# Module std::fmt

1.0.0 · source ·
Expand description

Utilities for formatting and printing `String`s.

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,
                                      //     )"

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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"

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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"

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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"

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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");

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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!");

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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")   !"

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### 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!");

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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`
- `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.

### 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);

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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");

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print three significantly different things:

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

### 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);

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## 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");

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## 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:

- nothing ⇒ `Display`
- `?` ⇒ `Debug`
- `x?` ⇒ `Debug` with lower-case hexadecimal integers
- `X?` ⇒ `Debug` with upper-case hexadecimal integers
- `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 {

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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"
    }

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#### `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\"");

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## 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 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.

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#### `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");

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#### `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");

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#### `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"));

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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

- 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 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
  Configuration for formatting.

## Enums

- Alignment
  Possible alignments returned by `Formatter::align`

## Traits

- Binary
  `b` formatting.
- Debug
  `?` formatting.
- Display
  Format trait for an empty format, `{}`.
- LowerExp
  `e` formatting.
- LowerHex
  `x` formatting.
- Octal
  `o` formatting.
- Pointer
  `p` formatting.
- UpperExp
  `E` formatting.
- UpperHex
  `X` formatting.
- Write
  A trait for writing or formatting into Unicode-accepting buffers or streams.

## Functions

- format
  The `format` function takes an `Arguments` struct and returns the resulting formatted string.
- write
  The `write` function takes an output stream, and an `Arguments` struct that can be precompiled with the `format_args!` macro.

## Type Aliases

- Result
  The type returned by formatter methods.

## Derive Macros

- Debug
  Derive macro generating an impl of the trait `Debug`.