alloc

Module fmt

1.36.0 · source
Expand description

Utilities for formatting and printing Strings.

This module contains the runtime support for the format! syntax extension. This macro is implemented in the compiler to emit calls to this module in order to format arguments at runtime into strings.

§Usage

The format! macro is intended to be familiar to those coming from C’s printf/fprintf functions or Python’s str.format function.

Some examples of the format! extension are:

format!("Hello");                 // => "Hello"
format!("Hello, {}!", "world");   // => "Hello, world!"
format!("The number is {}", 1);   // => "The number is 1"
format!("{:?}", (3, 4));          // => "(3, 4)"
format!("{value}", value=4);      // => "4"
let people = "Rustaceans";
format!("Hello {people}!");       // => "Hello Rustaceans!"
format!("{} {}", 1, 2);           // => "1 2"
format!("{:04}", 42);             // => "0042" with leading zeros
format!("{:#?}", (100, 200));     // => "(
                                  //       100,
                                  //       200,
                                  //     )"

From these, you can see that the first argument is a format string. It is required by the compiler for this to be a string literal; it cannot be a variable passed in (in order to perform validity checking). The compiler will then parse the format string and determine if the list of arguments provided is suitable to pass to this format string.

To convert a single value to a string, use the to_string method. This will use the Display formatting trait.

§Positional parameters

Each formatting argument is allowed to specify which value argument it’s referencing, and if omitted it is assumed to be “the next argument”. For example, the format string {} {} {} would take three parameters, and they would be formatted in the same order as they’re given. The format string {2} {1} {0}, however, would format arguments in reverse order.

Things can get a little tricky once you start intermingling the two types of positional specifiers. The “next argument” specifier can be thought of as an iterator over the argument. Each time a “next argument” specifier is seen, the iterator advances. This leads to behavior like this:

format!("{1} {} {0} {}", 1, 2); // => "2 1 1 2"

The internal iterator over the argument has not been advanced by the time the first {} is seen, so it prints the first argument. Then upon reaching the second {}, the iterator has advanced forward to the second argument. Essentially, parameters that explicitly name their argument do not affect parameters that do not name an argument in terms of positional specifiers.

A format string is required to use all of its arguments, otherwise it is a compile-time error. You may refer to the same argument more than once in the format string.

§Named parameters

Rust itself does not have a Python-like equivalent of named parameters to a function, but the format! macro is a syntax extension that allows it to leverage named parameters. Named parameters are listed at the end of the argument list and have the syntax:

identifier '=' expression

For example, the following format! expressions all use named arguments:

format!("{argument}", argument = "test");   // => "test"
format!("{name} {}", 1, name = 2);          // => "2 1"
format!("{a} {c} {b}", a="a", b='b', c=3);  // => "a 3 b"

If a named parameter does not appear in the argument list, format! will reference a variable with that name in the current scope.

let argument = 2 + 2;
format!("{argument}");   // => "4"

fn make_string(a: u32, b: &str) -> String {
    format!("{b} {a}")
}
make_string(927, "label"); // => "label 927"

It is not valid to put positional parameters (those without names) after arguments that have names. Like with positional parameters, it is not valid to provide named parameters that are unused by the format string.

§Formatting Parameters

Each argument being formatted can be transformed by a number of formatting parameters (corresponding to format_spec in the syntax). These parameters affect the string representation of what’s being formatted.

§Width

// All of these print "Hello x    !"
println!("Hello {:5}!", "x");
println!("Hello {:1$}!", "x", 5);
println!("Hello {1:0$}!", 5, "x");
println!("Hello {:width$}!", "x", width = 5);
let width = 5;
println!("Hello {:width$}!", "x");

This is a parameter for the “minimum width” that the format should take up. If the value’s string does not fill up this many characters, then the padding specified by fill/alignment will be used to take up the required space (see below).

The value for the width can also be provided as a usize in the list of parameters by adding a postfix $, indicating that the second argument is a usize specifying the width.

Referring to an argument with the dollar syntax does not affect the “next argument” counter, so it’s usually a good idea to refer to arguments by position, or use named arguments.

