Struct std::fmt::Formatter

pub struct Formatter<'a> { /* fields omitted */ }

A struct to represent both where to emit formatting strings to and how they should be formatted. A mutable version of this is passed to all formatting traits.

Methods

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.

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.

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

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

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

Writes some formatted information into this instance

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

pub fn align(&self) -> Alignment

🔬 This is a nightly-only experimental API. (fmt_flags_align #27726)

method was just created

Flag indicating what form of alignment was requested

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.

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)
        } else {
            write!(formatter, "Foo({})", self.0)
        }
    }
}

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 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_tuple(&'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_list(&'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()
    }
}

// prints "[10, 11]"
println!("{:?}", Foo(vec![10, 11]));

pub fn debug_set(&'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()
    }
}

// prints "{10, 11}"
println!("{:?}", Foo(vec![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: 'a, R: 'a>(&'a (L, R));
struct Table<'a, K: 'a, V: 'a>(&'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 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()
    }
}

// prints "{"A": 10, "B": 11}"
println!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)]));

Trait Implementations

impl<'a> Write for Formatter<'a>

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

Writes a slice of bytes 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.