Module std::fmt

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"
format!("{} {}", 1, 2);           // => "1 2"
format!("{:04}", 42);             // => "0042" with leading zeros

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.

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 which explicitly name their argument do not affect parameters which 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, although it must always be referred to with the same type.

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 which 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 argument:

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"

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

Argument types

Each argument's type is dictated by the format string. It is a requirement that every argument is only ever referred to by one type. For example, this is an invalid format string:

{0:x} {0:o}

This is invalid because the first argument is both referred to as a hexadecimal as well as an octal.

There are various parameters which do require a particular type, however. An example is the {:.*} syntax, which sets the number of decimal places in floating-point types:

let formatted_number = format!("{:.*}", 2, 1.234567);

assert_eq!("1.23", formatted_number)

If this syntax is used, then the number of characters to print precedes the actual object being formatted, and the number of characters must have the type usize. Although a usize can be printed with {}, it is invalid to reference an argument as such. For example this is another invalid format string:

{:.*} {0}

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:

• nothing ⇒ Display
• ? ⇒ Debug
• o ⇒ Octal
• x ⇒ LowerHex
• X ⇒ UpperHex
• p ⇒ Pointer
• b ⇒ Binary
• e ⇒ LowerExp
• E ⇒ UpperExp

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 f.buf stream. It is up to each format trait implementation to correctly adhere to the requested formatting parameters. The values of these parameters will be listed in the fields 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!("{:.*}", decimals, magnitude);
        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!("{:10.3b}", myvector); // => "     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\"");

Related macros

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 a &mut io::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
format_args! // described below.

write!

This 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 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");

format_args!

This is a curious macro which is 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 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.

Syntax

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> [ format <text> ] *
format := '{' [ argument ] [ ':' format_spec ] '}'
argument := integer | identifier

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

Formatting Parameters

Each argument being formatted can be transformed by a number of formatting parameters (corresponding to format_spec in the syntax above). These parameters affect the string representation of what's being formatted. This syntax draws heavily from Python's, so it may seem a bit familiar.

Fill/Alignment

The fill character is provided normally in conjunction with the width parameter. This indicates that if the value being formatted is smaller than width some extra characters will be printed around it. The extra characters are specified by fill, and the alignment can be one of the following options:

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

Note that alignment may not be implemented by some types. A good way to ensure padding is applied is to format your input, then use this resulting string to pad your output.

Sign/#/0

These can all be interpreted as flags for a particular formatter.

• + - This is intended for numeric types and indicates that the sign should always be printed. Positive signs are never printed by default, and the negative sign is only printed by default for the Signed trait. This flag indicates that the correct sign (+ or -) should always be printed.
• - - Currently not used
• # - This flag is indicates that the "alternate" form of printing should be used. The alternate forms are:
  • #? - pretty-print the Debug formatting
  • #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
• 0 - This is used to indicate for integer formats that the padding 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.

Width

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.

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

The value for the width can also be provided as a usize in the list of parameters by using the dollar syntax indicating that the second argument is a usize specifying the width, for example:

// 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);

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.

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: the first input holds the usize precision, and the second holds the value to print. Note that in this case, if one uses the format string {<arg>:<spec>.*}, then the <arg> part refers to the value to print, and the precision must come in the input preceding <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 ("x")} is {second of next two args (0.01) with precision
//                          specified in first of next two args (5)}
println!("Hello {} is {:.*}",    "x", 5, 0.01);

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

// Hello {next arg ("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 two significantly different things:

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

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 }}.

Structs

Arguments	This structure represents a safely precompiled version of a format string and its arguments. This cannot be generated at runtime because it cannot safely be done so, so no constructors are given and the fields are private to prevent modification.
DebugList	A struct to help with fmt::Debug implementations.
DebugMap	A struct to help with fmt::Debug implementations.
DebugSet	A struct to help with fmt::Debug implementations.
DebugStruct	A struct to help with fmt::Debug implementations.
DebugTuple	A struct to help with fmt::Debug implementations.
Error	The error type which is returned from formatting a message into a stream.
Formatter	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.

Traits

Binary	Format trait for the b character.
Debug	Format trait for the ? character.
Display	Format trait for an empty format, {}.
LowerExp	Format trait for the e character.
LowerHex	Format trait for the x character.
Octal	Format trait for the o character.
Pointer	Format trait for the p character.
UpperExp	Format trait for the E character.
UpperHex	Format trait for the X character.
Write	A collection of methods that are required to format a message into a stream.

Functions

format	The format function takes a precompiled format string and a list of arguments, to return the resulting formatted string.
write	The write function takes an output stream, a precompiled format string, and a list of arguments. The arguments will be formatted according to the specified format string into the output stream provided.

Type Definitions

Result

The type returned by formatter methods.