_struct<'b>(&'b mut self, name: &str) -> DebugStruct<'b, 'a>

Creates a DebugStruct builder designed to assist with creation of fmt::Debug implementations for structs.

Examples

use std::fmt;
use std::net::Ipv4Addr;

struct Foo {
    bar: i32,
    baz: String,
    addr: Ipv4Addr,
}

impl fmt::Debug for Foo {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("Foo")
            .field("bar", &self.bar)
            .field("baz", &self.baz)
            .field("addr", &format_args!("{}", self.addr))
            .finish()
    }
}

assert_eq!(
    "Foo { bar: 10, baz: \"Hello World\", addr: 127.0.0.1 }",
    format!("{:?}", Foo {
        bar: 10,
        baz: "Hello World".to_string(),
        addr: Ipv4Addr::new(127, 0, 0, 1),
    })
);

pub fn debug_struct_field1_finish<'b>( &'b mut self, name: &str, name1: &str, value1: &dyn Debug ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. debug_struct_fields_finish is more general, but this is faster for 1 field.

pub fn debug_struct_field2_finish<'b>( &'b mut self, name: &str, name1: &str, value1: &dyn Debug, name2: &str, value2: &dyn Debug ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. debug_struct_fields_finish is more general, but this is faster for 2 fields.

pub fn debug_struct_field3_finish<'b>( &'b mut self, name: &str, name1: &str, value1: &dyn Debug, name2: &str, value2: &dyn Debug, name3: &str, value3: &dyn Debug ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. debug_struct_fields_finish is more general, but this is faster for 3 fields.

pub fn debug_struct_field4_finish<'b>( &'b mut self, name: &str, name1: &str, value1: &dyn Debug, name2: &str, value2: &dyn Debug, name3: &str, value3: &dyn Debug, name4: &str, value4: &dyn Debug ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. debug_struct_fields_finish is more general, but this is faster for 4 fields.

pub fn debug_struct_field5_finish<'b>( &'b mut self, name: &str, name1: &str, value1: &dyn Debug, name2: &str, value2: &dyn Debug, name3: &str, value3: &dyn Debug, name4: &str, value4: &dyn Debug, name5: &str, value5: &dyn Debug ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. debug_struct_fields_finish is more general, but this is faster for 5 fields.

pub fn debug_struct_fields_finish<'b>( &'b mut self, name: &str, names: &[&str], values: &[&dyn Debug] ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. For the cases not covered by debug_struct_field[12345]_finish.

pub fn debug_tuple<'b>(&'b mut self, name: &str) -> DebugTuple<'b, 'a>

Creates a DebugTuple builder designed to assist with creation of fmt::Debug implementations for tuple structs.

Examples

use std::fmt;
use std::marker::PhantomData;

struct Foo<T>(i32, String, PhantomData<T>);

impl<T> fmt::Debug for Foo<T> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_tuple("Foo")
            .field(&self.0)
            .field(&self.1)
            .field(&format_args!("_"))
            .finish()
    }
}

assert_eq!(
    "Foo(10, \"Hello\", _)",
    format!("{:?}", Foo(10, "Hello".to_string(), PhantomData::<u8>))
);

pub fn debug_tuple_field1_finish<'b>( &'b mut self, name: &str, value1: &dyn Debug ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. debug_tuple_fields_finish is more general, but this is faster for 1 field.

pub fn debug_tuple_field2_finish<'b>( &'b mut self, name: &str, value1: &dyn Debug, value2: &dyn Debug ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. debug_tuple_fields_finish is more general, but this is faster for 2 fields.

pub fn debug_tuple_field3_finish<'b>( &'b mut self, name: &str, value1: &dyn Debug, value2: &dyn Debug, value3: &dyn Debug ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. debug_tuple_fields_finish is more general, but this is faster for 3 fields.

pub fn debug_tuple_field4_finish<'b>( &'b mut self, name: &str, value1: &dyn Debug, value2: &dyn Debug, value3: &dyn Debug, value4: &dyn Debug ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. debug_tuple_fields_finish is more general, but this is faster for 4 fields.

pub fn debug_tuple_field5_finish<'b>( &'b mut self, name: &str, value1: &dyn Debug, value2: &dyn Debug, value3: &dyn Debug, value4: &dyn Debug, value5: &dyn Debug ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. debug_tuple_fields_finish is more general, but this is faster for 5 fields.

pub fn debug_tuple_fields_finish<'b>( &'b mut self, name: &str, values: &[&dyn Debug] ) -> Result<(), Error>

🔬This is a nightly-only experimental API. (fmt_helpers_for_derive)

Used to shrink derive(Debug) code, for faster compilation and smaller binaries. For the cases not covered by debug_tuple_field[12345]_finish.

pub fn debug_list<'b>(&'b mut self) -> DebugList<'b, 'a>

Creates a DebugList builder designed to assist with creation of fmt::Debug implementations for list-like structures.

Examples

use std::fmt;

struct Foo(Vec<i32>);

impl fmt::Debug for Foo {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_list().entries(self.0.iter()).finish()
    }
}

assert_eq!(format!("{:?}", Foo(vec![10, 11])), "[10, 11]");

pub fn debug_set<'b>(&'b mut self) -> DebugSet<'b, 'a>

Creates a DebugSet builder designed to assist with creation of fmt::Debug implementations for set-like structures.

Examples

use std::fmt;

struct Foo(Vec<i32>);

impl fmt::Debug for Foo {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_set().entries(self.0.iter()).finish()
    }
}

assert_eq!(format!("{:?}", Foo(vec![10, 11])), "{10, 11}");

In this more complex example, we use format_args! and .debug_set() to build a list of match arms:

use std::fmt;

struct Arm<'a, L, R>(&'a (L, R));
struct Table<'a, K, V>(&'a [(K, V)], V);

impl<'a, L, R> fmt::Debug for Arm<'a, L, R>
where
    L: 'a + fmt::Debug, R: 'a + fmt::Debug
{
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        L::fmt(&(self.0).0, fmt)?;
        fmt.write_str(" => ")?;
        R::fmt(&(self.0).1, fmt)
    }
}

impl<'a, K, V> fmt::Debug for Table<'a, K, V>
where
    K: 'a + fmt::Debug, V: 'a + fmt::Debug
{
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_set()
        .entries(self.0.iter().map(Arm))
        .entry(&Arm(&(format_args!("_"), &self.1)))
        .finish()
    }
}

pub fn debug_map<'b>(&'b mut self) -> DebugMap<'b, 'a>

Creates a DebugMap builder designed to assist with creation of fmt::Debug implementations for map-like structures.

Examples

use std::fmt;

struct Foo(Vec<(String, i32)>);

impl fmt::Debug for Foo {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_map().entries(self.0.iter().map(|&(ref k, ref v)| (k, v))).finish()
    }
}

assert_eq!(
    format!("{:?}",  Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
    r#"{"A": 10, "B": 11}"#
 );

Trait Implementations

impl Write for Formatter<'_>

fn write_str(&mut self, s: &str) -> Result<(), Error>

Writes a string slice into this writer, returning whether the write succeeded.

fn write_char(&mut self, c: char) -> Result<(), Error>

Writes a char into this writer, returning whether the write succeeded.

fn write_fmt(&mut self, args: Arguments<'_>) -> Result<(), Error>

Glue for usage of the write! macro with implementors of this trait.