§Fill/Alignment

assert_eq!(format!("Hello {:<5}!", "x"),  "Hello x    !");
assert_eq!(format!("Hello {:-<5}!", "x"), "Hello x----!");
assert_eq!(format!("Hello {:^5}!", "x"),  "Hello   x  !");
assert_eq!(format!("Hello {:>5}!", "x"),  "Hello     x!");

The optional fill character and alignment is provided normally in conjunction with the width parameter. It must be defined before width, right after the :. This indicates that if the value being formatted is smaller than width some extra characters will be printed around it. Filling comes in the following variants for different alignments:

  • [fill]< - the argument is left-aligned in width columns
  • [fill]^ - the argument is center-aligned in width columns
  • [fill]> - the argument is right-aligned in width columns

The default fill/alignment for non-numerics is a space and left-aligned. The default for numeric formatters is also a space character but with right-alignment. If the 0 flag (see below) is specified for numerics, then the implicit fill character is 0.

Note that alignment might not be implemented by some types. In particular, it is not generally implemented for the Debug trait. A good way to ensure padding is applied is to format your input, then pad this resulting string to obtain your output:

println!("Hello {:^15}!", format!("{:?}", Some("hi"))); // => "Hello   Some("hi")   !"

§Sign/#/0

assert_eq!(format!("Hello {:+}!", 5), "Hello +5!");
assert_eq!(format!("{:#x}!", 27), "0x1b!");
assert_eq!(format!("Hello {:05}!", 5),  "Hello 00005!");
assert_eq!(format!("Hello {:05}!", -5), "Hello -0005!");
assert_eq!(format!("{:#010x}!", 27), "0x0000001b!");

These are all flags altering the behavior of the formatter.

  • + - This is intended for numeric types and indicates that the sign should always be printed. By default only the negative sign of signed values is printed, and the sign of positive or unsigned values is omitted. This flag indicates that the correct sign (+ or -) should always be printed.

  • - - Currently not used

  • # - This flag indicates that the “alternate” form of printing should be used. The alternate forms are:

    • #? - pretty-print the Debug formatting (adds linebreaks and indentation)
    • #x - precedes the argument with a 0x
    • #X - precedes the argument with a 0x
    • #b - precedes the argument with a 0b
    • #o - precedes the argument with a 0o

    See Formatting traits for a description of what the ?, x, X, b, and o flags do.

  • 0 - This is used to indicate for integer formats that the padding to width should both be done with a 0 character as well as be sign-aware. A format like {:08} would yield 00000001 for the integer 1, while the same format would yield -0000001 for the integer -1. Notice that the negative version has one fewer zero than the positive version. Note that padding zeros are always placed after the sign (if any) and before the digits. When used together with the # flag, a similar rule applies: padding zeros are inserted after the prefix but before the digits. The prefix is included in the total width. This flag overrides the fill character and alignment flag.

§Precision

For non-numeric types, this can be considered a “maximum width”. If the resulting string is longer than this width, then it is truncated down to this many characters and that truncated value is emitted with proper fill, alignment and width if those parameters are set.

For integral types, this is ignored.

For floating-point types, this indicates how many digits after the decimal point should be printed.

There are three possible ways to specify the desired precision:

  1. An integer .N:

    the integer N itself is the precision.

  2. An integer or name followed by dollar sign .N$:

    use format argument N (which must be a usize) as the precision.

  3. An asterisk .*:

    .* means that this {...} is associated with two format inputs rather than one:

    • If a format string in the fashion of {:<spec>.*} is used, then the first input holds the usize precision, and the second holds the value to print.
    • If a format string in the fashion of {<arg>:<spec>.*} is used, then the <arg> part refers to the value to print, and the precision is taken like it was specified with an omitted positional parameter ({} instead of {<arg>:}).

For example, the following calls all print the same thing Hello x is 0.01000:

// Hello {arg 0 ("x")} is {arg 1 (0.01) with precision specified inline (5)}
println!("Hello {0} is {1:.5}", "x", 0.01);

// Hello {arg 1 ("x")} is {arg 2 (0.01) with precision specified in arg 0 (5)}
println!("Hello {1} is {2:.0$}", 5, "x", 0.01);

// Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)}
println!("Hello {0} is {2:.1$}", "x", 5, 0.01);

// Hello {next arg -> arg 0 ("x")} is {second of next two args -> arg 2 (0.01) with precision
//                          specified in first of next two args -> arg 1 (5)}
println!("Hello {} is {:.*}",    "x", 5, 0.01);

