pub struct Formatter<'a> { /* private fields */ }
Expand description
Configuration for formatting.
A Formatter
represents various options related to formatting. Users do not
construct Formatter
s 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§
source§impl<'a> Formatter<'a>
impl<'a> Formatter<'a>
1.0.0 · sourcepub fn pad_integral(
&mut self,
is_nonnegative: bool,
prefix: &str,
buf: &str,
) -> Result<(), Error>
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");
1.0.0 · sourcepub fn pad(&mut self, s: &str) -> Result<(), Error>
pub fn pad(&mut self, s: &str) -> Result<(), Error>
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");
1.0.0 · sourcepub fn write_str(&mut self, data: &str) -> Result<(), Error>
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");
1.0.0 · sourcepub fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<(), Error>
pub fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<(), Error>
Glue for usage of the write!
macro with implementors of this trait.
This method should generally not be invoked manually, but rather through
the write!
macro itself.
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");
1.0.0 · sourcepub fn flags(&self) -> u32
👎Deprecated since 1.24.0: use the sign_plus
, sign_minus
, alternate
, or sign_aware_zero_pad
methods instead
pub fn flags(&self) -> u32
sign_plus
, sign_minus
, alternate
, or sign_aware_zero_pad
methods insteadReturns flags for formatting.
1.5.0 · sourcepub fn fill(&self) -> char
pub fn fill(&self) -> char
Returns the 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");
1.28.0 · sourcepub fn align(&self) -> Option<Alignment>
pub fn align(&self) -> Option<Alignment>
Returns a 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");
1.5.0 · sourcepub fn width(&self) -> Option<usize>
pub fn width(&self) -> Option<usize>
Returns the 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)");
1.5.0 · sourcepub fn precision(&self) -> Option<usize>
pub fn precision(&self) -> Option<usize>
Returns the 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)");
1.5.0 · sourcepub fn sign_plus(&self) -> bool
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)");
1.5.0 · sourcepub fn sign_minus(&self) -> bool
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)");
1.5.0 · sourcepub fn alternate(&self) -> bool
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");
1.5.0 · sourcepub fn sign_aware_zero_pad(&self) -> bool
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");
1.2.0 · sourcepub fn debug_struct<'b>(&'b mut self, name: &str) -> DebugStruct<'b, 'a>
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),
})
);
1.2.0 · sourcepub fn debug_tuple<'b>(&'b mut self, name: &str) -> DebugTuple<'b, 'a>
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>))
);
1.2.0 · sourcepub fn debug_list<'b>(&'b mut self) -> DebugList<'b, 'a>
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]");
1.2.0 · sourcepub fn debug_set<'b>(&'b mut self) -> DebugSet<'b, 'a>
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()
}
}
1.2.0 · sourcepub fn debug_map<'b>(&'b mut self) -> DebugMap<'b, 'a>
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}"#
);