Struct std::fmt::Formatter
pub struct Formatter<'a> { /* fields omitted */ }
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.
Implementations
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!("{:<4}", Foo), "Foo ");
assert_eq!(&format!("{:0>4}", Foo), "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!("{:0>8}", Foo), "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
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!("{:G>3}", Foo), "GGG");
assert_eq!(&format!("{:t>6}", Foo), "tttttt");pub fn align(&self) -> Option<Alignment>
Flag indicating what form of alignment was requested.
Examples
extern crate core;
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)
} 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()
}
}
assert_eq!(format!("{:?}", Foo(vec![10, 11])), "[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()
}
}
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: '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()
}
}
assert_eq!(
format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
r#"{"A": 10, "B": 11}"#
);Trait Implementations
impl<'_> Write for Formatter<'_>
pub fn write_str(&mut self, s: &str) -> Result<(), Error>
Writes a string slice into this writer, returning whether the write succeeded. Read more
pub fn write_char(&mut self, c: char) -> Result<(), Error>
pub fn write_fmt(&mut self, args: Arguments<'_>) -> Result<(), Error>
Auto Trait Implementations
impl<'a> !RefUnwindSafe for Formatter<'a>
impl<'a> !Send for Formatter<'a>
impl<'a> !Sync for Formatter<'a>
impl<'a> Unpin for Formatter<'a>
impl<'a> !UnwindSafe for Formatter<'a>
Blanket Implementations
impl<T> From<T> for T
pub fn from(t: T) -> T
Performs the conversion.
pub fn into(self) -> U
Performs the conversion.
type Error = Infallible
The type returned in the event of a conversion error.
pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>
Performs the conversion.
type Error = <U as TryFrom<T>>::Error
The type returned in the event of a conversion error.
pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>
Performs the conversion.
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/fmt/struct.Formatter.html