// Hello {arg 1 ("x")} is {arg 2 (0.01) with precision
//                          specified in next arg -> arg 0 (5)}
println!("Hello {1} is {2:.*}",  5, "x", 0.01);

// Hello {next arg -> arg 0 ("x")} is {arg 2 (0.01) with precision
//                          specified in next arg -> arg 1 (5)}
println!("Hello {} is {2:.*}",   "x", 5, 0.01);

// Hello {next arg -> arg 0 ("x")} is {arg "number" (0.01) with precision specified
//                          in arg "prec" (5)}
println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);

While these:

println!("{}, `{name:.*}` has 3 fractional digits", "Hello", 3, name=1234.56);
println!("{}, `{name:.*}` has 3 characters", "Hello", 3, name="1234.56");
println!("{}, `{name:>8.*}` has 3 right-aligned characters", "Hello", 3, name="1234.56");

print three significantly different things:

Hello, `1234.560` has 3 fractional digits
Hello, `123` has 3 characters
Hello, `     123` has 3 right-aligned characters

When truncating these values, Rust uses round half-to-even, which is the default rounding mode in IEEE 754. For example,

print!("{0:.1$e}", 12345, 3);
print!("{0:.1$e}", 12355, 3);

Would return:

1.234e4
1.236e4

§Localization

In some programming languages, the behavior of string formatting functions depends on the operating system’s locale setting. The format functions provided by Rust’s standard library do not have any concept of locale and will produce the same results on all systems regardless of user configuration.

For example, the following code will always print 1.5 even if the system locale uses a decimal separator other than a dot.

println!("The value is {}", 1.5);

§Escaping

The literal characters { and } may be included in a string by preceding them with the same character. For example, the { character is escaped with {{ and the } character is escaped with }}.

assert_eq!(format!("Hello {{}}"), "Hello {}");
assert_eq!(format!("{{ Hello"), "{ Hello");

§Syntax

To summarize, here you can find the full grammar of format strings. The syntax for the formatting language used is drawn from other languages, so it should not be too alien. Arguments are formatted with Python-like syntax, meaning that arguments are surrounded by {} instead of the C-like %. The actual grammar for the formatting syntax is:

format_string := text [ maybe_format text ] *
maybe_format := '{' '{' | '}' '}' | format
format := '{' [ argument ] [ ':' format_spec ] [ ws ] * '}'
argument := integer | identifier

format_spec := [[fill]align][sign]['#']['0'][width]['.' precision]type
fill := character
align := '<' | '^' | '>'
sign := '+' | '-'
width := count
precision := count | '*'
type := '' | '?' | 'x?' | 'X?' | identifier
count := parameter | integer
parameter := argument '$'

In the above grammar,

  • text must not contain any '{' or '}' characters,
  • ws is any character for which char::is_whitespace returns true, has no semantic meaning and is completely optional,
  • integer is a decimal integer that may contain leading zeroes and must fit into an usize and
  • identifier is an IDENTIFIER_OR_KEYWORD (not an IDENTIFIER) as defined by the Rust language reference.

§Formatting traits

When requesting that an argument be formatted with a particular type, you are actually requesting that an argument ascribes to a particular trait. This allows multiple actual types to be formatted via {:x} (like i8 as well as isize). The current mapping of types to traits is:

What this means is that any type of argument which implements the fmt::Binary trait can then be formatted with {:b}. Implementations are provided for these traits for a number of primitive types by the standard library as well. If no format is specified (as in {} or {:6}), then the format trait used is the Display trait.

When implementing a format trait for your own type, you will have to implement a method of the signature:

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

Your type will be passed as self by-reference, and then the function should emit output into the Formatter f which implements fmt::Write. It is up to each format trait implementation to correctly adhere to the requested formatting parameters. The values of these parameters can be accessed with methods of the Formatter struct. In order to help with this, the Formatter struct also provides some helper methods.

Additionally, the return value of this function is fmt::Result which is a type alias of Result<(), std::fmt::Error>. Formatting implementations should ensure that they propagate errors from the Formatter (e.g., when calling write!). However, they should never return errors spuriously. That is, a formatting implementation must and may only return an error if the passed-in Formatter returns an error. This is because, contrary to what the function signature might suggest, string formatting is 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.

An example of implementing the formatting traits would look like:

use std::fmt;

#[derive(Debug)]
struct Vector2D {
    x: isize,
    y: isize,
}

impl fmt::Display for Vector2D {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        // The `f` value implements the `Write` trait, which is what the
        // write! macro is expecting. Note that this formatting ignores the
        // various flags provided to format strings.
        write!(f, "({}, {})", self.x, self.y)
    }
}

// Different traits allow different forms of output of a type. The meaning
// of this format is to print the magnitude of a vector.
impl fmt::Binary for Vector2D {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let magnitude = (self.x * self.x + self.y * self.y) as f64;
        let magnitude = magnitude.sqrt();

        // Respect the formatting flags by using the helper method
        // `pad_integral` on the Formatter object. See the method
        // documentation for details, and the function `pad` can be used
        // to pad strings.
        let decimals = f.precision().unwrap_or(3);
        let string = format!("{magnitude:.decimals$}");
        f.pad_integral(true, "", &string)
    }
}

fn main() {
    let myvector = Vector2D { x: 3, y: 4 };

    println!("{myvector}");       // => "(3, 4)"
    println!("{myvector:?}");     // => "Vector2D {x: 3, y:4}"
    println!("{myvector:10.3b}"); // => "     5.000"
}

§fmt::Display vs fmt::Debug

These two formatting traits have distinct purposes:

  • fmt::Display implementations assert that the type can be faithfully represented as a UTF-8 string at all times. It is not expected that all types implement the Display trait.
  • fmt::Debug implementations should be implemented for all public types. Output will typically represent the internal state as faithfully as possible. The purpose of the Debug trait is to facilitate debugging Rust code. In most cases, using #[derive(Debug)] is sufficient and recommended.

Some examples of the output from both traits:

assert_eq!(format!("{} {:?}", 3, 4), "3 4");
assert_eq!(format!("{} {:?}", 'a', 'b'), "a 'b'");
assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\"");

There are a number of related macros in the format! family. The ones that are currently implemented are:

ⓘ
format!      // described above
write!       // first argument is either a &mut io::Write or a &mut fmt::Write, the destination
writeln!     // same as write but appends a newline
print!       // the format string is printed to the standard output
println!     // same as print but appends a newline
eprint!      // the format string is printed to the standard error
eprintln!    // same as eprint but appends a newline
format_args! // described below.

§write!

write! and writeln! are two macros which are used to emit the format string to a specified stream. This is used to prevent intermediate allocations of format strings and instead directly write the output. Under the hood, this function is actually invoking the write_fmt function defined on the std::io::Write and the std::fmt::Write trait. Example usage is:

use std::io::Write;
let mut w = Vec::new();
write!(&mut w, "Hello {}!", "world");

§print!

This and println! emit their output to stdout. Similarly to the write! macro, the goal of these macros is to avoid intermediate allocations when printing output. Example usage is:

print!("Hello {}!", "world");
println!("I have a newline {}", "character at the end");

§eprint!

The eprint! and eprintln! macros are identical to print! and println!, respectively, except they emit their output to stderr.

§format_args!

format_args! is a curious macro used to safely pass around an opaque object describing the format string. This object does not require any heap allocations to create, and it only references information on the stack. Under the hood, all of the related macros are implemented in terms of this. First off, some example usage is:

use std::fmt;
use std::io::{self, Write};

let mut some_writer = io::stdout();
write!(&mut some_writer, "{}", format_args!("print with a {}", "macro"));

fn my_fmt_fn(args: fmt::Arguments<'_>) {
    write!(&mut io::stdout(), "{args}");
}
my_fmt_fn(format_args!(", or a {} too", "function"));

The result of the format_args! macro is a value of type fmt::Arguments. This structure can then be passed to the write and format functions inside this module in order to process the format string. The goal of this macro is to even further prevent intermediate allocations when dealing with formatting strings.

For example, a logging library could use the standard formatting syntax, but it would internally pass around this structure until it has been determined where output should go to.

Structs§

Enums§

  • Alignment
    Possible alignments returned by Formatter::align

Traits§

Functions§

  • format
    Takes an Arguments struct and returns the resulting formatted string.
  • write
    Takes an output stream and an Arguments struct that can be precompiled with the format_args! macro.
  • from_fnExperimental
    Creates a type whose fmt::Debug and fmt::Display impls are provided with the function f.

Type Aliases§

  • Result
    The type returned by formatter methods.

Derive Macros§

  • Debug
    Derive macro generating an impl of the trait Debug.